CHAPTER – 5 DETAILS ABOUT WIRELESS COMMUNICATION Model of a communication system: The overall purpose of the communication system is to transfer information from one point to in space and time, called the source to another point, the user destination. As a rule, the message produced by a source is not electrical. Hence an input transducer is required for converting the message to a time varying electrical quantity called a message signal. At the destination point another transducer converts the electrical waveform to the appropriate message. The information source and the destination point are usually separated in space. The channel provides the electrical connection between the information source and the user. The channel can have many deferent forms such as a microwave radio link over free space a pair of wires, or an optical fiber. Regardless of its type the channel degrades the transmitted single in a number of ways. The degradation is a result of signal distortion due to imperfect response of the channel and due to undesirable electrical signals (noise) and interference. Noise and signal distortion are two basic problems of electrical communication. The transmitter and the receiver in a communication system are carefully designed to avoid signal distortion and minimize the effects of noise at the receiver so that a faithful reproduction of the message emitted by the source is possible. The transmitter couples the input message signal to the channel. While it may sometimes be possible to couple the input transducer directly to the channel, it is often necessary to process and modify the input signal for efficient transmission over the channel. Signal processing operations performed by the transmitter include amplification, filtering, and modulation. The most important of these operations is modulation a process designed to match the properties of the transmitted signal to the channel through the use of a carrier wave. Modulation is the systematic variation of some attribute of a carrier waveform such as the amplitude, phase, or frequency in accordance with a function of the message signal. Despite the multitude of modulation techniques, it is possible to identify two basic types of modulation: the continuous carrier wave (CW) modulation and the pulse nodulation. In continuous wave (CW) carrier modulation the carrier waveform is continuous (usually a sinusoidal waveform), and a parameter of the waveform is changed in proportion to the message signal. In pulse modulation the carrier waveform is a pulse waveform (often a rectangular pulse waveform), and a parameter of the pulse waveform is changed in proportion to the message signal. In both cases the carrier attribute can be changed in continuous or discrete fashion. Discrete pulse (digital) modulation is a discrete process and is best suited for messages that are discrete in nature such as the output of a teletypewriter. However, with the aid of sampling and quantization, continuously varying (analog) message signal can be transmitted using digital modulation techniques. Modulation is used in communication systems for matching signal characteristics to channel characteristics, for reducing noise and interference, for simultaneously transmitting several signals over a single channel, and for overcoming some equipment limitations. A considerable portion of this article is devoted to the study of how modulation schemes are designed to achieve the above tasks. The success of a communication system depends to a large extent on the modulation. The main function of the receiver is extract the input message signal from the degraded version of the transmitted signal coming from the channel. The receiver performs this function through the process of demodulation, the reverse of the transmitter’s modulation process. Because of the presence of noise and other signal degradations, the receiver cannot recover the message signal perfectly. Ways of approaching ideal recovery will be discussed later. In addition to demodulation, the receiver usually provides amplification and filtering. Based on the type of modulation scheme used and the nature of the output of the information source, we can divide communication systems into three categories: 1.analog communication systems designed to transmit analog information using analog modulation methods 2. digital communication systems designed for transmitting digital information using digital modulation schemes and 3. hybrid systems that use digital modulation schemes for transmitting sampled and quantized values of an analog message signal. Other ways of categorizing communication systems include the classification based on the frequency of the carrier and the nature or the communication channel With this brief description of a general model of a communication system, we will now take a detailed look at various components that make up a typical communication system using the digital communication system as an example. We will enumerate the important parameter of each functional block in a digital communication system and point out some of the limitations of the capabilities of various blocks. ELEMENTS OF A DIGITAL COMMUNICATION SYSTEM The overall purpose of the system is to transmit the messages (or sequences of symbols) coming out of a source to a destination point at as high a rate and accuracy as possible. The source and the destination point are physically separated in space and a communication channel of some sort connects the source to the destination point. The channel accepts electrical/electromagnetic signals, and the output of the channel is usually a smeared or distorted version of the input due to the non-ideal nature of the communication channel. In addition to the smearing, the information- bearing signal is also corrupted by unpredictable electrical signals (noise) from both man-made and natural causes. The smearing and noise introduce errors in the information being transmitted and limits the rate at which information can be communicated from the source to the destination. The probability of incorrectly decoding a message symbol at the receiver is often used as a measure of performance of digital communication system. The main function of the coder, the modulator, the demodulator, and the decoder is to combat the degrading effects of the channel on the signal and maximized the information rate and accuracy. Information source Information sources can be classified into two categories based on the nature of their outputs: Analog information sources, and discrete information sources. Analog information sources, such as a microphone actuated by speech, or a TV camera scanning a scene, emit one or more continuous amplitude signals (or functions of time). The output of discrete information sources such as a teletype or the numerical output of a computer consists of a sequence of discrete symbols or letters. An analog information source can be transformed onto a discrete information source through the process of sampling and quantizing. Discrete information sources ate characterized by the following parameters: 1. Source alphabet (symbols or letters) 2. Symbol rate 3. Source alphabet probabilities 4. Probabilistic dependence of symbols in a sequence From these parameters, we can construct a probabilistic model of the information source and define the source entropy (H) and source information rate (R) in bits per symbol and bits per second, respectively.(the term bid is used to denote a binary digit.) To develop a feel for what these quantities represent, let us consider a discrete information source-a teletype having 26 letters of the English alphabet plus six special characters. The source alphabet for this example consists of 32 symbols. The symbol rate refers to the rate at which the teletype produces characters: for purposes of discussion, let us assume that the teletype operates at a speed of 10 characters or 10 symbols/sec. If the teletype is producing messages consisting of symbol sequences in the English language, then we know that some letters will appear more often than others. We also know that the occurrence of a particular letter in a sequence is somewhat dependent on the letters preceding it. For example, the letter E will occur more often than letter Q and the occurrence of Q implies that the next letter in the sequence will most probably be the letter U, and so forth. These structural properties of symbol sequences can be characterized by probabilities of occurrence of individual symbols by the conditional probabilities of occurrence of symbols. An important parameter of a discrete source is its entropy. The entropy of a source, denoted by H, refers to the average information content per symbol in a long message and is given units of bits for symbol where bit is used as an abbreviation for a binary digit. In our example, if we assume that all symbols occur with equal probabilities in a statistically independent sequence, then the source entropy is five bits per symbols. However, the probabilistic dependence of symbols in a sequence, and the unequal probabilities of occurrence of symbols considerably reduce the average information content of the symbols. naturally we can justify the previous statement by convincing ourselves that in a symbol sequence QUE, the letter U carries little or no information because the occurrence of Q implies that the next letter in the sequence has to be a U. The source information rate is defined as the product of the source entropy and the symbol rate and has the units of bits per second. The information rate, denoted by R, represents the minimum number of bits per second that will be needed, on the average, to represent the information coming out of the discrete source. Alternately, R represents the Minimum average data rate needed to convey the information from the source to the destination. Source Encoder/Decoder The input to the source encoder (also referred to as the source coder) is a string of symbols occurring at a rate of rs symbols/sec. The source coder converts the symbol sequence into a binary sequence of 0’s and 1’s by assigning code words to the symbols in input sequence. The simplest way in which a source coder can perform this operation is to assign a fixed-length binary code word to each symbol in the input sequence. For the teletype example we have been discussing, this can be done by assigning 5-bit code world 00000 through 11111 for the 32 symbols in the source alphabet and replacing each symbol in the input sequence by its pre-assigned code word. With a symbol rate of 10 symbols/sec, the source coder output data rate will be 50 bits/sec. Fixed-length coding of individual symbols in a source output is efficient only if the symbols occur with equal probabilities in a statistically independent sequence. In most practical situation symbols in a sequence are statistically dependent, and they occur with unequal probabilities. In these situations the source coder takes a string of two or more symbols as a block and assigns variable-length code words to these block. The optimum source coder is designed to produce an output data rate approaching R, the source information rate. Due to practical constraints, the actual output rate of source encoders will be greater than the source information rate R. the important parameters of a source coder are black size, code word lengths, average data rate, and the efficiency of the coder (i.e., actual output data rate compared to the minimum achievable rate R). At the receiver the source decoder converts the binary output of the channel decoder into a symbol sequence. The decoder for a system using fixed-length code words is quite simple, but the decoder for a system using variable-length code words will be very complex. Decoders for such systems must be able to cope with a number of problems such as growing memory requirement and loss of synchronization due to bit errors. Communication Channel The Communication channel provides the electrical connection between the source and the destination. The channel may be a pair of wires or a telephone link or free space over which the information bearing signal is radiated. Due to physical limitations, communication channels have only finite bandwidth (B HZ), and the information bearing signal often suffers amplitude and phase distortion as it travels over the channel. In addition to the distortion, the signal power also decreases due to the attenuation of the channel. Furthermore, the signal is corrupted by unwanted, unpredictable electrical signals referred to as noise. While some of the degrading effects of the of the channel can be removed or compensated for, the effects of noise cannot be completely removed. From this point of view, the primary objective of a communication system design should be to suppress the bad effects of the noise as much as possible. One of the ways in which the effects of noise can be minimized is to increase the signal power. However, signal power cannot be increased beyond certain levels because of nonlinear effects that become dominant as the signal amplitude is increased. For this reason the signal-to-noise power ratio (S/N ), which can be maintained at the output of a communication channel, is an important parameter of the system. Other important parameters of the channel are the usable bandwidth (B), amplitude an phase response, and the statistical properties of the noise. If the parameters of a communication channel are known, then we can compute the channel capacity C, which represents the maximum rate at which nearly errorless data transmission is theoretically possible. For certain types of communication channels it has been shown that c is equal to B log2 (1+S/N) bits/sec. The channel capacity C has to be greater than the average information rate R of the source for errorless transmission. The capacity c represents a theoretical limit, and the practical usable data rate will be much smaller than C. as an example, for a typical telephone link with a usable bandwidth of 3KHz and S/N = 103, the channel capacity is approximately 30,000 bits/sec. At the present time, the actual data rate on such channels ranges from 150 to 9600 bits/sec. Modulator The modulator accepts a bit stream as its input and converts it to an electrical waveform suitable for transmission over the communication channel. Modulation is one of the most powerful tools in the hands of a communication systems designer. It can be effectively used to minimize the effects of channel noise, to match the frequency spectrum of the transmitted signal with channel characteristics, to provide the capability to multiplex many signals, and to overcome some equipment limitations. The important parameters of the modulator are the types of waveforms used, the duration of the waveforms, the power level, and the bandwidth used. The modulator accomplishes the task of minimizing the effects of channel noise by the use of large signal power and bandwidth, and by the use of waveforms that last for longer durations. While the use of increasingly large amounts of signal power and bandwidth to combat the effects of noise is an obvious method, these parameters cannot be increased indefinitely because of equipment and channel limitations. The use of waveforms of longer time duration to minimize the effects of channel noise is based on the well-known statistical law of large numbers. The law of large numbers states that while the outcome of a single random experiment may fluctuate wildly, the overall result of many repetitions of a random experiment can be predicted accurately. In data communications, this principle can be used to advantage by making the duration of signaling waveforms long. By averaging over longer durations of time, the effects of noise can be minimized. To illustrate the above principle, assume that the input to the modulator consists of 0’s and 1’s occurring at a rate of 1 bit/sec. The modulator can assign waveforms once every second. Notice that the information contained in the input bit is now contained in the frequency of the output waveform. To employ waveforms of longer duration, the modulator can assign waveforms once every four seconds. The number of distinct waveforms the modulator has to generate (hence the number of waveforms the demodulator has to detect) increases exponentially as the duration of the waveforms increases. This leads to an increase in equipment complexity and hence the duration cannot be increased indefinitely. The number of waveforms used in commercial digital modulators available at the present time ranges from 2 to 16. Demodulator Modulation is a reversible process, and the demodulator accomplishes the extraction of the message from the information bearing waveform produced by the modulator. For a given type of modulation, the most important parameter of the demodulator is the method of demodulation. There are a variety of techniques available for demodulating a given modulated waveform: the actual procedure used determines the equipment complexity needed and the accuracy of demodulation. Given the type and duration of waveforms used by the modulator, the power level at the modulator, he physical and noise characteristics of the channel, and the type of demodulation, we can derive unique relationship between data rate, power bandwidth requirements, and the probability of incorrectly decoding a message bit. A considerable portion of this text is devoted to the derivation of these important relationships and their use in system design. Channel Encoder/Decoder Digital channel coding is a practical method of realizing high transmission reliability and efficiency that otherwise may be achieved only by the use of signals of longer duration in the modulation/demodulation process. With digital coding, a relatively a small set of analog signals, often two, is selected for transmission over the channel and the demodulator has the conceptually simple task of distinguishing between two different waveforms of known shapes. The channel coding operation that consists of systematically adding extra bits to the output of the source coder accomplishes error control. While these extra bits themselves convey no information, they make it possible for the receiver to detect and/or correct some of the errors in the information bearing bits. There are two methods of performing the channel coding operation. In the first method, called the block coding method, the encoder takes a block of k information bits from the source encoder and adds r error control bits. The number of error control bits added will depend on the value of k and the error control capabilities desired. In the second method, called the convolutional coding method, the information bearing message stream is encoded in a continuous fashion by continuously interleaving information bits and error control bits. Both methods require storage and processing of binary data at the encoder and decoder. While this requirement was a limiting factor in the early days of data communication, it is no longer such a problem because of the availability of solid-state memory and microprocessor devices at reasonable prices. The important parameters of a channel encoder are the method of coding. Rate or efficiency of the coder (as measured by the ratio of data rate at input to the data rate at the output), error controls capabilities, and complexity of the encoder. The channel decoder recovers the information bearing bits from the coded binary stream. The channel decoder also performs error detection and possible correction. The decoder operates either in a block mode or in a continuous sequential mode depending on the type of coding used in the system. The complexity of the decoder and the time delay involved in the decoder are important design parameter.