COMM 1208 Unit 5 Baseband Communications Baseband Communications 1. Bandwidth ............................................................................................................................ 2 1.1 Bandwidth definitions (Google definitions) .................................................................... 2 1.2 Minimum Bandwidth Requirements .............................................................................. 3 2. Line Coding .......................................................................................................................... 4 2.1 Requirements .................................................................................................................. 4 2.2 Analogue Telephone Line Considerations ...................................................................... 4 2.3 Digital Signalling Formats ............................................................................................. 4 2.3.1 Unipolar Non Return to Zero (NRZ) ........................................................................ 4 2.3.2 Bipolar NRZ ............................................................................................................. 5 2.3.3 Unipolar Return to Zero (RZ) .................................................................................. 5 2.3.4 Bipolar RZ ................................................................................................................ 5 2.3.5 Alternate Mark Inversion (AMI) RZ Signalling ...................................................... 6 2.3.6 Manchester Coding .................................................................................................. 6 2.3.7 Coding comparison ................................................................................................... 7 3. M-Ary Line Coding ............................................................................................................. 8 4. Line Transmission Systems .............................................................................................. 9 4.1 Equaliser ....................................................................................................................... 10 5. Transmission Line Impairments ................................................................................... 10 5.1 Amplitude distortion and phase distortion .................................................................. 11 5.2 Inter Symbol Interference ............................................................................................. 11 Page 1 of 12 COMM 1208 Unit 5 Baseband Communications 1. Bandwidth 1.1 Bandwidth definitions (Google definitions) A measure of the capacity of a communications channel. The higher a channel's bandwidth, the more information it can carry. www.tamu.edu/ode/glossary.html A relative range of frequencies that can carry a signal on a transmission medium. www.adaptivedigital.com/services/serv_definitions.htm A measure of spectrum (frequency) use or capacity. For instance, a voice transmission by telephone requires a bandwidth of about 3000 cycles per second (3 KHz). A TV channel occupies a bandwidth of 6 million cycles per second (6 MHz) in terrestrial Systems. In satellite based systems a larger bandwidth of 17.5 to 72 MHz is used to spread or "dither" the television signal in order to prevent interference. www.spidersat.net/glossary/glossary_b.htm The range of frequencies, expressed in hertz (Hz), that can pass over a given transmission channel. The bandwidth determines the rate at which information can be transmitted through the circuit. www.ssloral.com/html/products/glossary.html The complete range of frequencies over which a circuit or electronic system is allocated to function. In transmission, the US analog and digital television channel bandwidth is 6 MHz. www.wgcu.org/watch/hdtv_glossaryofterms.html The range of frequencies a channel can carry. The higher the frequency, the higher the bandwidth and the greater the capacity of a channel. In Internet terms, higher bandwidth means a higher ability to transmit and receive data. www.7designavenue.com/glossary.htm 2 16/02/2016 COMM 1208 Unit 5 Baseband Communications 1.2 Minimum Bandwidth Requirements Any signal can be treated as if it were made up of an infinite number of frequency components. We have seen the spectrum of a square wave already in which case the (significant) frequency components include much higher frequencies than the fundamental frequency. To transmit the waveform without significant distortion would therefore require a channel with a considerable bandwidth, together with a suitable phase change characteristic. A pulse train does not have to be received undistorted in order to make +V correct decisions about its binary states. Time In fact, in the case illustrated, so long as the fundamental component, at f=l/(T) Hz, of the square wave corresponding to Period = T the bit stream ...01010101… can be Bit Interval B = T/2 T B transmitted, then correct decisions can be made about the binary states. It is possible, in theory at least, to transmit 2/T symbols per second over a channel of bandwidth 2/(T) Hz. B=2xf Bits per second = 2 x bandwidth For example a 64 kbit/s data stream, could be sent and recovered over a 32 kHz bandlimited channel. This is an important general rule (due to Nyquist) for digital waveforms. Example: A primary ISDN signal has a bit rate of 2.048 Mbit/s. What would be the minimum theoretical bandwidth required to transmit this signal? Answer: Minimum bandwidth = 2.048 Mbit/s = 1.024 MHz Example: A spectrum analyser and antenna is used to record the radiation pattern from a TDM system which contains clock generators. There are two peaks, at 153.088 MHz and 154.112 MHz. (a) What is the frequency of the clock generator which is responsible for these peaks? (b) What is the order of these harmonics? (c) How might these peaks be reduced without affecting the performance of the system? Answer: (a) A clock generator outputs a square wave which contains only odd harmonics. Therefore these frequencies are odd multiples of the clock frequency, and the difference between them is twice the clock frequency (or perhaps 4 or six times - but much less likely). The difference is 154.112 - 153.088 MHz = 1.024 MHz. Therefore the clock is 512 kHz. (b) 154.122 MHz is the 301st harmonic (154.122/.512 = 301). The other one is the 299th harmonic (c) The output from the clock generator should be filtered e.g. by using a ferrite bead in series, or a small capacitor to earth. Track layout is important, keeping all tracks as short as possible. Or the circuit could be screened - effectively put in a metal box. 3 16/02/2016 COMM 1208 Unit 5 Baseband Communications 2. Line Coding 2.1 Requirements Digital data can be transmitted by various pulse waveforms, also called line codes. The following properties are desirable for a line code: It is important that the pulses stream to be transmitted does not have a DC component. It can case baseline wander or Galvanic Corrosion. It should be relatively easy to recover the data clock. The line coding scheme should be bandwidth efficient. The line code should be robust in the presence of noise. It should be possible to recognise a line coding error, sometimes called a line violation. (In some signalling protocols, a line violation is deliberately generated to mark the start of a frame) 2.2 Analogue Telephone Line Considerations To review the telephone line. At the local exchange a voltage is applied, via inductors and resistors to R the copper pair. This allows the transmitting + equipment to sink current. The variations in C L current correspond to change in the voltage signal Telephone RX TX on the line. The receiver terminal reads this Line voltage. In most cases the receiving terminal is R Terminal allowed to take a DC feed from the line itself. + L The bandwidth of an analogue telephone line connection is 300 Hz to 3.4 kHz. A square wave or Local Exchange any pulse train with very fast rise times will be distorted if it is sent along a telephone line. Therefore an analogue telephone line is not suitable for sending digital pulses as all frequency components outside the 300 - 3 kHz range will be removed. This bandwidth limitation is not caused totally by the copper pair but by the filters in the local exchange which are part of the analogue to digital process. In the past some analogue telephone lines also had loading coils (inductors) on the line which were intended to give a flat frequency response. On a digital telephone line all analogue filters are removed so the usable bandwidth of the copper pair itself is much greater and can extend to a few Megahertz. These lines are suitable for pulse transmission e.g. ISDN. 2.3 Digital Signalling Formats 2.3.1 Unipolar Non Return to Zero (NRZ) Symbol 1 is represented by transmitting a pulse of constant amplitude for the entire duration of the bit interval, and symbol 0 is represented by no pulse. NRZ indicates that the assigned amplitude level is maintained throughout the entire bit period. This allows for long series without change, which makes synchronization difficult (difficult to recover the clock). Unipolar also contains a strong DC component. . From www.wikipedia.com 4 16/02/2016 COMM 1208 Unit 5 Baseband Communications In telecommunication, a non-return-to-zero (NRZ) line code is a binary code in which "1's" are represented by one significant condition and "0's" are represented by the other significant condition, with no other neutral or rest condition. The pulses have more energy than a RZ code, but it does not have a rest state, which means a synchronization signal must also be sent alongside the code. For a given data signaling rate, i.e., bit rate, the NRZ code requires only half the bandwidth required by the Manchester code. When used to represent data in an asynchronous communication scheme, the absence of a neutral state requires other mechanisms for data recovery, to replace methods used for error detection when using synchronization information when a separate clock signal is available. 2.3.2 Bipolar NRZ Pulses of equal positive and negative amplitudes represent symbols 1 and 0. (e.g. ± 5 volts, ± 12 volts) In either case, the assigned pulse amplitude level is maintained throughout the bit interval. Because of the positive and negative levels the average voltage will tend towards zero and hence little DC component. Again synchronisation will be difficult. 2.3.3 Unipolar Return to Zero (RZ) Symbol 1 is represented by a positive pulse that returns to zero before the end of the bit interval and symbol 0 is represented by the absence of pulse. 2.3.4 Bipolar RZ Positive and negative pulses of equal amplitude are used for symbol 1 and symbol 0. In either case the pulse returns to 0 before the end of the bit interval. From www.wikipedia.com Return-to-zero (RZ) describes a line code used in telecommunications signals in which the signal drops (returns) to zero between each pulse. This takes place even if a number of consecutive zeros or ones occur in the signal. The signal is self-clocking. This means that a separate clock does not need to be sent alongside the signal, but suffers from using twice the bandwidth to achieve the same data-rate as compared to non-return-to-zero format. The "zero" between each bit is a neutral or rest condition, such as a zero amplitude in pulse amplitude modulation (PAM). 5 16/02/2016 COMM 1208 Unit 5 Baseband Communications 2.3.5 Alternate Mark Inversion (AMI) RZ Signalling Positive and negative pulses (of equal amplitude) are used for alternative symbols 1 .No pulse is used for symbol 0. In either case the pulse returns to 0 before the end of the bit interval. An advantage of AMI is that it is easy to recognise a line violation. From www.wikipedia.com A binary 0 is encoded as zero volts as in unipolar encoding. A binary 1 is encoded alternately as a positive voltage and a negative voltage. This prevents a significant build-up of DC, as the positive and negative pulses average to zero volts. Little or no DC-component is considered an advantage because the cable may then be used for longer distances and to carry power for intermediate equipment such as line repeaters. The DC-component can be easily and cheaply removed before the signal reaches the decoding circuitry. Bipolar encoding is preferable to non-return-to-zero where signal transitions are required to maintain synchronization between the transmitter and receiver. Other systems must synchronize using some form of out-of-band communication, or add frame synchronization sequences that don't carry data to the signal. These alternative approaches require either an additional transmission medium for the clock signal or a loss of performance due to overhead, respectively. A bipolar encoding is an often good compromise: runs of ones will not cause a lack of transitions, however long sequences of zeroes are still an issue. Long sequences of zero bits result in no transitions and a loss of synchronization. Where frequent transitions are a requirement, a self-clocking encoding such as return-to-zero or some other more complicated line code may be more appropriate, though they introduce significant overhead. 2.3.6 Manchester Coding Symbol 1 is represented by a positive pulse followed by a negative pulse - with each pulse being of equal amplitude and duration of half a pulse. For symbol 0 the polarities of these pulses are reversed. An advantage of this coding is that it is easy to recover the original data clock. From www.wikipedia.com Manchester coding provides a simple way to encode arbitrary binary sequences without ever having long periods without level transitions, thus preventing the loss of clock synchronization, or bit errors from low-frequency drift on poorly-equalized analog links (see ones-density). If transmitted as a bipolar signal (i.e. where the two signaling levels are of opposite polarity), the DC component of the encoded signal is zero, again preventing baseline drift of the repeated signal, making it easy to regenerate and preventing waste of energy. 6 16/02/2016 COMM 1208 Unit 5 Baseband Communications Time is divided into periods, and one bit is transmitted per period A "0" is expressed by a low-to-high transition, a "1" by high-to-low transition (according to G.E. Thomas' convention--in the IEEE 802.3 convention, the reverse is true) The transitions signifying 0 or 1 occur at the midpoint of a period Manchester codes always have a transition at the middle of each bit period, and depending on the state of the signal, may have a transition at the beginning of the period as well. The direction of the mid-bit transition is what carries the data, with a low-to-high transition indicating one binary value, and a high-to-low transition indicating the other. 2.3.7 Coding comparison 1 0 1 1 0 0 1 0 Unipolar NRZ Bipolar NRZ Unipolar RZ Biplolar RZ AMI Manchester 7 16/02/2016 COMM 1208 Unit 5 Baseband Communications 3. M-Ary Line Coding The utilisation of bandwidth can be made more efficient by adopting an M-Ary format for the representation of the input binary data . A Binary code consists of two symbols- '1' and '0'. A quaternary (i.e. 4 level) code would consist of 4 symbols. The 4 symbols could be assigned to 00, 01, 10 and 11 for example. This would allow us to half the symbol rate on a transmission line compared to one bit per symbol. Note that binary data rate is measured in bits/second whereas the symbol rate is measured in Baud. (Symbols per second). An example of M-Ary Line coding is 2B1Q line code, as above, used on ISDN basic rate telephone lines between a subscribers premises and the local telephone exchange. In this case the baud rate will be half the 11 bit rate. (There are 4 possible symbols, each of which requires 2 10 bits. If the probability of each symbol is ¼ then the information in each symbol is log2 (1/¼) = log2 4 = 2 bits, so that the Time information rate is 2 * baud rate = bit rate). For example, the input binary sequence 11100001 is viewed as a 01 new sequence of dibits (pairs of bits); 11 10 00 01. Each dibit 00 symbol is assigned one of 4 levels. If we increase the number of levels there will be a trade-off between noise performance and bandwidth. Example: Explain how a ternary line coding system can code 3 bits per symbol. Answer: At 3 bits per symbol, 23 = 8, therefore we need at least 8 symbols. A single ternary pulse would only allow one of 3 symbols to be represented. Two ternary pulses in a particular order, however would allow for 9 combinations of levels. This code could be abbreviated as 3B2T. It is not as efficient as 2B1Q but one advantage is that zero volts is one of the levels and the wave form would resemble a binary bipolar format, Note that 4B3T coding is also used on ISDN lines in some countries. 8 16/02/2016 COMM 1208 Unit 5 Baseband Communications 4. Line Transmission Systems Problems are encountered when a digital signal is sent through a channel. The diagram shows the basic stages in a digital signal transmission. A simple non-return-to-zero (NRZ) code is assumed, as already defined. The transmission medium might be a coaxial cable or a copper twisted pair, as often used in local area networks or digital telephone systems. Similar principles apply, however, to systems using other transmission media and/or more complicated codes. Retiming Extracot Noise Digital Information clocked at fc - may be source and/or channel coded Transmitter E A Channel e.g. Co-ax Copper pair Equaliser B C Threshold Detector D Retiming Sector Received Digital Information F Typical waveforms at the points labelled A to F in the system are shown. After passing down the cable the original waveform A is attenuated and a noise component is added. Also because of a finite system response time and propagation delays, the clear transition between voltage levels become indistinct. To counteract the distortion illustrated, the system includes an equaliser which sharpens the received waveform, so that the B Received signal from TX channel relationship of the equaliser output C to the original binary symbols is much clearer. The equaliser would C Output from equaliser normally consist of an amplifier stage combined with a filter to D Output from threshold detector reduce unwanted frequency components. For example, it is quite common for copper cables to E Output from timing extractor pick up a 50 Hz noise component from the mains. It is essential that F Output from re-timing circuit the equaliser removes this 1 0 1 1 0 1 0 component. Also, in certain configurations copper cables will pick up electromagnetic interference, which must be filtered out. Note also that the input to the equaliser must be protected from over-voltages such as induced lightning and other transients. Passing the equalised waveform through a threshold detector (e.g. a Schmitt trigger) generates a binary signal very similar to the transmitted one. If the threshold settings are too small then noise will trigger the detector. If the settings are too large then the data may not trigger the detector. It is important that the slew rate of the comparator used in the detector is fast enough for the data rate. Provided that the noise levels are sufficiently low, and the equaliser and threshold detector are properly designed, then the only difference between binary waveforms A and D is that the transitions of the latter are not perfectly in step with those of the former. The transitions of D will correspond to the threshold-crossings of waveform C which will not precisely mirror those of the original binary waveform. The final stage is the re-timing of the received waveform. If this were not carried out, then the irregularities (jitter) in the waveforms would soon build up to cause error over a long link. 1 0 1 1 0 1 0 0 Binary information A NRZ Line code from Transmitter 9 16/02/2016 COMM 1208 Unit 5 Baseband Communications A regular timing reference signal F - the data clock - is derived from the received waveform itself by a special circuit, (the timing extraction circuit which is based on a Phase Locked Loop). The clock signal and the output from the threshold detector are then processed to give a final regenerated digital signal F whose transitions now coincide with the instants at which the clock signal goes from low to high. A comparison of waveforms C and F shows that the combined effect of threshold detection and re-timing is equivalent to sampling waveform C near its peaks and troughs to determine the appropriate binary states. So even in the presence of noise, regenerated signal F can be an almost perfect (delayed) replica of the transmitted signal, provided only that the noise is not sufficient to cause an incorrect decision to be made at the threshold detector. 4.1 Equaliser From www.wikipedia.com An equalization (EQ) filter is a filter, usually adjustable, chiefly meant to compensate for the unequal frequency response of some other signal processing circuit or system. An EQ filter typically allows the user to adjust one or more parameters that determine the overall shape of the filter's transfer function. It is generally used to improve the fidelity of sound, to emphasize certain instruments, to remove undesired noises, or to create completely new and different sounds. 5. Transmission Line Impairments Until now we have assumed that the transmitter sends a rectangular pulse. A transmission line acts like a filter so the output response of the transmission line to a rectangular pulse can be quite distorted. The distortion can mean that pulses can become overlapped thus causing receiver errors. We therefore need to model the effects of transmitting pulses through a transmission line. a) Rectangular pulse response of a first order lowpass V input pulse filter, where the duration of the pulse is approximately equal to the filter time constant. pulse response b) Response of the same filter to a binary waveform. The figure shows the response to a single pulse, and (a) Time the superimposed pulse responses corresponding to an input pulse train (binary waveform). Note that 1 0 1 1 0 1 0 0 because the response to a single pulse takes longer to decay that the duration of a symbol period, the output waveform gradually accumulates a DC offset. In the absence of further processing this would clearly cause problems for threshold detection. Even in the (b) positions corresponding to a binary 0 there can be a considerable output voltage. 10 16/02/2016 COMM 1208 Unit 5 Baseband Communications This figure, on the other hand, shows a much more desirable overall pulse response for a telecommunications channel. It shows the possible pulse response response of a telecommunications channel to (a) a rectangular pulse and (b) a binary waveform. Here the (a) Time system response to a bit stream could be decoded without difficulty, owing to the clear distinction in the 1 0 1 1 0 1 0 0 combined response between binary 1 and 0. An alternative approach to modelling a linear channel or component is based on the second definition of linearity. Any practical message signal can be described in terms of its frequency content - or, to be (b) more precise, modelled as a frequency spectrum. Similarly, any linear system can be completely specified by its frequency response function, which is a description of amplitude and phase shifts introduced by the system for all frequencies. V input pulse 5.1 Amplitude distortion and phase distortion An ideal transmission channel would pass all frequency components of a signal with their amplitude and phase relationships unchanged The simplest frequency domain model of such behaviour would be a constant amplitude ratio and zero phase shift for all frequencies Transmitted Signal Time of interest. For example a square wave would be unaffected by the transmission Phase and Pulse has spread channel. In practice, the higher order Amplitude Distortion due to phase delay harmonics will be greatly attenuated by the for harmonics transmission channel. Also the phase shift Amplitude Distortion will be different for each harmonic. For Only example, the fundamental harmonic may have a phase shift of 45 degrees whereas the fifth harmonic could have a phase shift of 80 degrees. This will cause components of the pulse to be delayed or stretched. Specifications for digital receiver systems usually include limits for phase delay. 5.2 Inter Symbol Interference t Intersymbol Interferencce Due to the fact that the transmission channels are bandlimited, the transmitted pulses tend to spread during transmission. This pulse spreading or dispersion causes overlap of pulses into adjacent pulse time slots. This signal overlap may result in an error at the point where the receiver makes a decision as to which pulse has been transmitted, especially when other impairments are present (such as noise, interference). This effect of pulse overlap and the resultant difficulty of discriminating between symbols at the receiver are termed inter symbol interference (ISI). From www.wikipedia.com In telecommunication, intersymbol interference (ISI) means a form of distortion of a signal that causes the previously transmitted symbols to have an effect on the currently received symbol. This is usually an unwanted phenomenon as the previous symbols have similar effect as noise, thus making the communication less reliable. ISI is usually caused by echoes or non11 16/02/2016 COMM 1208 Unit 5 Baseband Communications linear frequency response of the channel. Ways to fight against intersymbol interference include adaptive equalization or error correcting codes. In a digital transmission system, distortion of the received signal, which is manifested in the temporal spreading and consequent overlap of individual pulses to the degree that the receiver cannot reliably distinguish between changes of state, i.e., between individual signal elements. At a certain threshold, intersymbol interference will compromise the integrity of the received data. 12 16/02/2016