See upload Assignment Example | Topics and Well Written Essays - 500 words

See upload - Assignment Example Tax returns, copies thereof, or other records may be sufficient to establish the use of the method of accounting used in the preparation of the taxpayers income tax returns (i)  Cash receipts and disbursements method.  Generally, under the cash receipts and disbursements method in the computation of taxable income, all items which constitute gross income (whether in the form of cash, property, or services) are to be included for the taxable year in which actually or constructively received. Expenditures are to be deducted for the taxable year in which actually made. For rules relating to constructive receipt, see  §1.451-2. For treatment of an expenditure attributable to more than one taxable year, see section 461(a) and paragraph (a)(1) of  §1.461-1. (ii)  Accrual method.  (A) Generally, under an accrual method, income is to be included for the taxable year when all the events have occurred that fix the right to receive the income and the amount of the income can be determined with reasonable accuracy. Except as provided in section 5.02(2) of this revenue procedure for certain short taxable years, this revenue procedure does not permit deferral to a taxable year later than the next succeeding taxable year The court, distinguishing from the holding in Schlude v. Commissioner, held that accrual method taxpayers are not required to include prepayments in gross income when there is certainty as to when performance would occur. Verdict: the Court held that, under the accrual method, taxpayers must include as income in a particular year advance payments by way of cash, negotiable notes, and contract installments falling due but remaining unpaid during that year. Verdict: It was held that the Commissioner of Internal Revenue did not abuse his discretion in determining that the prepaid dues were taxable as income in the year in which they were actually received and in rejecting

Battered woman Syndrome Essay Example | Topics and Well Written Essays - 500 words

Battered woman Syndrome - Essay Example Francine Hughes was married to an abusive man for thirteen years. She tried to leave, even divorcing her husband. However, her ex-husband moved back in. When Francine tried to go to school, her ex-husband would rip up her books. Francine would try to go back home, to the Department of Human Services, and even the police. No one could or would help. Finally, after her ex-husband raped her, Francine set his bed on fire. Francine was not verbally abused, but beaten, raped, and put in the hospital over and over for thirteen years. When she went to trial her defense was the Battered-woman Defense. A jury found her not guilty by reason of insanity. Francines case generated much interest in the United States, with the focus on domestic violence (Westervelt, 102). The book and movie The Burning Bed caused Americans to stop and think about abused women for the first time. This story happened in the late 1970s, with the book coming out in the early 1980s. At that time battered women needed a defense for protecting themselves from abusive spouses. Yet, as the 1980s wore on, the Battered-woman Defense started to be misused. Betty Broderick definitely misused this defense. Betty Broderick was not a battered woman, but a scorned one. It began when her husband, Dan, began an affair with his secretary. Betty felt that the deception was mental abuse. Finally, Bettys husband left her for his secretary. After a nasty legal battle, wherein she felt ganged up on, because Dan was an attorney, Betty was left with nothing. She had left her four children on Dans doorstep, so he could understand how she felt. It backfired. Dan kept the children. However, the courts did order him to pay $16,000 a month, plus insurance and other bills. Betty in the meantime started breaking into Dans home and vandalizing it. The final straw was when Dan married his secretary. Betty stole her daughters keys, let herself into Dans house, and

High Performance Wireless Telecommunications Modulation

High Performance Wireless Telecommunications Modulation Introduction The primary goal of the project is to analyze of OFDM system and to assess the suitability of OFDM as a modulation technique for wireless communications. In the part of project is covered two leading successfully implementation of OFDM based technologies are Digital Video Broadcasting (DVB-T and DVB-H) and Long Term Evolution (LTE advanced for 4G). Wireless communications is an emerging field, which has seen enormous growth in the last several years. The huge uptake rate of mobile phone technology, Wireless Local Area Networks (WLAN) and the exponential growth of the Internet have resulted in an increased demand for new methods of obtaining high capacity wireless networks. For cellular mobile applications, we will see in the near future a complete convergence of mobile phone technology, computing, Internet access, and potentially many multimedia applications such as video and high quality audio. In fact, some may argue that this convergence has already largely occurred, with the advent of being able to send and receive data using a notebook computer and a mobile phone. The goal of third and fourth generation mobile networks is to provide users with a high data rate, and to provide a wider range of services, such as voice communications, videophones, and high speed Internet access. The higher data rate of future mobile networks will be achieved by increasing the amount of spectrum allocated to the service and by improvements in the spectral efficiency. OFDM is a potential candidate for the physical layer of fourth generation mobile systems. Basic Principles of OFDM OFDM overview The Orthogonal Frequency Division Multiplexing (OFDM) is a modulation technique where multiple low data rate carriers are combined by a transmitter to form a composite high data rate transmission. The first commercial use of OFDM in the communication field was in the 1980s, and it was later widely used in the broadcast audio and video field in the 1990s in such areas as, ADSL, VHDSL, ETSI standard digital audio broadcast (DAB), digital video broadcast (DVB), and high-definition digital TV (HDTV). Digital signal processing makes OFDM possible. To implement the multiple carrier scheme using a bank of parallel modulators would not be very efficient in analog hardware. However, in the digital domain, multi-carrier modulation can be done efficiently with currently available DSP hardware and software. Not only can it be done, but it can also be made very flexible and programmable. This allows OFDM to make maximum use of available bandwidth and to be able to adapt to changing system requirements. Figure 1 is illustrated, Instead of separate modulators; the outgoing waveform is created by executing a high-speed inverse DFT on a set of time-samples of the transmitted data (post modulation). The output of the DFT can be directly modulated onto the outgoing carrier, without requiring any other components. Each carrier in an OFDM system is a sinusoid with a frequency that is an integer multiple of a base or fundamental sinusoid frequency. Therefore, each carrier is like a Fourier series component of the composite signal. In fact, it will be shown later that an OFDM signal is created in the frequency domain, and then transformed into the time domain via the Discrete Fourier Transform (DFT). Two periodic signals are orthogonal when the integral of their product, over one period, is equal to zero. This is true of certain sinusoids as illustrated in Equation 1. Definition of Orthogonal The carriers of an OFDM system are sinusoids that meet this requirement because each one is a multiple of a fundamental frequency. Each one has an integer number of cycles in the fundamental period. [2, 145-153; 6] The importantance of being orthogonal The main concept in OFDM is orthogonality of the sub-carriers.Since the carriers are all sine/cosine wave, we know that area under one period of a sine or a cosine wave is zero. Lets take a sine wave of frequency m and multiply it by a sinusoid (sine or a cosine) of a frequency n, where both m and n are integers. The integral or the area under this product is given by These two components are each a sinusoid, so the integral is equal to zero over one period. When we multiply a sinusoid of frequency n by a sinusoid of frequency m/n the area under the product is zero. In general for all integers n and m , sin(mx), cos(mx), cos(nx) , sin(nx) are all orthogonal to each other. These frequencies are called harmonics. Making the subcarriers mathematically orthogonal was a breakthrough for OFDM because it enables OFDM receivers to separate the subcarriers via an FFT and eliminate the guard bands. As figure 3 shows, OFDM subcarriers can overlap to make full use of the spectrum, but at the peak of each subcarrier spectrum, the power in all the other subcarriers is zero. OFDM therefore offers higher data capacity in a given spectrum while allowing a simpler system design. Creating orthogonal subcarriers in the transmitter is easy using an inverse FFT. To ensure that this orthogonality is maintained at the receiver (so that the subcarriers are not misaligned), the system must keep the transmitter and receiver clocks closely synchronizedwithin 2 parts per million in 802.11a systems. The 802.11a standard therefore dedicates four of its 52 subcarriers as pilots that enable phase-lock loops in the receiver to track the phase and frequency of the incoming signal. The 802.11a standard therefore dedicates four of its 52 subcarriers as pilots that enable phase-lock loops in the receiver to track the phase and frequency of the incoming signal. This method also eliminates low-frequency phase noise.Separating the subcarriers via an FFT require about an order of magnitude fewer multiply-accumulate operations than individually filtering each carrier. In general, an FFT implementation is much simpler than the RAKE receivers used for CDMA and the decision-feedback equalizers for TDMA.This idea are key to understanding OFDM. The orthogonality allows simultaneously transmission on a lot of sub- carriers in a tight frequency space without interference form each other. In essence this is similar to CDMA, where codes are used to make data sequences independent (also orthogonal) which allows many independent users to transmitin same space successfully.[2, 153-154; 6 ; 7] OFDM Operation Preliminary Concepts When the DFT (Discrete Fourier Transform) of a time signal is taken, the frequency domain results are a function of the time sampling period and the number of samples as shown in Figure 4. The fundamental frequency of the DFT is equal to 1/NT (1/total sample time). Each frequency represented in the DFT is an integer multiple of the fundamental frequency. Parameter Mapping from Time to Frequency for the DFT The maximum frequency that can be represented by a time signal sampled at rate 1/T is fmax = 1/2T as given by the Nyquist sampling theorem. This frequency is located in the center of the DFT points. All frequencies beyond that point are images of the representative frequencies. The maximum frequency bin of the DFT is equal to the sampling frequency (1/T) minus one fundamental (1/NT).The IDFT (Inverse Discrete Fourier Transform) performs the opposite operation to the DFT. It takes a signal defined by frequency components and converts them to a time signal. The parameter mapping is the same as for the DFT. The time duration of the IDFT time signal is equal to the number of DFT bins (N) times the sampling period (T).It is perfectly valid to generate a signal in the frequency domain, and convert it to a time domain equivalent for practical use (The frequency domain is a mathematical tool used for analysis. Anything usable by the real world must be converted into a real, time domain signal). This is how modulation is applied in OFDM. In practice the Fast Fourier Transform (FFT) and IFFT are used in place of the DFT and IDFT, so all further references will be to FFT and IFFT.[1 ,118 ; 4] Definition of Carriers The maximum number of carriers used by OFDM is limited by the size of the IFFT. This is determined as follows in Equation 2. OFDM Carrier Count In order to generate a real-valued time signal, OFDM (frequency) carriers must be defined in complex conjugate pairs, which are symmetric about the Nyquist frequency (fmax). This puts the number of potential carriers equal to the IFFT size/2. The Nyquist frequency is the symmetry point, so it cannot be part of a complex conjugate pair. The DC component also has no complex conjugate. These two points cannot be used as carriers so they are subtracted from the total available. If the carriers are not defined in conjugate pairs, then the IFFT will result in a time domain signal that has imaginary components. This must be a viable option as there are OFDM systems defined with carrier counts that exceed the limit for real-valued time signals given in Equation 2.In general, a system with IFFT size 256 and carrier count 216. This design must result in a complex time waveform. Further processing would require some sort of quadrature technique (use of parallel sine and cosine processing paths). In this report, only real-value time signals will be treated, but in order to obtain maximum bandwidth efficiency from OFDM, the complex time signal may be preferred (possibly an analogous situation to QPSK vs. BPSK). Equation 2, for the complex time waveform, has all IFFT bins available as carriers except the DC bin. Both IFFT size and assignment (selection) of carriers can be dynamic. The transmitter and receiver just have to use the same parameters. This is one of the advantages of OFDM. Its bandwidth usage (and bit rate) can be varied according to varying user requirements. A simple control message from a base station can change a mobile units IFFT size and carrier selection.[2,199-206; 4] Modulation Binary data from a memory device or from a digital processing stream is used as the modulating (baseband) signal. The following steps may be carried out in order to apply modulation to the carriers in OFDM: combine the binary data into symbols according to the number of bits/symbol selected convert the serial symbol stream into parallel segments according to the number of carriers, and form carrier symbol sequences apply differential coding to each carrier symbol sequence convert each symbol into a complex phase representation assign each carrier sequence to the appropriate IFFT bin, including the complex conjugates take the IFFT of the result OFDM modulation is applied in the frequency domain. Figure 5 and Figure 6 give an example of modulated OFDM carriers for one symbol period, prior to IFFT. OFDM Carrier Magnitude prior to IFFT For this example, there are 4 carriers, the IFFT bin size is 64, and there is only 1 bit per symbol. The magnitude of each carrier is 1, but it could be scaled to any value. The phase for each carrier is either 0 or 180 degrees, according to the symbol being sent. The phase determines the value of the symbol (binary in this case, either a 1 or a 0). In the example, the first 3 bits (the first 3 carriers) are 0, and the 4th bit (4th carrier) is a 1. OFDM Carrier Phase prior to IFFT Note that the modulated OFDM signal is nothing more than a group of delta (impulse) functions, each with a phase determined by the modulating symbol. In addition, note that the frequency separation between each delta is proportional to 1/N where N is the number of IFFT bins. The frequency domain representation of the OFDM is described in Equation 3. OFDM Frequency Domain Representation (one symbol period) After the modulation is applied, an IFFT is performed to generate one symbol period in the time domain. The IFFT result is shown in 7. It is clear that the OFDM signal has varying amplitude. It is very important that the amplitude variations be kept intact as they define the content of the signal. If the amplitude is clipped or modified, then an FFT of the signal would no longer result in the original frequency characteristics, and the modulation may be lost. This is one of the drawbacks of OFDM, the fact that it requires linear amplification. In addition, very large amplitude peaks may occur depending on how the sinusoids line up, so the peak-to-average power ratio is high. This means that the linear amplifier has to have a large dynamic range to avoid distorting the peaks. The result is a linear amplifier with a constant, high bias current resulting in very poor power efficiency. OFDM Signal, 1 Symbol Period Figure 8 is provided to illustrate the time components of the OFDM signal. The IFFT transforms each complex conjugate pair of delta functions (each carrier) into a real-valued, pure sinusoid. Figure 8 shows the separate sinusoids that make up the composite OFDM waveform given in Figure 7. The one sinusoid with 180 phase shift is clearly visible as is the frequency difference between each of the 4 sinusoids. Transmission The key to the uniqueness and desirability of OFDM is the relationship between the carrier frequencies and the symbol rate. Each carrier frequency is separated by a multiple of 1/NT (Hz). The symbol rate (R) for each carrier is 1/NT (symbols/sec). The effect of the symbol rate on each OFDM carrier is to add a sin(x)/x shape to each carriers spectrum. The nulls of the sin(x)/x (for each carrier) are at integer multiples of 1/NT. The peak (for each carrier) is at the carrier frequency k/NT. Therefore, each carrier frequency is located at the nulls for all the other carriers. This means that none of the carriers will interfere with each other during transmission, although their spectrums overlap. The ability to space carriers so closely together is very bandwidth efficient. OFDM Time Waveform Figure 9 shows the OFDM time waveform for the same signal. There are 100 symbol periods in the signal. Each symbol period is 64 samples long (100 x 64 = 6400 total samples). Each symbol period contains 4 carriers each of which carries 1 symbol. Each symbol carries 1 bit. Note that Figure 9 again illustrates the large dynamic range of the OFDM waveform envelope. OFDM Spectrum Figure 10 shows the spectrum for of an OFDM signal with the following characteristics: 1 bit / symbol 100 symbols / carrier (i.e. a sequence of 100 symbol periods) 4 carriers 64 IFFT bins spectrum averaged for every 20 symbols (100/20 = 5 averages) Red diamonds mark all of the available carrier frequencies. Note that the nulls of the spectrums line up with the unused frequencies. The four active carriers each have peaks at carrier frequencies. It is clear that the active carriers have nulls in their spectrums at each of the unused frequencies (otherwise, the nulls would not exist). Although it cannot be seen in the figure, the active frequencies also have spectral nulls at the adjacent active frequencies. It is not currently practical to generate the OFDM signal directly at RF rates, so it must be up converted for transmission. To remain in the discrete domain, the OFDM could be upsampled and added to a discrete carrier frequency. This carrier could be an intermediate frequency whose sample rate is handled by current technology. It could then be converted to analog and increased to the final transmit frequency using analog frequency conversion methods. Alternatively, the OFDM modulation could be immediately converted to analog and directly increased to the desired RF transmits frequency. Either way, the selected technique would have to involve some form of linear AM (possibly implemented with a mixer). [1, 122-125; 6] Reception and Demodulation The received OFDM signal is down converted (in frequency) and taken from analog to digital. Demodulation is done in the frequency domain (just as modulation was). The following steps may be taken to demodulate the OFDM: partition the input stream into vectors representing each symbol period take the FFT of each symbol period vector extract the carrier FFT bins and calculate the phase of each calculate the phase difference, from one symbol period to the next, for each carrier decode each phase into binary data sort the data into the appropriate order OFDM Carrier Magnitude following FFT Figure 11 and Figure 12 show the magnitude and spectrum of the FFT for one received OFDM symbol period. For this example, there are 4 carriers, the IFFT bin size is 64, there is 1 bit per symbol, and the signal was sent through a channel with AWGN having an SNR of 8 dB. The figures show that, under these conditions, the modulated symbols are very easy to recover. OFDM Carrier Phase following FFT In Figure 12 that the unused frequency bins contain widely varying phase values. These bins are not decoded, so it does not matter, but the result is of interest. Even if the noise is removed from the channel, these phase variations still occur. It must be a result of the IFFT/FFT operations generating very small complex values (very close to 0) for the unused carriers. The phases are a result of these values. [1, 125 -128; 3] OFDM transceiver OFDM signals are typically generated digitally due to the difficulty in creating large banks of phase lock oscillators and receivers in the analog domain. Figure 13 shows the block diagram of a typical OFDM transceiver. The transmitter section converts digital data to be transmitted, into a mapping of subcarrier amplitude and phase. It then transforms this spectral representation of the data into the time domain using an Inverse Discrete Fourier Transform (IDFT). The Inverse Fast Fourier Transform (IFFT) performs the same operations as an IDFT, except that it is much more computationally efficiency, and so is used in all practical systems. In order to transmit the OFDM signal the calculated time domain signal is then mixed up to the required frequency. Block diagram showing a basic OFDM transceiver [3] The receiver performs the reverse operation of the transmitter, mixing the RF signal to base band for processing, then using a Fast Fourier Transform (FFT) to analyze the signal in the frequency domain. The amplitude and phase of the subcarriers is then picked out and converted back to digital data. The IFFT and the FFT are complementary function and the most appropriate term depends on whether the signal is being received or generated. In cases where the Signal is independent of this distinction then the term FFT and IFFT is used interchangeably. [1, 125 -128, 3] Analysis of OFDM characteristics Guard Period OFDM demodulation must be synchronized with the start and end of the transmitted symbol period. If it is not, then ISI will occur (since information will be decoded and combined for 2 adjacent symbol periods). ICI will also occur because orthogonality will be lost (integrals of the carrier products will no longer be zero over the integration period), To help solve this problem, a guard interval is added to each OFDM symbol period. The first thought of how to do this might be to simply make the symbol period longer, so that the demodulator does not have to be so precise in picking the period beginning and end, and decoding is always done inside a single period. This would fix the ISI problem, but not the ICI problem. If a complete period is not integrated (via FFT), orthogonality will be lost. The effect of ISI on an OFDM signal can be further improved by the addition of a guard period to the start of each symbol. This guard period is a cyclic copy that extends the length of the symbol waveform. Each subcarrier, in the data section of the symbol, (i.e. the OFDM symbol with no guard period added, which is equal to the length of the IFFT size used to generate the signal) has an integer number of cycles. Because of this, placing copies of the symbol end-to-end results in a continuous signal, with no discontinuities at the joins. Thus by copying the end of a symbol and appending this to the start results in a longer symbol time. Addition of a guard period to an OFDM signal [3] In Figure 14, The total length of the symbol is Ts=TG + TFFT, where Ts is the total length of the symbol in samples, TG is the length of the guard period in samples, and TFFT is the size of the IFFT used to generate the OFDM signal. In addition to protecting the OFDM from ISI, the guard period also provides protection against time-offset errors in the receiver. For an OFDM system that has the same sample rate for both the transmitter and receiver, it must use the same FFT size at both the receiver and transmitted signal in order to maintain subcarrier orthogonality. Each received symbol has TG + TFFT samples due to the added guard period. The receiver only needs TFFT samples of the received symbol to decode the signal. The remaining TG samples are redundant and are not needed. For an ideal channel with no delay spread the receiver can pick any time offset, up to the length of the guard period, and still get the correct number of samples, without crossing a symbol boundary. Function of the guard period for protecting against ISI [3] Figure 15 shows this effect. Adding a guard period allows time for the transient part of the signal to decay, so that the FFT is taken from a steady state portion of the symbol. This eliminates the effect of ISI provided that the guard period is longer than the delay spread of the radio channel. The remaining effects caused by the multipath, such as amplitude scaling and phase rotation are corrected for by channel equalization. In order to avoid ISI and ICI, the guard period must be formed by a cyclic extension of the symbol period. This is done by taking symbol period samples from the end of the period and appending them to the front of the period. The concept of being able to do this, and what it means, comes from the nature of the IFFT/FFT process. When the IFFT is taken for a symbol period (during OFDM modulation), the resulting time sample sequence is technically periodic. This is because the IFFT/FFT is an extension of the Fourier Transform which is an extension of the Fourier Series for periodic waveforms. All of these transforms operate on signals with either real or manufactured periodicity. For the IFFT/FFT, the period is the number of samples used. Guard Period via Cyclic Extension With the cyclic extension, the symbol period is longer, but it represents the exact same frequency spectrum. As long as the correct number of samples are taken for the decode, they may be taken anywhere within the extended symbol. Since a complete period is integrated, orthogonality is maintained. Therefore, both ISI and ICI are eliminated. Note that some bandwidth efficiency is lost with the addition of the guard period (symbol period is increased and symbol rate is decreased) [2,154-160, 3] Windowing The OFDM signal is made up of a series of IFFTs that are concatenated to each other. At each symbol period boundary, there is a signal discontinuity due to the differences between the end of one period and the start of the next. These discontinuities can cause high frequency spectral noise to be generated (because they look like very fast transitions of the time waveform). To avoid this, a window function (Hamming, Hanning, Blackman, ) may be applied to each symbol period. The window function would attenuate the time waveform at the start and the end of each period, so that the discontinuities are smaller, and the high frequency noise is reduced. However, this attenuation distorts the signal and some of the desired frequency content is lost.[1, 121;2 154] Multipath Characteristics OFDM avoids frequency selective fading and ISI by providing relatively long symbol periods for a given data rate. This is illustrated in Figure 17. For a given transmission channel and a given source data rate, OFDM can provide better multipath characteristics than a single carrier. OFDM vs. Single Carrier, Multipath Characteristic Comparison However, since the OFDM carriers are spread over a frequency range, there still may be some frequency selective attenuation on a time-varying basis. A deep fade on a particular frequency may cause the loss of data on that frequency for a given time, but the use of Forward Error Coding can fix it. If a single carrier experienced a deep fade, too many consecutive symbols may be lost and correction coding may be ineffective. [8] Bandwidth A comparison of RF transmits bandwidth between OFDM and a single carrier is shown in Figure 18 (using the same example parameters as in Figure 17). OFDM Bandwidth Efficiency In Figure 18, the calculations show that OFDM is more bandwidth efficient than a single carrier. Note that another efficient aspect of OFDM is that a single transmitters bandwidth can be increased incrementally by addition of more adjacent carriers. In addition, no bandwidth buffers are needed between transmit bandwidths of separate transmitters as long as orthogonality can be maintained between all the carriers.[2, 161-163; 8; 9] Physical Implementation Since OFDM is carried out in the digital domain, there are many ways it can be implemented. Some options are provided in the following list. Each of these options should be viable given current technology: ASIC (Application Specific Integrated Circuit) ASICs are the fastest, smallest, and lowest power way to implement OFDM Cannot change the ASIC after it is built without designing a new chip General-purpose Microprocessor or MicroController PowerPC 7400 or other processor capable of fast vector operations Highly programmable Needs memory and other peripheral chips Uses the most power and space, and would be the slowest Field-Programmable Gate Array (FPGA) An FPGA combines the speed, power, and density attributes of an ASIC with the programmability of a general purpose processor. An FPGA could be reprogrammed for new functions by a base station to meet future (currently unknown requirements).This should be the best choice.[9] OFDM uses in DVB (Digital Video Broadcasting) DVB (Digital Video Broadcast) is a set of standards for the digital transmission of video and audio streams, and also data transmission. The DVB standards are maintained by the DVB Project, which is an industry-led consortium of over 260 broadcasters, manufacturers, network operators, software developers, regulatory bodies and others in over 35 countries. DVB has been implemented over satellite (DVB-S, DVB-S2), cable (DVB-C), terrestrial broadcasting (DVB-T), and handheld terminals (DVB-H). the DVB standard following the logical progression of signal processing steps, as well as source and channel coding, COFDM modulation, MPEG compression and multiplexing methods, conditional access and set-top box Technology. In this project is presented an investigation of two OFDM based DVB standards, DVB-T and DVB-H. DVB-T (Digital Video Broadcasting Terrestrial) The first Terrestrial Digital Video Broadcasting pilot transmissions were started in the late 90s, and the first commercial system was established in Great Britain. In the next few years the digital broadcasting system has been set up in many countries, and the boom of the digital terrestrial transmission is estimated in the next few years, while the analogue transmission will be cancelled within about 15 years. The greatest advantage of the digital system is the effective use of the frequency spectrum and its lower radiated power in comparison with the analogue transmission, while the covered area remains the same. Another key feature is the possibility of designing a so-called Single Frequency Network (SFN), which means that the neighboring broadcast stations use the same frequency and the adjacent signals dont get interfered. The digital system transmits a data stream, which means that not only television signals but data communication (e.g. Internet service) may be used according to the demands. The data stream consists of an MPEG-2 bit stream, which means a compression is used, enabling the transfer of even 4 or 5 television via the standard 8 MHz wide TV channel. For the viewer, the main advantages are the perfect, noise-free picture, CD quality sound, and easier handling, as well as services like Super Teletext, Electronic Programme Guide, interactivity and mobility.[11, 251-253] Modulation technique in DVB-T The DVB-T Orthogonal Frequency Division Multiplexing (OFDM) modulation system uses multi-carrier transmission. There are 2 modes, the so-called 2k and 8k modes, using 1705 and 6817 carriers respectively, with each carrier modulated separately and transmitted in the 8 MHz TV channel. The common modulation for the carriers is typically QPSK, 16-QAM or 64-QAM. Each signal can be divided into two, so-called „In Phase (I) and „Quadrature Phase components, being a 90Â ° phase shift between them. The constellation diagram and the bit allocation is shown in bellow 16-QAM constellation diagram and bit allocation [6] This modulation can be demonstrated in the constellation diagram, where the 2 axes represent the 2 components (I and Q). In case of using 16-QAM modulation, the number of states is 16, so 1 symbol represents 4 bits. [11, 255; 6; 14] Bir errors If we simulate all the carriers in the constellation diagram we get not just 1 discrete point, but many points, forming a „cloud and representing each state. In case of additive noise the „cloud gets bigger and the receiver may decide incorrectly, resulting in bit errors. Figure 2 shows the measured constellation diagram without and with additive noise. Measured 16-QAM constellation diagram a) without additive noise b) with additive noise [6] To ensure perfect picture quality, the DVB-T system uses a 2 level error correction (Reed-Solomon and Viterbi). This corrects the bad bits at an even 10-4 Bit Error Rate (BER) and enables error-free data transmission. [13, 32-36] The multi-carrier structure The structure of carriers can be illustrated also in the function of time (Figure 20). The horizontal axis is the frequency and the vertical axis is the time. The 8 MHz channel consists of many carriers, placed 4462 Hz or 1116 Hz far from each other according to the modulation mode (2k or 8k). Structure of OFDM carriers [13] There are some reserved, so-called Transmission Parameter Signalling (TPS) carriers that do not transfer payload, just provide transmission mode information for the receiver, so the total number of useful carriers is 1512 and 6048 respectively in the two transmission modes, and the resultant bit rate is between 4,97 and 31,66 Mbit/s, depending on the modulation (QPSK, 16-QAM or 64-QAM), the transmission mode (2k or 8k), the Code Rate (CR) used for error correction and the selected Guard Interval (GI). This guard interval means that there is a small time gap between each symbol, so the transmission is not continuous. This guarding time enables perfect reception by eliminating the errors caused by multipath propagation.[4, 79-90; 13] Frequency spectrum In 2k mode, 1705 carriers are modulated in the 8 MHz TV channel, so each carrier is 4462 Hz far from its neighbor, while in 8k mode this distance is 1116 Hz. In digital broadcasting, there are no vision and sound carriers, so the power for each carrier is the same. This mean High Performance Wireless Telecommunications Modulation High Performance Wireless Telecommunications Modulation Introduction The primary goal of the project is to analyze of OFDM system and to assess the suitability of OFDM as a modulation technique for wireless communications. In the part of project is covered two leading successfully implementation of OFDM based technologies are Digital Video Broadcasting (DVB-T and DVB-H) and Long Term Evolution (LTE advanced for 4G). Wireless communications is an emerging field, which has seen enormous growth in the last several years. The huge uptake rate of mobile phone technology, Wireless Local Area Networks (WLAN) and the exponential growth of the Internet have resulted in an increased demand for new methods of obtaining high capacity wireless networks. For cellular mobile applications, we will see in the near future a complete convergence of mobile phone technology, computing, Internet access, and potentially many multimedia applications such as video and high quality audio. In fact, some may argue that this convergence has already largely occurred, with the advent of being able to send and receive data using a notebook computer and a mobile phone. The goal of third and fourth generation mobile networks is to provide users with a high data rate, and to provide a wider range of services, such as voice communications, videophones, and high speed Internet access. The higher data rate of future mobile networks will be achieved by increasing the amount of spectrum allocated to the service and by improvements in the spectral efficiency. OFDM is a potential candidate for the physical layer of fourth generation mobile systems. Basic Principles of OFDM OFDM overview The Orthogonal Frequency Division Multiplexing (OFDM) is a modulation technique where multiple low data rate carriers are combined by a transmitter to form a composite high data rate transmission. The first commercial use of OFDM in the communication field was in the 1980s, and it was later widely used in the broadcast audio and video field in the 1990s in such areas as, ADSL, VHDSL, ETSI standard digital audio broadcast (DAB), digital video broadcast (DVB), and high-definition digital TV (HDTV). Digital signal processing makes OFDM possible. To implement the multiple carrier scheme using a bank of parallel modulators would not be very efficient in analog hardware. However, in the digital domain, multi-carrier modulation can be done efficiently with currently available DSP hardware and software. Not only can it be done, but it can also be made very flexible and programmable. This allows OFDM to make maximum use of available bandwidth and to be able to adapt to changing system requirements. Figure 1 is illustrated, Instead of separate modulators; the outgoing waveform is created by executing a high-speed inverse DFT on a set of time-samples of the transmitted data (post modulation). The output of the DFT can be directly modulated onto the outgoing carrier, without requiring any other components. Each carrier in an OFDM system is a sinusoid with a frequency that is an integer multiple of a base or fundamental sinusoid frequency. Therefore, each carrier is like a Fourier series component of the composite signal. In fact, it will be shown later that an OFDM signal is created in the frequency domain, and then transformed into the time domain via the Discrete Fourier Transform (DFT). Two periodic signals are orthogonal when the integral of their product, over one period, is equal to zero. This is true of certain sinusoids as illustrated in Equation 1. Definition of Orthogonal The carriers of an OFDM system are sinusoids that meet this requirement because each one is a multiple of a fundamental frequency. Each one has an integer number of cycles in the fundamental period. [2, 145-153; 6] The importantance of being orthogonal The main concept in OFDM is orthogonality of the sub-carriers.Since the carriers are all sine/cosine wave, we know that area under one period of a sine or a cosine wave is zero. Lets take a sine wave of frequency m and multiply it by a sinusoid (sine or a cosine) of a frequency n, where both m and n are integers. The integral or the area under this product is given by These two components are each a sinusoid, so the integral is equal to zero over one period. When we multiply a sinusoid of frequency n by a sinusoid of frequency m/n the area under the product is zero. In general for all integers n and m , sin(mx), cos(mx), cos(nx) , sin(nx) are all orthogonal to each other. These frequencies are called harmonics. Making the subcarriers mathematically orthogonal was a breakthrough for OFDM because it enables OFDM receivers to separate the subcarriers via an FFT and eliminate the guard bands. As figure 3 shows, OFDM subcarriers can overlap to make full use of the spectrum, but at the peak of each subcarrier spectrum, the power in all the other subcarriers is zero. OFDM therefore offers higher data capacity in a given spectrum while allowing a simpler system design. Creating orthogonal subcarriers in the transmitter is easy using an inverse FFT. To ensure that this orthogonality is maintained at the receiver (so that the subcarriers are not misaligned), the system must keep the transmitter and receiver clocks closely synchronizedwithin 2 parts per million in 802.11a systems. The 802.11a standard therefore dedicates four of its 52 subcarriers as pilots that enable phase-lock loops in the receiver to track the phase and frequency of the incoming signal. The 802.11a standard therefore dedicates four of its 52 subcarriers as pilots that enable phase-lock loops in the receiver to track the phase and frequency of the incoming signal. This method also eliminates low-frequency phase noise.Separating the subcarriers via an FFT require about an order of magnitude fewer multiply-accumulate operations than individually filtering each carrier. In general, an FFT implementation is much simpler than the RAKE receivers used for CDMA and the decision-feedback equalizers for TDMA.This idea are key to understanding OFDM. The orthogonality allows simultaneously transmission on a lot of sub- carriers in a tight frequency space without interference form each other. In essence this is similar to CDMA, where codes are used to make data sequences independent (also orthogonal) which allows many independent users to transmitin same space successfully.[2, 153-154; 6 ; 7] OFDM Operation Preliminary Concepts When the DFT (Discrete Fourier Transform) of a time signal is taken, the frequency domain results are a function of the time sampling period and the number of samples as shown in Figure 4. The fundamental frequency of the DFT is equal to 1/NT (1/total sample time). Each frequency represented in the DFT is an integer multiple of the fundamental frequency. Parameter Mapping from Time to Frequency for the DFT The maximum frequency that can be represented by a time signal sampled at rate 1/T is fmax = 1/2T as given by the Nyquist sampling theorem. This frequency is located in the center of the DFT points. All frequencies beyond that point are images of the representative frequencies. The maximum frequency bin of the DFT is equal to the sampling frequency (1/T) minus one fundamental (1/NT).The IDFT (Inverse Discrete Fourier Transform) performs the opposite operation to the DFT. It takes a signal defined by frequency components and converts them to a time signal. The parameter mapping is the same as for the DFT. The time duration of the IDFT time signal is equal to the number of DFT bins (N) times the sampling period (T).It is perfectly valid to generate a signal in the frequency domain, and convert it to a time domain equivalent for practical use (The frequency domain is a mathematical tool used for analysis. Anything usable by the real world must be converted into a real, time domain signal). This is how modulation is applied in OFDM. In practice the Fast Fourier Transform (FFT) and IFFT are used in place of the DFT and IDFT, so all further references will be to FFT and IFFT.[1 ,118 ; 4] Definition of Carriers The maximum number of carriers used by OFDM is limited by the size of the IFFT. This is determined as follows in Equation 2. OFDM Carrier Count In order to generate a real-valued time signal, OFDM (frequency) carriers must be defined in complex conjugate pairs, which are symmetric about the Nyquist frequency (fmax). This puts the number of potential carriers equal to the IFFT size/2. The Nyquist frequency is the symmetry point, so it cannot be part of a complex conjugate pair. The DC component also has no complex conjugate. These two points cannot be used as carriers so they are subtracted from the total available. If the carriers are not defined in conjugate pairs, then the IFFT will result in a time domain signal that has imaginary components. This must be a viable option as there are OFDM systems defined with carrier counts that exceed the limit for real-valued time signals given in Equation 2.In general, a system with IFFT size 256 and carrier count 216. This design must result in a complex time waveform. Further processing would require some sort of quadrature technique (use of parallel sine and cosine processing paths). In this report, only real-value time signals will be treated, but in order to obtain maximum bandwidth efficiency from OFDM, the complex time signal may be preferred (possibly an analogous situation to QPSK vs. BPSK). Equation 2, for the complex time waveform, has all IFFT bins available as carriers except the DC bin. Both IFFT size and assignment (selection) of carriers can be dynamic. The transmitter and receiver just have to use the same parameters. This is one of the advantages of OFDM. Its bandwidth usage (and bit rate) can be varied according to varying user requirements. A simple control message from a base station can change a mobile units IFFT size and carrier selection.[2,199-206; 4] Modulation Binary data from a memory device or from a digital processing stream is used as the modulating (baseband) signal. The following steps may be carried out in order to apply modulation to the carriers in OFDM: combine the binary data into symbols according to the number of bits/symbol selected convert the serial symbol stream into parallel segments according to the number of carriers, and form carrier symbol sequences apply differential coding to each carrier symbol sequence convert each symbol into a complex phase representation assign each carrier sequence to the appropriate IFFT bin, including the complex conjugates take the IFFT of the result OFDM modulation is applied in the frequency domain. Figure 5 and Figure 6 give an example of modulated OFDM carriers for one symbol period, prior to IFFT. OFDM Carrier Magnitude prior to IFFT For this example, there are 4 carriers, the IFFT bin size is 64, and there is only 1 bit per symbol. The magnitude of each carrier is 1, but it could be scaled to any value. The phase for each carrier is either 0 or 180 degrees, according to the symbol being sent. The phase determines the value of the symbol (binary in this case, either a 1 or a 0). In the example, the first 3 bits (the first 3 carriers) are 0, and the 4th bit (4th carrier) is a 1. OFDM Carrier Phase prior to IFFT Note that the modulated OFDM signal is nothing more than a group of delta (impulse) functions, each with a phase determined by the modulating symbol. In addition, note that the frequency separation between each delta is proportional to 1/N where N is the number of IFFT bins. The frequency domain representation of the OFDM is described in Equation 3. OFDM Frequency Domain Representation (one symbol period) After the modulation is applied, an IFFT is performed to generate one symbol period in the time domain. The IFFT result is shown in 7. It is clear that the OFDM signal has varying amplitude. It is very important that the amplitude variations be kept intact as they define the content of the signal. If the amplitude is clipped or modified, then an FFT of the signal would no longer result in the original frequency characteristics, and the modulation may be lost. This is one of the drawbacks of OFDM, the fact that it requires linear amplification. In addition, very large amplitude peaks may occur depending on how the sinusoids line up, so the peak-to-average power ratio is high. This means that the linear amplifier has to have a large dynamic range to avoid distorting the peaks. The result is a linear amplifier with a constant, high bias current resulting in very poor power efficiency. OFDM Signal, 1 Symbol Period Figure 8 is provided to illustrate the time components of the OFDM signal. The IFFT transforms each complex conjugate pair of delta functions (each carrier) into a real-valued, pure sinusoid. Figure 8 shows the separate sinusoids that make up the composite OFDM waveform given in Figure 7. The one sinusoid with 180 phase shift is clearly visible as is the frequency difference between each of the 4 sinusoids. Transmission The key to the uniqueness and desirability of OFDM is the relationship between the carrier frequencies and the symbol rate. Each carrier frequency is separated by a multiple of 1/NT (Hz). The symbol rate (R) for each carrier is 1/NT (symbols/sec). The effect of the symbol rate on each OFDM carrier is to add a sin(x)/x shape to each carriers spectrum. The nulls of the sin(x)/x (for each carrier) are at integer multiples of 1/NT. The peak (for each carrier) is at the carrier frequency k/NT. Therefore, each carrier frequency is located at the nulls for all the other carriers. This means that none of the carriers will interfere with each other during transmission, although their spectrums overlap. The ability to space carriers so closely together is very bandwidth efficient. OFDM Time Waveform Figure 9 shows the OFDM time waveform for the same signal. There are 100 symbol periods in the signal. Each symbol period is 64 samples long (100 x 64 = 6400 total samples). Each symbol period contains 4 carriers each of which carries 1 symbol. Each symbol carries 1 bit. Note that Figure 9 again illustrates the large dynamic range of the OFDM waveform envelope. OFDM Spectrum Figure 10 shows the spectrum for of an OFDM signal with the following characteristics: 1 bit / symbol 100 symbols / carrier (i.e. a sequence of 100 symbol periods) 4 carriers 64 IFFT bins spectrum averaged for every 20 symbols (100/20 = 5 averages) Red diamonds mark all of the available carrier frequencies. Note that the nulls of the spectrums line up with the unused frequencies. The four active carriers each have peaks at carrier frequencies. It is clear that the active carriers have nulls in their spectrums at each of the unused frequencies (otherwise, the nulls would not exist). Although it cannot be seen in the figure, the active frequencies also have spectral nulls at the adjacent active frequencies. It is not currently practical to generate the OFDM signal directly at RF rates, so it must be up converted for transmission. To remain in the discrete domain, the OFDM could be upsampled and added to a discrete carrier frequency. This carrier could be an intermediate frequency whose sample rate is handled by current technology. It could then be converted to analog and increased to the final transmit frequency using analog frequency conversion methods. Alternatively, the OFDM modulation could be immediately converted to analog and directly increased to the desired RF transmits frequency. Either way, the selected technique would have to involve some form of linear AM (possibly implemented with a mixer). [1, 122-125; 6] Reception and Demodulation The received OFDM signal is down converted (in frequency) and taken from analog to digital. Demodulation is done in the frequency domain (just as modulation was). The following steps may be taken to demodulate the OFDM: partition the input stream into vectors representing each symbol period take the FFT of each symbol period vector extract the carrier FFT bins and calculate the phase of each calculate the phase difference, from one symbol period to the next, for each carrier decode each phase into binary data sort the data into the appropriate order OFDM Carrier Magnitude following FFT Figure 11 and Figure 12 show the magnitude and spectrum of the FFT for one received OFDM symbol period. For this example, there are 4 carriers, the IFFT bin size is 64, there is 1 bit per symbol, and the signal was sent through a channel with AWGN having an SNR of 8 dB. The figures show that, under these conditions, the modulated symbols are very easy to recover. OFDM Carrier Phase following FFT In Figure 12 that the unused frequency bins contain widely varying phase values. These bins are not decoded, so it does not matter, but the result is of interest. Even if the noise is removed from the channel, these phase variations still occur. It must be a result of the IFFT/FFT operations generating very small complex values (very close to 0) for the unused carriers. The phases are a result of these values. [1, 125 -128; 3] OFDM transceiver OFDM signals are typically generated digitally due to the difficulty in creating large banks of phase lock oscillators and receivers in the analog domain. Figure 13 shows the block diagram of a typical OFDM transceiver. The transmitter section converts digital data to be transmitted, into a mapping of subcarrier amplitude and phase. It then transforms this spectral representation of the data into the time domain using an Inverse Discrete Fourier Transform (IDFT). The Inverse Fast Fourier Transform (IFFT) performs the same operations as an IDFT, except that it is much more computationally efficiency, and so is used in all practical systems. In order to transmit the OFDM signal the calculated time domain signal is then mixed up to the required frequency. Block diagram showing a basic OFDM transceiver [3] The receiver performs the reverse operation of the transmitter, mixing the RF signal to base band for processing, then using a Fast Fourier Transform (FFT) to analyze the signal in the frequency domain. The amplitude and phase of the subcarriers is then picked out and converted back to digital data. The IFFT and the FFT are complementary function and the most appropriate term depends on whether the signal is being received or generated. In cases where the Signal is independent of this distinction then the term FFT and IFFT is used interchangeably. [1, 125 -128, 3] Analysis of OFDM characteristics Guard Period OFDM demodulation must be synchronized with the start and end of the transmitted symbol period. If it is not, then ISI will occur (since information will be decoded and combined for 2 adjacent symbol periods). ICI will also occur because orthogonality will be lost (integrals of the carrier products will no longer be zero over the integration period), To help solve this problem, a guard interval is added to each OFDM symbol period. The first thought of how to do this might be to simply make the symbol period longer, so that the demodulator does not have to be so precise in picking the period beginning and end, and decoding is always done inside a single period. This would fix the ISI problem, but not the ICI problem. If a complete period is not integrated (via FFT), orthogonality will be lost. The effect of ISI on an OFDM signal can be further improved by the addition of a guard period to the start of each symbol. This guard period is a cyclic copy that extends the length of the symbol waveform. Each subcarrier, in the data section of the symbol, (i.e. the OFDM symbol with no guard period added, which is equal to the length of the IFFT size used to generate the signal) has an integer number of cycles. Because of this, placing copies of the symbol end-to-end results in a continuous signal, with no discontinuities at the joins. Thus by copying the end of a symbol and appending this to the start results in a longer symbol time. Addition of a guard period to an OFDM signal [3] In Figure 14, The total length of the symbol is Ts=TG + TFFT, where Ts is the total length of the symbol in samples, TG is the length of the guard period in samples, and TFFT is the size of the IFFT used to generate the OFDM signal. In addition to protecting the OFDM from ISI, the guard period also provides protection against time-offset errors in the receiver. For an OFDM system that has the same sample rate for both the transmitter and receiver, it must use the same FFT size at both the receiver and transmitted signal in order to maintain subcarrier orthogonality. Each received symbol has TG + TFFT samples due to the added guard period. The receiver only needs TFFT samples of the received symbol to decode the signal. The remaining TG samples are redundant and are not needed. For an ideal channel with no delay spread the receiver can pick any time offset, up to the length of the guard period, and still get the correct number of samples, without crossing a symbol boundary. Function of the guard period for protecting against ISI [3] Figure 15 shows this effect. Adding a guard period allows time for the transient part of the signal to decay, so that the FFT is taken from a steady state portion of the symbol. This eliminates the effect of ISI provided that the guard period is longer than the delay spread of the radio channel. The remaining effects caused by the multipath, such as amplitude scaling and phase rotation are corrected for by channel equalization. In order to avoid ISI and ICI, the guard period must be formed by a cyclic extension of the symbol period. This is done by taking symbol period samples from the end of the period and appending them to the front of the period. The concept of being able to do this, and what it means, comes from the nature of the IFFT/FFT process. When the IFFT is taken for a symbol period (during OFDM modulation), the resulting time sample sequence is technically periodic. This is because the IFFT/FFT is an extension of the Fourier Transform which is an extension of the Fourier Series for periodic waveforms. All of these transforms operate on signals with either real or manufactured periodicity. For the IFFT/FFT, the period is the number of samples used. Guard Period via Cyclic Extension With the cyclic extension, the symbol period is longer, but it represents the exact same frequency spectrum. As long as the correct number of samples are taken for the decode, they may be taken anywhere within the extended symbol. Since a complete period is integrated, orthogonality is maintained. Therefore, both ISI and ICI are eliminated. Note that some bandwidth efficiency is lost with the addition of the guard period (symbol period is increased and symbol rate is decreased) [2,154-160, 3] Windowing The OFDM signal is made up of a series of IFFTs that are concatenated to each other. At each symbol period boundary, there is a signal discontinuity due to the differences between the end of one period and the start of the next. These discontinuities can cause high frequency spectral noise to be generated (because they look like very fast transitions of the time waveform). To avoid this, a window function (Hamming, Hanning, Blackman, ) may be applied to each symbol period. The window function would attenuate the time waveform at the start and the end of each period, so that the discontinuities are smaller, and the high frequency noise is reduced. However, this attenuation distorts the signal and some of the desired frequency content is lost.[1, 121;2 154] Multipath Characteristics OFDM avoids frequency selective fading and ISI by providing relatively long symbol periods for a given data rate. This is illustrated in Figure 17. For a given transmission channel and a given source data rate, OFDM can provide better multipath characteristics than a single carrier. OFDM vs. Single Carrier, Multipath Characteristic Comparison However, since the OFDM carriers are spread over a frequency range, there still may be some frequency selective attenuation on a time-varying basis. A deep fade on a particular frequency may cause the loss of data on that frequency for a given time, but the use of Forward Error Coding can fix it. If a single carrier experienced a deep fade, too many consecutive symbols may be lost and correction coding may be ineffective. [8] Bandwidth A comparison of RF transmits bandwidth between OFDM and a single carrier is shown in Figure 18 (using the same example parameters as in Figure 17). OFDM Bandwidth Efficiency In Figure 18, the calculations show that OFDM is more bandwidth efficient than a single carrier. Note that another efficient aspect of OFDM is that a single transmitters bandwidth can be increased incrementally by addition of more adjacent carriers. In addition, no bandwidth buffers are needed between transmit bandwidths of separate transmitters as long as orthogonality can be maintained between all the carriers.[2, 161-163; 8; 9] Physical Implementation Since OFDM is carried out in the digital domain, there are many ways it can be implemented. Some options are provided in the following list. Each of these options should be viable given current technology: ASIC (Application Specific Integrated Circuit) ASICs are the fastest, smallest, and lowest power way to implement OFDM Cannot change the ASIC after it is built without designing a new chip General-purpose Microprocessor or MicroController PowerPC 7400 or other processor capable of fast vector operations Highly programmable Needs memory and other peripheral chips Uses the most power and space, and would be the slowest Field-Programmable Gate Array (FPGA) An FPGA combines the speed, power, and density attributes of an ASIC with the programmability of a general purpose processor. An FPGA could be reprogrammed for new functions by a base station to meet future (currently unknown requirements).This should be the best choice.[9] OFDM uses in DVB (Digital Video Broadcasting) DVB (Digital Video Broadcast) is a set of standards for the digital transmission of video and audio streams, and also data transmission. The DVB standards are maintained by the DVB Project, which is an industry-led consortium of over 260 broadcasters, manufacturers, network operators, software developers, regulatory bodies and others in over 35 countries. DVB has been implemented over satellite (DVB-S, DVB-S2), cable (DVB-C), terrestrial broadcasting (DVB-T), and handheld terminals (DVB-H). the DVB standard following the logical progression of signal processing steps, as well as source and channel coding, COFDM modulation, MPEG compression and multiplexing methods, conditional access and set-top box Technology. In this project is presented an investigation of two OFDM based DVB standards, DVB-T and DVB-H. DVB-T (Digital Video Broadcasting Terrestrial) The first Terrestrial Digital Video Broadcasting pilot transmissions were started in the late 90s, and the first commercial system was established in Great Britain. In the next few years the digital broadcasting system has been set up in many countries, and the boom of the digital terrestrial transmission is estimated in the next few years, while the analogue transmission will be cancelled within about 15 years. The greatest advantage of the digital system is the effective use of the frequency spectrum and its lower radiated power in comparison with the analogue transmission, while the covered area remains the same. Another key feature is the possibility of designing a so-called Single Frequency Network (SFN), which means that the neighboring broadcast stations use the same frequency and the adjacent signals dont get interfered. The digital system transmits a data stream, which means that not only television signals but data communication (e.g. Internet service) may be used according to the demands. The data stream consists of an MPEG-2 bit stream, which means a compression is used, enabling the transfer of even 4 or 5 television via the standard 8 MHz wide TV channel. For the viewer, the main advantages are the perfect, noise-free picture, CD quality sound, and easier handling, as well as services like Super Teletext, Electronic Programme Guide, interactivity and mobility.[11, 251-253] Modulation technique in DVB-T The DVB-T Orthogonal Frequency Division Multiplexing (OFDM) modulation system uses multi-carrier transmission. There are 2 modes, the so-called 2k and 8k modes, using 1705 and 6817 carriers respectively, with each carrier modulated separately and transmitted in the 8 MHz TV channel. The common modulation for the carriers is typically QPSK, 16-QAM or 64-QAM. Each signal can be divided into two, so-called „In Phase (I) and „Quadrature Phase components, being a 90Â ° phase shift between them. The constellation diagram and the bit allocation is shown in bellow 16-QAM constellation diagram and bit allocation [6] This modulation can be demonstrated in the constellation diagram, where the 2 axes represent the 2 components (I and Q). In case of using 16-QAM modulation, the number of states is 16, so 1 symbol represents 4 bits. [11, 255; 6; 14] Bir errors If we simulate all the carriers in the constellation diagram we get not just 1 discrete point, but many points, forming a „cloud and representing each state. In case of additive noise the „cloud gets bigger and the receiver may decide incorrectly, resulting in bit errors. Figure 2 shows the measured constellation diagram without and with additive noise. Measured 16-QAM constellation diagram a) without additive noise b) with additive noise [6] To ensure perfect picture quality, the DVB-T system uses a 2 level error correction (Reed-Solomon and Viterbi). This corrects the bad bits at an even 10-4 Bit Error Rate (BER) and enables error-free data transmission. [13, 32-36] The multi-carrier structure The structure of carriers can be illustrated also in the function of time (Figure 20). The horizontal axis is the frequency and the vertical axis is the time. The 8 MHz channel consists of many carriers, placed 4462 Hz or 1116 Hz far from each other according to the modulation mode (2k or 8k). Structure of OFDM carriers [13] There are some reserved, so-called Transmission Parameter Signalling (TPS) carriers that do not transfer payload, just provide transmission mode information for the receiver, so the total number of useful carriers is 1512 and 6048 respectively in the two transmission modes, and the resultant bit rate is between 4,97 and 31,66 Mbit/s, depending on the modulation (QPSK, 16-QAM or 64-QAM), the transmission mode (2k or 8k), the Code Rate (CR) used for error correction and the selected Guard Interval (GI). This guard interval means that there is a small time gap between each symbol, so the transmission is not continuous. This guarding time enables perfect reception by eliminating the errors caused by multipath propagation.[4, 79-90; 13] Frequency spectrum In 2k mode, 1705 carriers are modulated in the 8 MHz TV channel, so each carrier is 4462 Hz far from its neighbor, while in 8k mode this distance is 1116 Hz. In digital broadcasting, there are no vision and sound carriers, so the power for each carrier is the same. This mean

Portrait :: essays research papers

Portrait of the Artist as a Young Man Stephen Dedalus is born of a woman, created of the earth; pure in his childhood innocence. From this beginning stems the birth of an artist, and from this the novel, A Portrait of the Artist as a Young Man, James Joyce recounts Stephen's story. His journey is followed from childhood to maturity, and thus his transformation from secular to saintly to an awakening of what he truly is. The novel evolves from simple, childlike diction, to sophisticated, higher ideas and thoughts as Dedalus completes his transition into an artist. In the beginning, Dedalus sees the world in an almost sing-song nursery rhyme sense, with a "moocow" coming down the road. By the end of the novel, Dedalus is mature and worldly; a man who stands tall and who feels confident with "Old father, old artificer, stand me now and ever in good stead." (238). Through the use of the symbols of woman and earth, and white and purification, Joyce gives his novel depth and wonder. These symbols follow an array of transformations, changing throughout the novel much like Stephen himself. The figure woman goes from the mother figure, to that of the whore, and finally to the representation of freedom itself. As a child, the image of the mother figure is strong. It is nurturing and supportive, that of "a woman standing at the half-door of a cottage with a child in her arms . . ." (10) who shelters and protects and makes Stephen afraid to "think of how it was" to be without a mother. As Stephen grows, however, like any child his dependency of him mother begins to dwindle, as does his awe for her. He begins to question his relationship with her and she is suddenly seen as a dirty figure, beginning the transformation of Stephen's image of women; from that of mother to whore. He first begins to questions the purity of his mother, his creator, his earth, when confronted by class mates, who taunt and confuse the innocent act of kissing his mother. He suddenly wonders, "Was it right to kiss his mother or wrong to kiss his mother? What did that mean, to kiss? You put your face up like that to say good night and then his mother put her face down. That was to kiss." (24) However, later in the novel the image of the pure and novel mother appears once more, but not in the figure of Stephen's own mother.

Mrs Elizabeth Dole’s Presidential Election

As President of the American Red Cross, Elizabeth Dole has led an extraordinary public service career in which she has served six United States Presidents and has been named by the Gallup Poll as one of the world†s ten most admired women. Born and raised in Salisbury, North Carolina, Elizabeth Dole was apparently always diligent. She obtained excellent grades and won the prize in an essay writing competition offered annually by the Daughters of the Confederacy. Her classmates voted her â€Å"Most Likely to Succeed,† and would often remark that she would one day be a First Lady or a President. Following in her brother†s footsteps, she attended Duke University. She was elected president of the Women†s Student Government Association. Elizabeth Dole left Duke with a bachelor†s degree in political science, with recognition as Student Leader of the Year, Phi Beta Kappa and was the May Queen. She then went on to earn her law degree from Harvard Law School as well as obtaining a master†s in education and government from Harvard. Elizabeth Dole headed the White House Office of Consumer Affairs under both Presidents Johnson and Nixon. It was there that she began a career-long dedication to public safety, for which she received the National Safety Council†s Distinguished Service Award in 1989. By 1974, Nixon had appointed her a Federal Trade Commissioner. She and Bob Dole were married in 1975 while she was still with the FTC, and when he became the Vice Presidential candidate under Jerry Ford, she took a leave of absence to campaign for him. In 1980, the now married Elizabeth Dole, impressed Ronald Reagan to the extent that he appointed her director of his transition team†s human services group and a year later, promoted her to head of the White House Office of Public Liaison. In February 1983, Elizabeth Dole joined President Reagan†s Cabinet as Secretary of Transportation – the first woman to hold that position. During her four years at Transportation, the United States enjoyed the safest years in its history in all three major areas – rail, air, and highway. Some of her many safety initiatives included a new regulation which required air bags or automatic safety belts in all new cars and spawned safety belt laws in 36 states and the District of Columbia. She led the crusade to raise the drinking age to 21; directed the overhaul of the aviation safety inspection system; and imposed tougher aviation security measures at the U.S. airports, which led to tightened security measures around the world. She also oversaw the sale of CONRAIL, the government-owned freight railroad that returned $1.2 billion dollars to the U.S. Treasury. In January of 1989, President Bush swore in Elizabeth Dole as the nation†s 20th Secretary of Labor. As Labor Secretary, she served as the President†s chief adviser on labor and work force issues. She has worked to help shatter the â€Å"glass ceiling† for America†s working women and minorities, increase safety and health in the workplace, upgrade the skills of the American work force, and improve relations between labor and management, playing a key role in bringing the parties together to resolve the bitter eleven month Pittston Coal Strike. In 1993, Women Executives in State Government honored Elizabeth Dole with their Lifetime Achievement Award for her many achievements in helping women and minorities break through the â€Å"glass ceiling.† Also this year, she was selected for induction into the Safety and Health Hall of Fame International for her numerous transportation, workplace, and blood safety accomplishments. She went on to receive the North Carolina Press Association†s first â€Å"North Carolinian of the Year† Award. As President of the American Red Cross, Elizabeth Dole oversaw nearly 30,000 staff members and more than 1.5 million volunteers who comprise the world†s foremost humanitarian organization. She was a member of that volunteer force in 1991, taking no salary her first year. The American Red Cross provides 52% of America†s blood supply. While blood is â€Å"overwhelmingly safe,† to quote the Food and Drug Administration, four months into her presidency, Elizabeth Dole secured approval of the organization†s Board of Governors to launch a sweeping $148 million state of the art blood system which will be able to quickly and efficiently incorporate medical technology as it evolves. Following two years of record breaking natural disasters, Elizabeth Dole launched an aggressive relief campaign that raised $172 million dollars in 1992 to assist victims of disasters including Hurricanes Andrew and Iniki. Elizabeth Dole certainly has the political credentials as well as strong other values. She understands how to be powerful and yet remain human, warm and sincere. She understands the importance of integrity, morality, and accountability in government. With all the scandal that Bill Clinton has brought to Washington, observers say that Mrs. Dole†s strong religious and traditional values could work as a remedy. If our country will ever be ready for a female in the Oval Office it is now, with Elizabeth Dole. There will be, however, significant electoral, institutional, and constitutional ramifications if she is elected. First of all, the Electoral College will be jumbled. As Elizabeth Dole is a strong member of the Republican Party, electing a woman to the presidential office is a very democratic move. Therefore, many of the Democratic electoral voters may cast their votes in the direction of Elizabeth Dole, rather than their own presidential candidate, and vice versa for the Republican electoral voters. These electoral voters will be in a cross-pressured situation that will blur the outcome of the election to a certain degree. The institutional effects of Elizabeth Dole†s election to office will be in two major parts: (1) Her leadership of the American Red Cross as well as her association with and involvement in the American political system will adhere to a knowledge of those and similar institutions, and (2) the mass media will curb the campaigns with an instance never before been seriously tampered with. Although many may argue against Elizabeth Dole†s ability to act as Commander in Chief of the Army and Navy of the United States, she seems to have the confidence and the aid to do so. She is very much in favor of restoring America†s Defense capability. â€Å"The readiness of our troops is in question and a whole generation of outdated military equipment is waiting to be replaced†¦. I believe there is an urgent need to refurbish our military and resolve to develop and deploy a strategic missile defense system at the earliest possible date.† Furthermore, the Presidency has become an institution itself, containing many aids, helping in the decision-making procedure and the management of domestic policy, economic policy, foreign affairs, congressional relations, and public relations. Her knowledge both of executive power as well as working closely with executives and their aids (referring to U.S. Presidents) has given her tremendously valuable experience that readies her for her tasks as a President of the United States. Now, the mass media always has a great influence in the public opinion of politics due to their coverage and choice of material presented to this public. This can be looked upon as an advantage for Elizabeth Dole. The media will, without fail, give special attention to her campaign, for she is the first woman in American history to have a prospect of securing the Presidency. Statistics have shown that voters tend to favor those candidates who have a combination of sufficient media coverage and charisma, the latter of which Elizabeth Dole undeniably possesses. Therefore, with this ensemble and her qualifications, Elizabeth Dole will be giving the public eye something they†ve been waiting to see in a presidential candidate†¦the background, the experience, the disposition, the intelligence and the integrity to run our country with our full faith. The Constitutional effects have much to do with Elizabeth Dole†s platform as well as the intermingling of powers. The issue of a Republican woman elected President being a Democratic move could induce a more efficient process of law making in Congress. Furthermore, Elizabeth Dole is a firm believer in rolling back the bureaucracy. This refers directly to the tenth amendment of the Constitution: â€Å"The powers not delegated to the United States by the Constitution, nor prohibited by it to states, are reserved to the states respectively, or to the people.† The founding fathers inserted this amendment for fear of the development and consolidation of a powerful and meddlesome federal government. These days, our federal government maintains numerous and indefinite powers as the states hold few. The Federal Government has become too big, too complex, too bureaucratic. Decisions once made in state legislatures, in city halls and around kitchen tables are now made in Washington†¦. What we need to do, it seems, is to remember the wisdom of our country†s founders, and the tenth Amendment to the Constitution: those powers not specifically delegated to the federal government or prohibited to the states are reserved for the states and for â€Å"we the people† – you and me! Elizabeth Dole is not a power hungry politician like the ones we today to whom we are so accustomed. She is a politically knowledgeable and powerful woman who has the ability to stand strong as the head of the world†s most powerful nation.

Environmental Class Project Lab

These social notations might be encouraged in less developed countries exposing the coo entry to more modern cultures or the distribution of and education on birth control options. 3. Early, middle, and late demographic transition map the concepts of first, SE Condo, and third world countries because early countries are usually third world and late count rye's first. 4. The most developed countries have shapes that are more like blocks and the e least developed countries have steeper triangular shapes. 5.If a country has a steeper triangular shape than there are more children the n those in the ‘prime of their life' can take care of, causing the quality of life to go down. 6. The Use's demographic pattern about 1 00 years ago would be similar to thou SE countries in the Mechanization of Agriculture/ arbitration like Mexico or Nigeria. 7 . China would be in the presidential Age because their change occurred moor e rapidly. Factors that prompt women to have few children later in life include the rise I n birth control and social equality.Lesson 2 Responses to Questions: 1 . Population momentum is an important factor to consider when studying the demographics Of a country. The shape changes from being a steep triangle to being mostly blob click during all the generations after the change if a less developed country is given the birth rate of a more developed country. This is because the birth rates are from a more demographer hectically stable country, causing the shape to look more stable. Because of population meme mount. The change continues to affect the shape in subsequent generations.When the average c hill bearing age is increased, the population decreased because when women start having babe later in life, they have less time to have healthy children. Conversely, when the age is decrease d, the population increased, as women have more time to have children. â€Å"First world† country's tend to have older childbearing women do to the cultural f actors of social gender equality and birth control. 2. The results from Italy were not what predicted. I thought the changes woo old have a greater effect on birth rate than they did. This is probably because the population pry amid of Italy is more stable than that of Nigeria.Monetary incentives to employees who have ultimate children would encourage more child birth in Italy. 3. The government might want to do this because their birth rates are decree sing. This would affect their demographics because a baby boom would make Italy's pyramid s deeper. Lesson 3 1 . Another factor that may be explored when considering the demographics o f a country is how they change when the birth and date rate are changed. 2. My prediction of how much the birth rate would have to be lowered and ho w much the death rate would have to go up to give Egypt a 0% population growth in 2050 ere far too low.To make my prediction, I compared Egypt population growth rate in 205 O, birth rate, and death rate to other countries and then tried to gauge how much the birth rate would have to decrease and the death rate increase to achieve a 0% growth rate. Then a adjusted the values based on the results. In order to achieve a zero growth rate, Egypt would have to either lower the birth rate or increase the death rate. Obviously, decreasing the birth rate I s the logical choice as there are ethical implications of artificially increasing the death rate. However, the Egyptian government would also face ethical debates on deck reassign the birth rate if they tried to enact laws against having a certain number of children. Egg yap would need to change more than the Mexico and a lot more than the LIST to achieve a 0% p population increase in 2050 because it is further from having a 0% population growth UN deer current conditions. 4. Mexico has a higher birth rate and a lower death rate than the United State s. This is probably because the United States is further along in the transition than Mix I CC is.