Unformatted text preview: Trellis Coded Modulation (TCM) 1 Intuitive Guide to Principles of Communications www.complextoreal.com Trellis Coded Modulation (TCM)
Trellis Coded Modulation Tutorial Before reading this tutorial, make sure you have read the tutorial on Convolutional coding and Modulation. This is essential. What does a rate 1/2 convolutional code do? It takes one bit, codes it, puts out 2 bits. This is something that is done at the digital level. These 2 bits are then modulated (i.e. converted to analog form via a sinusoidal carrier) and transmitted. The coding is a digital function and modulation which is an analog function, are done separately and independently in most modulations. In Trellis Coded Modulation (TCM), however the two are combined in one function. TCM concepts Euclidean Distance A straight line distance between any two points is called the Euclidean distance. For a point p1 at (x1, y1) and another point p2 at (x2, y2), the Euclidean distance is given by the familiar formula ( x1 − x2 ) 2 + ( y1 − y2 ) 2
It is implicit that this distance is the shortest distance between these points. The Euclidean distance is an analog concept, the very concept of distance that we normally use daytoday in the world of real numbers. For signals, we define this distance in the IQ plane. In Figure 1 we have a 8PSK signal constellation. The radius is equal to 1 and represents the maximum amplitude. Each point of the constellation is a certain combination of a particular amplitude and phase. The distance between these points is can be measured in the manner described above and these are given in the Figure below. The distances given in Figure 1 are squared and are called Squared Euclidean Distance (SED). The smallest of these distances is called the Minimum Squared 2 Euclidean Distance (MSED), designated as d min for a particular constellation.
2 3 2.0 3.414 4.0 7 5 6 1 0 .586 4 Figure 1 – 8 PSK constellation and squared Euclidean distances between symbols Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 2 Hamming distance Just as real numbers have a concept of distance, so do the binary numbers. Take two binary numbers, 011011 and 101101. The distance between these is the number of places these two numbers differ. And that number is 4. This distance is called the Hamming distance between these numbers. The distance would be zero, if these two numbers were the same. A zero distance means the numbers are the same, same interpretation as in Euclidean concept of distance. We distinguish these two types of distances by recognizing that one belongs to the analog world of real numbers and the other to the binary world. Both concepts are useful in signal processing. In coding Hamming distance is most often used as a performance metric whereas it is Euclidean distance in the analog world. Distance between sequences We can also talk about Euclidean distances between sequences by comparing distances between corresponding points of the sequences. Let’s take for example an 8PSK signal that consists of a sequence of these symbols. S0 S3 S2 S1 S0 In bits, we can map these as: 000 011 010 101 100 000
2 3 2.0 3.414 4.0 7 5 6 1 0 .586 4(100) 7(111) 5(101) 6(110) 3(011) 2(010) 1(001) 0(000) 4 Figure 2 – Euclidean and sequence Hamming distance The Euclidean distance for this sequence is the distance between each symbol in this sequence and a reference sequence. If we designate the allzerosymbols as the reference sequence, then the squared Euclidean distance (SED) is the distance between each one of these symbols and the symbol S0. s0 to s0 = 0.0, s0 to s1 = .586, s0 to s2 = 2.0, s0 to s3 = 3.414 The Sum of the Squared Euclidean Distances (SSED), also called d2free of this sequence, from the all zero sequence is 3.414 + 2.0 + 0.586 = 6.0 This cumulative distance gives a feeling of how easy or difficult it would be to mistake one sequence for another. For the reference sequence we could have used any other sequence than the all zero, and the results would be the same. However using an allzero sequence is convenient and conventional. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 3 Trellis Coded Modulation (TCM) TCM uses many diverse concepts from signal processing. In simplest terms it is a combination of coding and modulation, hence it name Trellis Coded Modulation, where the word trellis stands for the use of trellis (also called convolutional) codes. Whereas we normally talk about coding and modulation as two independent aspects of the communications link, in TCM they are combined. TCM is a complex concept to understand particulalry due to the nonlinear nature of the performance. It uses ideas from modulation and coding as well as dynamic programming, lattice structures and matrix math. A convolutional code that has optimum performance when used independently may not be optimum in TCM. Gray coding is helpful in uncoded signalling and constellation mapping, but not always so in TCM. So it is a not an easy topic and my hats off (if I wore one) to Mr. Ungerboeck and others who came up with it. Fortunately, there is not a lot of math to deal with here. But you will need to know concepts of convolutional codes, trellis, lattice, cosets, and coset generators. Communications theory says that it is best to design codes in long sequences of messages. The allowed sequences should be very different from each other. The receiver can then make a decision between sequences using their statistics rather than on symbolbysymbol basis. When decoding this way, the probability of error is an inverse function of the sequence length. In general form the probability of error between sequences is given by the expression pe ∼ e − d 2 min / 2σ 2 where dmin is the sequence Euclidean distance between sequences and is σ2 the noise power. We measure the performance of TCM (and many other schemes) by Asymptotic Coding Gain (ACG). This is the gain obtained over some baseline performance at high SNR in a Gaussian environment. AGC is not achievable in practice because we do not transmit signals at high SNRs, have hardware and channel imperfections that depart from Additive White Gaussian Noise (AWGN) assumptions. So recognize that all gains quoted herein are maximum possible only in theory. Actual numbers are determined by test and simulation in the given environment. The functions of a TCM consist of a Trellis code and a constellation mapper as shown in Figure 3. TCM combines the functions of a convolutional coder of rate R = k / k + 1 and a Mary signal mapper that maps M = 2 k input points into a larger constellation of M = 2 k +1 constellation points.
Trellis Code Convolutional Encoder Rate k/(k+1) k+1 bits . . . . Constellation Mapper MPSK or MQAM or MASK Modulator M = 2k+1 MPSK or MQAM or MASK symbols k bits . . . . Carrier Figure 3 – A general trellis coded modulation Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 4 For k = 2, we have a code of rate 2/3 that takes a QPSK signal (M = 4) and putsout a 8PSK signal (M = 8). So instead of expanding the bandwidth as the signal goes from QPSK to 8PSK, it instead doubles the constellation points. It is kind of an upgrading system, where you take a chosen signal and upgrade it to another with larger number of constellation points as shown in Figure 4 below. s1 (0 1) s0 (0 0) s3 s2 s1 s4 s0 s5 s2 ( 11 ) s3 (1 0) s6 s7 Figure 4 – Constellation doubling in TCM. A QPSK signal transmitted using a 8PSK constellation. Main points of a TCM are 1. TCM is bandwidth efficient modulation which accomplishes this by the use of convolutional coding. 2. It conserves bandwidth by doubling the number of constellation points of the signal. This way the bit rate increases but the symbol rate stays the same. 3. Convolutional coding constrains allowed symbol transitions, creating sequence coding. 4. Unlike a true Convolutional coding, not all incoming bits are coded. 5. Increasing the constellation size reduces Euclidean distances between the constellation points but sequence coding offers a coding gain that overcomes the power disadvantage of going to the higher constellation. 6. Performance is measured by coding gain over an uncoded signal. 7. The decoding metric is the Euclidean distance and not Hamming distance. 8. Ungerboeck originally proposed TCM which used setpartitioning and small number of states with code rates that varied with the input signal type. 9. Pragmatic TCM uses a less than perfect rate ½ convolutional code with constraint length equal to 7 or 9. This is a widely available code and its use makes TCM less expensive to implement. 10. The constellation mapping in set partitioning is based on natural numbering where as gray coding is preferred in pragmatic TCM. TCM is a general concept and by varying k, we can create a QPSK, 8PSK or higher level signals as shown in Figure 5. All of these are types of TCM. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 5 k=1 BPSK in (Trellis) Convolutional 2 Encoder bits Rate (1/2) 4PSK Constellation Mapper M = 22 = 4 Carrier QPSK symbol BPSK to QPSK TCM k=2 QPSK in (Trellis) Convolutional 3 bits Encoder Rate (2/3) 8PSK Constellation Mapper M = 23 = 8 Carrier 8PSK symbol QPSK to 8PSK TCM k=3 8PSK in (Trellis) Convolutional 4 bits Encoder Rate (3/4) 16QAM Constellation Mapper M = 2 4 = 16 Carrier 16QAM symbol 8PSK to 16QAM TCM Figure 5 – General trellis coded modulation (a) BPSK, code rate 1/2, output QPSK (b) QPSK, code rate = 2/3, output 8PSK (c) 8PSK, code rate = 3/4, output 16QAM Notice that in each case, the code rate is different. But they are all called TCM. The coding adds just one extra bit to the symbol bit size. The symbol size increases from k bits to k + 1 bits. If coding increases the bit rate by 1 extra bit, then we need to double the constellation size to accommodate this bit as we see below, where L is original number of bits per symbol. 2 L +1 = 2 × 2 L
So if original signal is BPSK, then a TCM encoder will put out a QPSK, a QPSK will become 8PSK and if 8PSK was chosen, it becomes a 16QAM signal. As constellation expands but the signal energy is kept the same, the distance between the symbols decreases. That implies a worsening of a performance, not improving. So where is the improvement coming from? We start with a given bandwidth B, because in real life this is a big constraint. From this bandwidth, we determine the maximum possible symbol rate, which is never more than 2B but usually less. Now determine the size of the alphabet that can deliver the needed signal BER at the given available power. The uncoded QPSK signal (k = 2) is TCM'd Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 6 Let’s say that we intend to transmit a QPSK signal. Figure 6 shows the constellation of a QPSK signal of amplitude = 1. The minimum Euclidean distance between the four constellation points is d min = 2 . Now we draw the trellis of this QPSK signal which is a trivial case as in Figure 6 (b).
s1 (01) s0 (00) s0 (00) s1 (01) s2 (11) s3 (10) s2 (11) s3 (10) t=0 t=1 t=2 Figure 6 – QPSK signal constellation and its trellis (a) Four constellation points of QPSK, (b) the signal trellis of a QPSK signal which allows all transitions at each time period. This trellis shows how symbols are transmitted in an uncoded QPSK signal. Sixteen paths are shown in two time periods. Any of these 16 paths is possible for an uncoded signal. At time t = 0, we can pick any of the four possible symbols, s0 to s3, and then same at each subsequent time tick, again we can pick any of the four possible symbols. It is trivial but I want to get across the point that there are no transition restrictions in an uncoded signal. All sequences are possible ergo, this signal has no sequence coding.
s1 (01) s0 (00) s0 (00) s1 (01) s2 (11) s3 (10) s2 (11) s3 (10) t=0 t=1 t=2 Figure 7 – SQPSK signal constellation and its trellis (b) Four constellation points of SQPSK, (b) the signal trellis of a SQPSK signal which allows only three transitions from each point at each time period. In Figure 7, we show the trellis for a staggered QPSK (also called offset QPSK) and we see that here we do have restrictions on what transitions are allowed. From each state, we can only go to the two adjoining symbols. The Minimum Squared Euclidean Distance (MSED), is the minimum squared distance between all points of a constellation and is usually the distance between any two adjacent points. The decoder on the receive side makes the decision about which symbol was sent based on which decision region the signal falls in sort like a dartboard. The error performance is a function of the Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 7 MSED, d2min which for signals of Figure 6 and 7, is 1.414. LOG BER vs Eb/No Ideal curves 16PSK
1 2 3 4 5 6 7 8 9 6 8 10 12 14 16 18 20 22 24 QPSK 8PSK 16QAM 64QAM Eb/No (dB) Figure 8  Ideal BER vs. Eb/N0 in AWGN environment Let’s say we want to transmit a QPSK signal, uncoded with a BER of 105. This is going to require 9.6 dB of energy per the ideal Eb/No vs. BER relationship. If that much power is not available because the transmitter is small, then an options is to add a code of rate 2/3 to reduce the BER which will give this BER at a smaller Eb/N0. But then we have a another problem. If we keep the same bit rate for the information bits and let the coded bit rate increase to accommodate the overhead bits, then bandwidth requirements increase by the 1/R. So addition of coding increases the bandwith by 3/2. If we cannot allow the bandwidth to change, then information rate will have to decrease by the same proportion. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 8 Bit Rate = 2 QPSK Modulator Bit Rate = 2 Symbol Rate = 1 Bit Rate = 2 Rate 1/2 Convolutional code QPSK Modulator Bit Rate = 3 Symbol Rate = 1.5 Bit Rate = 2 Rate 2/3 Convolutional code 8PSK Modulator Bit Rate = 3 Symbol Rate = 1.0 Figure 9 – Adding independent convolutional coding to a QPSK signal increases its bit rate and required bandwidth by the code rate (a) uncoded signal (b) Add a rate ½ code infront of the QPSK modulator but this increases the symbol rate. (c) add rate 2/3 code and transmit 8PSK signal. For the QPSK example, before coding, the information bit rate was 2 bps, and after adding one bit for coding, the bit rate becomes 3 bps. The symbol rate which was 1 sps before coding is 1.5 sps afterwards. What happens to bandwidth when you increase the symbol rate from 1 to 1.5? As symbol rate is increased, the bandwith (as measured by the frequency of the first null) also increases. A signal with symbol rate of 1 sps has a lowpass bandwidth of 1 Hz, a signal of 2 sps has a bandwidth of 2 Hz and so on. The bandwidth of the signal increases from 1 Hz for the uncoded signal to 1.5 Hz for the coded signal.
Power Spectrum PSK signals with varying Symbol Rates, 1, 2, 3 sps
0 10 Lo ss, dB 20 30 40 50 60 0 0.5 1 1.5 2 Frequency 2.5 3 3.5 4 Figure 10 – Symbol rate vs. required bandwidth of a PSK signal. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 9 For PSK as long as the symbol rate is the same, all MPSK modulations have the same bandwidth, whatever the M. Now look at Figure 10 where required bandwidth is a function of the bit rate instead of symbol rate. All three of these signals carry the same bit rate but 8PSK requires the least bandwidth. The bandwidth requirements for a given bit rate decrease as M is increased in a MPSK signal which is what is meant when these are called bitefficient modulations.
P o w e r S p e ctru m P S K si gna ls o f sa m e Bi t Ra te , 1 b p s 10
BPS K 0 10 L o ss, d B 20 30 40 50 60 0 0. 5 1 F re que ncy QP S K 8 PSK 1. 5 2 Figure 11 – For same bit rate, the required bandwidth decreases as M increases in a MPSK signal. In third option in Figure 9, we move up in modulation order and in this way can transmit a signal which has coding but without changing the bandwidth or the spectrum. Now the problem is that 8PSK symbols are closer together. Looking at the curve from Figure 8 we see that it needs app. 3.5 dB higher power to deliver the same BER. Can a code of rate 2/3 give us a coding gain of at least 3.5 dB? Because that is the only way this idea will be work. Otherwise we have not gained anything over the uncoded QPSK option. The coding gain of coded signal over an uncoded signal The coding gain of a coded signal is given by d 2 / coded Es / coded γ = 2free d min/ uncoded Es / uncoded (1) where dfree/coded is equal to the coded sequence Euclidean distance and dmin/uncoded is minimum distance between the signals in the uncoded constellation as previously defined. This coding gain is referenced to the baseline signal. We are starting with QPSK and want a BER of 105. We said that this requires a Eb/N0 of 9.6 dB. The number we come up for coding gain from Eq. 1 is counted as reduction from the baseline Eb/N0. Fact that 8PSK actually requires a higher Eb/N0 than the QPSK baseline is accounted for in Eq. 1. So even though d min/ coded < d min/ uncoded , if we can increase d free / coded > d min/ uncoded we will get a positive gain. We know what dmin is, but what is dfree? Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 10 How do you measure the Euclidean distance of a coded signal when in fact, its constellation Euclidean distance is smaller than dmin. Now we need to talk about distances between sequences rather than distances between signals. dfree is defined as the Euclidean distance of a coded signal in terms of the smallest possible distance between all allowed sequences. Instead of checking all possible combinations, this distance is measured from the allzero sequence, same as the measurement of Hamming distance and Hamming weight, which are both referenced to an allzero code word. Let say that we transmit an allzero message. During decoding, errors occur and wrong trellis path is followed by the decoder. The only way, the decoder can get depart and then back to the correct path, which is the allzero path, is by diverging and then remerging as shown below. s0 (00) s1 (01) s2 (11) s3 (10) Figure 12 – diverging and remerging of alternate sequences. The correctly decoded sequences is the upper straight line. The shortest incorrectly decoded sequences is the darker 2 segment path. The Euclidean distance of a coded signal is defined as the smallest distance between an allzero sequence and one that diverges from it and then remerges. What exactly does that mean? Remember that each sequence is a set of demodulated symbols. When two paths diverge, it means, there was an error and the decoder made a wrong decision. Assuming that the probability of such error is small, at subsequent junctions, the further decisions should take the path back to the correct one. A small allowed path which incorrect is more likely than a long one, so the distance of this small path is a measure of the error correcting capability of the code. This is the same concept as when a receiver is most likely to pick neighboring signal points because they are most like the correct signal, rather than picking one that is quite different, as such the fundamental idea behind Maximum Likelihood Decoding (MLD). In the example of the simple trellis above we see that if an error is made at the time t = 0 and the path diverges, then it can get back to the correct sequence in only two segments as shown. The sum of the squared Euclidean distanced for this path is called the free distance, dfree of the code. The Euclidean distance here is determined by looking at the diverging path followed and its minimum distance from the allzero path, symbol by symbol. For the example above, this is sum of SED of each of the two segments.
2 2 d 2 free / coded = d min + d min = 2+2 =4 Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 11 Let’s compute dfree and the coding gain of a rate 2/3 convolutional code to see what it can do. + + + Figure 13 – a Rate 2/3 convolutional encoder This is a four state convolutional code. Looking at the above diagram you can write down its transfer function. It is a systematic encoder since two uncoded bits go through and are appended by one parity bit. We are going to look at the performance of this code based on two different bits to symbol mappings, in (b) the symbols are mapped in the natural binary order and in (c) they are mapped with Gray coding so that adjacent bits differ by one bit only.
2 3 2.0 3.414 4.0 7 5 6 1 0 .586 4(100) 7(111) 5(101) 6(110) 4(110) 7(100) 5(111) 6(101) 3(011) 2(010) 1(001) 0(000) 3(010) 2(011) 1(001) 0(000) 4 Figure 14 – (a) Mapping the 8PSK symbols (b) Natural mapping, (c) Gray coding Note that although d2min for a QPSK signal was 2.0, the same number is 0.586, nearly 75% smaller for 8PSK. We write down the trellis of the 2/3 convolutional code by stating with 0’s in the memory registers and then putting in all three bit combinations through the registers. (See Convolutional Coding tutorial on how to do this.) Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 12 State 1 = 00 000 110 011 101 011 101 000 110 100 010 111 001 111 001 100 010 000 110 011 101 0.0 2.0 3.414 3.414 3.414 3.414 0.0 2.0 4.0 2.0 0.586 0.586 0.586 0.586 4.0 2.0 2.0 State 2 = 10 2.0 State 3 = 01 0.586 State 4 = 11 (a) (b) (c) Figure 15  Trellis of a rate 2/3 convolutional code, (a) Trellis with bit assignments (b) each path replaced with its SED (c) Three segments that diverge and then merge back to state 00. In (a) the trellis shows the four possible states of the registers with each line diverging in a bit combination in order from top to bottom. In (b) we replace the symbols by their squared Euclidean distance from the zero symbol reference. (These distances are written on the side rather on the lines to keep the diagram easy to decipher.) At each state, there are four different diverging paths. In order to determine the coding gain, we need to know the dfree of this code. To determine the minimum distance path, we follow from each state the path with the smallest squared distance (but not 0). This is 2.0 for the path starting at state 00. This takes us to state 2. From here we follow again the minimum distance path, which is 2.0. This takes us to state 4 and from here we return to state 0 via a path that has a squared distance of 0 .586. There is no other path that can take us back to 00 state and has a smaller total distance. The total minimum squared Euclidean distance (MSED) for this sequence is the sum all three of these squared distances. 2 + 2 + .586 = 4.586. To determine the coding gain, we divide this distance by the d min , the minimum distance of the uncoded QPSK constellation which 2.0 The coding gain from Eq. 1 (assuming that both coded and uncoded signals have same energies, is
2 ⎛ 4.56 ⎞ 10 log ⎜ ⎟ = 3.6 dB ⎝2⎠
This is great but maybe we can do better. Let’s try an alternate mapping of the symbols (Gray coding) as shown in (c). This gives the trellis of Figure 15. As before, we convert each symbol to its distance from the 0 symbol and then we follow the minimum path. This time to our chagrin, we only get a total MSED of .586 + .586 + .586 = 1.758, or actually a loss of approximately 0.5 dB. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 13 So we see that mapping is an important consideration. Gray coding was not helpful here. Adding any code of rate 2/3 in front of a 8PSK modulator is not beneficial. Now see how TCM can help improve on this. State 1 = 00 000 110 011 101 011 101 000 110 100 010 111 001 111 001 100 010 000 110 011 101 0.0 4.0 2.0 0.586 2.0 0.586 0.0 4 2.0 0.586 3.414 0.586 3.414 0.586 0.586 0.586 0.586 0.586 State 2 = 10 0.586 State 3 = 01 State 4 = 11 Figure 16 – Euclidean distance change if the symbol to bit mapping is changed. Ungerboeck’s brain storm Ungerboeck had an idea to improve upon this code by a mapping the signals in a special way called set partitioning. The basic idea is to map k information bits to 2k+1constellation points such that we can limit the transitions to occur only along the largest SED. As the subsets are partitioned, the signals get further apart increasing the Euclidean distance between the signals in that set. The Figure below shows this process. 8PSK signal constellation is partitioned (using lattice structure and terminology). We have three incoming bits from the code. They are b1 b2 b3 Proceed with bit b3 and using this bit, proceed down the decision tree as shown below. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 14 2 d 0 = .586 2 d 1 = 2.0 b3 = 0 b3 = 1 2 d 2 = 4.0 b2 = 0 b2 = 1 b2 = 0 b2 = 1 b1 = 0 b1 = 1 b1 = 0 b1 = 1 b1 = 0 b1 = 1 b1 = 0 b1 = 1 S0 S1 S2 S3 S4 S5 S6 S7 b3 b2 b1 b1 b2 b3 000 000 001 100 010 010 011 110 100 001 101 101 110 011 111 111 Figure 17 – Partition of a 8PSK constellation to ever increasing Euclidean distance subsets. This Figure shows how the 8 points of 8PSK are successively portioned into disjoint cosets such that the SEDs are increasing at each level. There are total of four partitions counting the first unpartitioned set. At topmost level, the MSED is 0.586. At the next level, where there are only four points in each of the two cosets, the MSED has increased to 2.0 and at the last level, the MSED is 4.0. Each subset is also called a coset and by the lattice terminology, we can show the partition with its coset generators in this way. b3 = 1 b3 = 0 b2 = 1 b2 = 0 b2 = 1 b2 = 0 Figure 18 – Lattice structure and coset generators for the 8PSK set partition (Top 3 levels) Since the top two levels have smaller distances, these errors are more likely, we will use the coded bits to traverse through this part. b3 to and b2 which are coded can be used to decide which partition (or coset) to choose and then we can use the uncoded bit at the last level to pick the signal transmitted. The most significant bit b1, used at the last level, has a large Euclidean distance from its complement and would require an error of 180 degrees to be corrupted. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 15 Example: 010 bits would be mapped to symbol s2 using this mapping. Ungerboeck approach leaves the most significant bit uncoded and lets it take care of itself via its large Euclidean distance. This turns out to be the key to larger coding gains of this approach. Only the bits that decide at the top levels with smaller SEDs are coded, thus reducing coding rates and increasing bit efficiency. OK, so here is how we can do TCM using a lower code rate leaving the MSB uncoded. Figure 18 – 4 state rate 1/2 convolutional code as a basis of TCM Now instead of using a rate 2/3 code on both incoming bits, we leave one bit uncoded and then use a rate ½ code on the other bit. Out come three bits same as with a rate 2/3 code. However, the second approach is more promising since a rate ½ code has a better coding gain. This is TCM at its most basic, to selectively code only some of the bits and take advantage of the increasing Euclidean distances obtained by set partitioning, then leaving uncoded those bits that are protected naturally by their large SED. We start with a four state encoder. First we draw the trellis of the rate ½ convolutional coder. As we did for the rate 2/3 code, this trellis is converted to squared distances and then a minimum length path is identified. State 1 = 00 0(00) 1(01) 0(0.0) 1(2.0) 1(2.0) 0(0.0) 3(2.0) 2(4.0) 2(4.0) 3(2.0) 3(2.0) 1(2.0) 1(2.0) State 2 = 10 1(01) 0(00) State 3 = 01 0(10) 1(11) 1(11) State 4 = 11 0(10) Figure 19 – Trellis of the rate 1/2 code (one bit in, 2 bits out) The minimum length path is labeled as 01 10 01. The corresponding distances from the 00 bit symbol are 2.0, 2.0, 2.0, the sum of which is 6.0 The MSED for this code is 6. The coding gain is Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 16 ⎛6⎞ 10 log ⎜ ⎟ = 4.77dB ⎝2⎠
This is better than the 3.6 dB gain we had from the rate 2/3 code, so this looks promising. But not so fast! This is just the coding gain of the rate 1/2 code. What about the uncoded bit. We have not accounted for that. Modifying the code trellis to account for the uncoded bit Now comes the complicated part. We want to modify this trellis to show the effect of the uncoded bit. We have 3 bits going into the modulator, b1, b2, b3. b2 and b3 are the result of coding. b1 is uncoded. At each state we have two coded bits incoming as well as one uncoded bit, so each path doubles to account for the two choices for the uncoded bit. At state 00, if coded bits are 10, then we can get either 110 if uncoded bit is 1 or 010 if it is 0. This doubling of choices is called parallel transitions. (This is of consequence only in a 4state code.)
10(100) 00 0(00) 1(10) 00(000) 11(110) 01(010) 10 0(01) 1(11)
00(001) 10(101) 11(111) 01(011) 01 11 Figure 20 – (a) Code rate 1/2 trellis, (b) code rate 1/2 trellis modified to include the third uncoded bit. Modify the whole trellis to incorporate parallel transitions at each state. Now we draw the trellis for the TCM which has both coded and uncoded bits. At each junction, we now have an option of getting either a 0 or 1 uncoded bit in addition to the 2 coded bits. Each trellis path is now doubled. The dashed lines in Figure 20, 21 indicate the uncoded bit is a 0 and the solid line means the bit is 1. Now there are 3 bits inside the parenthesis which is symbol. The first bit is the uncoded bit and the last two are the coded bits from the top trellis. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 17 10(100) 00 00(000) 11(110) 01(010) 01(001) S4 S0 S6 S2 S2 S6 0, 4 2.0, 2.0 1(2.0) 2.0 2.0 .586 .586 .586 .586 10 11(101) 10(100) 00(000) 00(001) 10(101) 11(111) 11(111) 01(011) 01(011) S3 S7 S3 S7 S1 S3 S0 S4 S5 S1 0, 4.0 .586, 3.414 3.414, .586 .586, 3.414 3.414, .586 01 11 00(001) 10(101) Figure 21 – (a) Modified trellis for the uncoded bit , (b) replace each bit combination by chosen symbol map, )c) for each path, pick the one with the lowest Euclidean distance to symbol S0, (c) Find shortest path that starts at state 00 and then remerges back (in dark lines) A parallel decision making process occurs in decoding the incoming signal. First the decoder makes a decision about the coded bits and then the uncoded bit, sending it down one of the four paths at each state. Even if the decoded decision is correct, there would still be a possibility that the uncoded bit will be decoded incorrectly. Now we need to determine which of these two independent events i.e. the error probability of the coded vs. the uncoded bits is larger. The dfree of this code is dfree = 2 + .586 + 2 = 4.77 Now look at the parallel paths at t = 0, state 00. We have two parallel pairs, 100 and 000 pair and the 110 and 010 pair. If you check these against the set partition of Figure 17, (Note that in Figure 17, the bit order is b3 b2 b1 whereas in the trellis is it b1 b2 b3!) Each pair is 180 degree phase shift apart. But this corresponds to a MSED of 4.0, and it is smaller that the sequence SMSED (4.77), this error is more likely. In other words, it is more likely that the uncoded bits will be decoded incorrectly that the two coded bits, simple because they have a larger SMSED. Of course, this is not always true in TCM, This is also called a single stage error.
2 δ min = 4.0 (not dmin, but just the SED at the bottom level of the partition.) 2 The terminology is: δ 2 e is the sum of the MSED of the coded bits and δ min is distance at the fre lowest level which contains the uncoded bit. It is the smaller of the two that determine the overall performance.
2 d 2 ee = min ⎡δ f2ree , δ min ⎤ = min [ 4.77, 4.0] fr ⎣ ⎦
2 We take the minimum of these two and then divide by d min/ uncoded to get the coding gain. 2 d min/ uncoded is equal to 2.0 for QPSK. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 18 Coding gain = 10 log ⎜ ⎟ = 3dB This is not as good as 3.6 dB gain we got when we used the rate 2/3 code alone, but we did get this gain without any increase in bandwidth or symbol rate, which is a remarkable thing. The reason why this number is low is because this a limiting case due to single state errors and this limits the coding gain to 3 dB. No matter what code we use, as long as we have parallel transitions, the coding gain is limited to 3 dB. With four state convolutional code, we cannot do any better than that. It is our limiting case. So now we have to admit that although we computed the coding gain of the rate ½ code as 4.77 dB, we are not going to see this gain in TCM because it is trumped by the smaller Euclidean distance of the uncoded bit. We need to do better than that. We can do that by eliminating the parallel transitions. One way to eliminate parallel transitions is to increase the number of states. This way we can assign paths so that they do not have parallel transition. Figure below shows the redone trellis for the same code but one that has 8 states instead of 4. (This is done by increasing the number of memory registers from 2 to 3.)
0426
0 4 4 2 6 4 6 6 0 2 0 6 0 ⎛4⎞ ⎝2⎠ 1537 2 4062 5173 7 5 1 1 35 2604 7 3 6 2 4 0 2(010) 1(001) 3(011) 4(100) 0(000) 3715 6240 7(111) 5(101) 6(110) 7351 Figure 22 – Trellis of a code of rate 1/2 with eight states (b) bits to symbol mapping With more states available, we assign the four paths of Figure 21 to other states so there are no parallel transitions. In the Figure above, the left side lists the symbol numbers possible at each state. At state 0, we take path 6, because it has the smallest distance, from there we go to symbol 7 and then symbol 6 again. Without proving, we state that this path has the smallest SED. The sum of each of the distances is s0 to s6 = 2.0, s0 to s7 = .586, s0 to s6 = 2.0 Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 19 2 + .586 + 2 = 4.586 and now the coding gain is 10 log (4.586/2) = 3.7 dB This is an improvement over the case of four states which had a coding gain of only 3 dB and also an improvement over the rate 2/3 code by itself. More states improve this yet further. Here is a list of coding gains possible as number of states is increased still further. We conclude that any TCM with number of states greater than 4 can beat the performance of using the convolutional code alone. Table I – Coding gain vs. number of states in a 8PSK TCM Number of States Coding Gain, dB 4 3.0 8 3.6 16 4.1 64 4.8 128 5 256 5.4 512 5.7 The examples have all been for 8PSK. But in TCM we can we can start with a 8PSK and then go to 16QAM as the final signal or any other such doubling. In V.32 modulation a constellation of 32 points is used. To examine how 8PSK to 16QAM would work, we take 3 incoming bits.(u stands for uncoded.) ub1 ub2 ub3 Of these, we would code bits ub2 and ub3 with rate 2/3 encoder to get three bits, and now we have ub1 cb2 cb3 cb4 Starting with cb4, we would then select a signal using the partition in Figure 23. Example: we have 1011, this would map to symbol s11. Note that the last mapping is done based on the first uncoded bit. It has a ED which is 16 times larger than at the first level and hence offers good error protection on its own. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 20 dmin = 2a d2min = 4a2
0 1 d2min = 8a2 0 1 0 1 d2min = 16a2
0 1 0 1 0 1 0 1 d2min = 32a2 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0000 0001 S0 S1 0010 0011 S2 S3 0100 0101 S4 S5 0110 S6 0111 S7 1000 1001 S8 S9 1010 1011 S10 S11 1100 S12 1101 S13 1110 S14 1111 S15 Figure 23 – 16 QAM partition If the at the lowest distance is 2a with SED equal to 4a2, then at the next level it is 8a2, then 16a2 and then 32a2 and so on, doubling each time. Compare this to the SED of QPSK to which we would compare this to determine the coding gain. The coding gain based on this would be 4.4 dB A list of coding gains possible for 16QAM are given below. Number of states and coding gain for 16QAM States 4 8 16 64 128 Gain, dB 4.4 5.3 6.1 6.8 7.4 The Pragmatic TCM Each version of TCM as created by Ungerboeck, requires a different rate code. To avoid this problem, Viterbi (see Ref 5) suggested the use of the 64 state standard rate ½ convolutional code which had been a de facto standard in communications for quite a while. The other advantage of using this “offtheshelf” code is that with minor modification it can be used to decode both the coded and the uncoded bit. The use of this code and not the custom codes of the original concept is called the Pragmatic Approach. Pragmatic, meaning practical but not necessarily optimum, because you use what you have available. Viterbi algorithms can be used to decode the pragmatic TCM so this approach is popular, although it is not as effective as the setpartitioning approach of Ungerboeck. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 21 The pragmatic approach uses the common ½ rate coder (which can be punctured to provide all other rates.) and can create the QPSK, 8PSK and 16QAM signal TCMs.
4PSK
+ 2 bits Modulator M = 2k = 4 Carrier 8PSK QPSK symbols Rate 1/2, K = 7 + BPSK to QPSK TCM + Modulator 3 bits M = 2k = 8 Carrier 8PSK symbols Rate 1/2, K = 7 + QPSK to 8PSK TCM + Rate 1/2, K = 7 + + 4 bits 16QAM Modulator M = 2k = 16 Carrier 16QAM symbols x 8PSK to 16QAM TCM
Rate 1/2, K = 7 + Figure 24  Pragmatic TCM uses a standard rate 1/2 convolutional code with modified Viterbi decoding. Decoding using the Viterbi Algorithm Viterbi algorithm uses a metric and tracks this metric for several trellis paths at once. The path with larger metric is dropped when it merges with another. In harddecision Viterbi decoding, this is done using the Hamming distance as a metric. In TCM the decoding is done with softdecision algorithm and Euclidean distance is used as the metric. The objective is to track n possible sequences, keep track of cumulative MSEDs. When paths merge at a state, follow only the one with the smallest metric. Multidimensional TCM BPSK, PAM are one dimensional modulations. QPSK is a two dimensional modulation composed of two BPSK signals in quadrature. 8PSK similarly is also a 2D modulation. The TCM we have been talking about so far is called 2LDMPSKTCM, with L = 1, a 2LD modulation with a dimensionality factor L = 1. A method of increasing the dimensionality of TCM (L > 1) signals was proposed by Wei. (See really excellent paper on this by Pietrobon, Ref. 4) The term L denotes L dimensions of 2D MPSK signals. With L = 1, we transmit just one TCM symbol. With L = 2, we transmit 2 symbol, so that the number of symbols transmitted is equal to L. Another way multiD is referred is by L x MPSK. So if we say 3 x MPSK, that is 3 symbols, and a 4 x MPSK is four symbols. All of these are 2D signals so, the total dimension is 2L. The 4 x MPSK is also called 8D TCM. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 22 The main concept in multidimensionality is increasing the number of symbols created in one processing period. The transmitted symbols are generated together and this cogeneration creates dependence and allows better performance. The term multidimensionality does not mean anything other than a form of multiprocessing. We do not have a multidimensional signal in the usual sense here. In fact typically multidimensionality implies independence and here we have the opposite. So the term is confusing. It should have been called higherorder TCM instead. The advantage of multi dimensional TCM are 1. We can transmit fractional information rates. Instead of the effective code rate being 2/3 as it is in 1 x 8PSK, here it can be higher. We can reduce the code overhead by effecting more than one symbol so we can use code rates like 5/6, 8/9 and 11/12. 2. Better bit efficiency is possible. We define bit efficiency as number of input information bits divided by the number of symbols transmitted in one processing period. 3. Smaller Peak to average ratio The peak to average ratios are seen to go down for these signals since random coherence problems are lessened. 4. No additional hardware complexity, we can use standard rate ½ codes (punctured) with standard decoding. MultiD TCM typically uses the pragmatic version of TCM.
Z1
+ b1 b2 b3 b4 x5 x4 x3 x2 Code R = 1/2 x1 x0 Z1
+ + Z2 Z2 Constellation Mapper Carrier Figure 25 – 2 x 8PSK, 4 input bits, 2 remain uncoded, 2 are coded to produce 4 bits for a total of 6 bits going into the constellation mapper. The constellation mapper uses a algorithm to reorder these bits to output 2 symbols. This Figure shows the conFigureuration for generating a 2 x 8PSK signal. 4 bits come in. 2 of these go into a rate ½ encoder and generate 2 parity bits, for a total of 6 bits. These six bits are then mapped in a special way by the constellation mapper. It is this mapping function that creates the symbol interdependency. The effective code rate is 2/3 and the bit efficiency as defined by number of information bits per symbol is 2 bits per symbol. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 23 b1 b2 b3 b4 b5 x5 x4 x3 x2 x1 Code R = 2/3 x0 Z1
+ Z1
+ + Z2 Z2 Constellation Mapper Carrier Figure 26  2 x 8PSK bit efficiency = 2.5 bits per symbol, code rate of 5/6 By playing with code rate, such as a rate 2/3 encoder in Figure 26, with five input bits producing the 2 symbols, we can change the effective code rate. In example of Figure 26, the code rate is 5/6 and bit efficiency is 5/2 = 2.5 bits per symbol. The mapping function of the constellation mapper is bit more complex than in a 1 x 8PSK. We do not just take the six incoming bits and map them sequentially to the two symbols. There is processing going on inside this box. We will now examine how this mapping is done. Example: Constellation mapping of a rate 2/3 2 X 8PSK TCM (L = 2) Now imagine that we have two parallel 8PSK symbol producers. Each is numbered from 0 to 7. If we transmit a pair of symbols, then there are total of 64 possible pairs. Any one of these pairs can be mapped to the 6 input bits. Figure 27 – A pairs of symbols are being produced in one processing period in a 2 x 8PSK (or also 4 D TCM) We write the output vector of two symbols as ⎡ Symbol1 ⎤ ⎡ Y1 ⎤ ⎡ y12 Y == ⎢ ⎥=⎢ ⎥=⎢ 2 ⎣ Symbol2 ⎦ ⎣Y2 ⎦ ⎣ y2 1 y1 1 y2 y10 ⎤ 0⎥ y2 ⎦ Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 24 Table II – 2 x 8PSK signal set partitioning (a) Top level
00 10 20 30 40 50 60 70 01 11 21 31 41 51 61 71 02 12 22 32 42 52 62 72 03 13 23 33 43 53 63 73 04 14 24 34 44 54 64 74 05 15 25 35 45 55 65 75 06 16 26 36 46 56 66 76 07 17 27 37 47 57 67 77 (b) First partition
00 10 20 30 40 50 60 70 01 11 21 31 41 51 61 71 02 12 22 32 42 52 62 72 03 13 23 33 43 53 63 73 04 14 24 34 44 54 64 74 05 15 25 35 45 55 65 75 06 16 26 36 46 56 66 76 07 17 27 37 47 57 67 77 In this table II we list all 64 possible pairs of symbols. Let’s take the two neighbor points (00) and (01). These two sets of symbol sequences are about as close as two sequences can get. The MSED between them is .586. (The distance between the first symbols 0 and 0 is 0.0, the distance between the second symbols 0 and 1 is .586 for a total of .586) This is the smallest distance at this level where all pairs are possible. Now partition top level (a) into two cosets as shown in two different colors in Table II (b). There are two cosets in this partition. One coset can be transformed into the other by coset generator g0 = (0, 1). If you take any symbol from the one set and if you add 0 to the first symbol and 1 to the second, you will shift position the symbol from one coset to the other. If you examine the sequence distance of points in the colored coset (b) you see that they are now further apart than at the top level. For points 00 and 11, which are two closest neighbors, the sum of MSED is (.586 + .586) = 1.172 at this level. (c)
00 11 20 31 40 51 60 71 62 73 42 53 64 75 22 33 44 55 66 77 02 13 24 35 46 57 60 62 64 66 04 15 26 37 40 42 44 46 06 17 20 22 24 26 (d)
00 02 04 06 Now partition again. We again get two cosets, related by the coset generator g1 = {1, 1). as in (c). The distance between the points in the new coset has increased again. Now the two nearest points are 00 and 02. The distance between these is 2.0. Now partition the shaded coset again, we get Partition (d). These points are now 4.0 apart. Now the two closest points are 00 and 22. The SMSED is (2 + 2) = 4. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 25 We do this two more times to get to an individual signal. At the last level, the points are (4 + 4) = 8 units apart. We partitioned the set six times to get to a single signal. Partition (e)
00 04 Partition (f)
00 22 26 40 44 44 62 66 How the math works All of this is easily coded into a chip using simple math. Let’s take at look at each of the coset generator which is making decisions about which assignment to make based on the received bit. We have six incoming bits (after all incoming bits have been coded) and we have six partitions in response to each of these bits. Let’s write down all the coset generators as we will need them, g 0 = (0,1), g1 = (1,1) g 2 = (0, 2), g 3 = (2, 2) g 4 = (0, 4), g 5 = (4, 4)
Now we take the six input bits and put them through this transformation to come up with the coded and related symbols. Take for example an incoming vector x = 010101 in form of = x 5 , x 4 , x 3 , x 2 , x1 , x 0 = (0,1, 0,1, 0,1)
We multiply each bit by its coset generator to determine which coset it falls in. We get (0, 7) if math is done mod(8). = x 5 (4, 4) + x 4 (0, 4) + x 3 (2, 2) + x 2 (0, 2) + x1 (1,1) + x 0 (0,1) = (0, 7)
This looks a bit incomprehensive until you do it in binary form. Let's rewrite the coset generators in binary form by replacing the ordinal number with binary equivalents.
⎡1 1 ⎤ ⎡0 1 ⎤ ⎡0 0 ⎤ ⎢0 0⎥ g = ⎢0 0⎥ g 3 = ⎢1 1 ⎥ g5 = ⎢ ⎢ ⎥4⎢ ⎥ ⎥ ⎢0 0 ⎥ ⎢0 0 ⎥ ⎢0 0 ⎥ ⎣ ⎣ ⎣ ⎦ ⎦ ⎦ Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 26 ⎡0 0 ⎤ ⎡0 0 ⎤ ⎡0 0 ⎤ ⎢0 1 ⎥ g = ⎢0 0 ⎥ g = ⎢0 0 ⎥ g2 = ⎢ ⎥1⎢ ⎥0⎢ ⎥ ⎢0 0 ⎥ ⎢1 1 ⎥ ⎢0 1 ⎥ ⎣ ⎣ ⎣ ⎦ ⎦ ⎦ The output two symbols and their constituent bits are given by ⎡Y ⎤ ⎡ y 2 Y = ⎢ 1 ⎥ = ⎢ 12 ⎣Y2 ⎦ ⎣ y2 1 y1 1 y2 y10 ⎤ or 0⎥ y2 ⎦ 1 2 1 0 Y = (Y1 , Y2 ) ) = ⎡( y12 , y1 , y10 ), ( y2 , y2 , y2 ) ⎤ ⎣ ⎦ Each bit falls in a particular coset and the total vector is sum of each bit times its corresponding coset generator. There are six generators so index goes from 0 to 5.
Y = (y
1 ,y 2) = 0≤ p≤5 ∑ x p g p For the example bit sequence for (0,1,0,1,0,1), x5, x3, and x1 are zero. Crossing these out, we get ⎡1 1 ⎤ ⎡0 1⎤ ⎡ 0 0⎤ ⎡ 0 0⎤ ⎡ 0 0⎤ ⎡ 0 0⎤ ⎢ 0 0 ⎥ ⊕ x 4 ⎢ 0 0 ⎥ ⊕ x 3 ⎢ 1 1 ⎥ ⊕ x 2 ⎢ 0 1 ⎥ ⊕ x1 ⎢ 0 0 ⎥ ⊕ x 0 ⎢ 0 0 ⎥ =x ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢0 0⎥ ⎢ 0 0⎥ ⎢ 0 0⎥ ⎢ 0 0⎥ ⎢1 1 ⎥ ⎢0 1⎥ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦
5 ⎡0 1⎤ ⎡0 0⎤ ⎡ 0 0⎤ ⎢ 0 0⎥ ⊕ x 2 ⎢0 1⎥ ⊕ x 0 ⎢ 0 0⎥ =x ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢0 0⎥ ⎢0 0⎥ ⎢0 1⎥ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦
4 ⎡0 1 ⎤ ⎡0 0⎤ ⎡0 0⎤ ⎡0 1⎤ = 1⎢0 0⎥ ⊕ 1⎢0 1 ⎥ ⊕ 1⎢0 0⎥ = ⎢0 1⎥ or ⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥ ⎢0 0⎥ ⎢0 0⎥ ⎢0 1 ⎥ ⎢0 1⎥ ⎣ ⎦⎣ ⎦⎣ ⎦⎣ ⎦ Y = ⎡(000) , (111) ⎤ ⎣ ⎦
and this is the same as Y = [ 0, 7 ] in ordinal numbers. So for that given input sequence two output symbols are s0 and S7.
If you take the following equation and put it in a picture form as in Figure 27, you will see that the symbol mapper is doing just this math. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 27 ⎡1 1⎤ ⎡0 1⎤ ⎡ 0 0⎤ ⎡ 0 0⎤ ⎡ 0 0⎤ ⎡ 0 0⎤ ⎢ 0 0 ⎥ ⊕ x 4 ⎢ 0 0 ⎥ ⊕ x 3 ⎢ 1 1 ⎥ ⊕ x 2 ⎢ 0 1 ⎥ ⊕ x1 ⎢ 0 0 ⎥ ⊕ x 0 ⎢ 0 0 ⎥ =x ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢0 0⎥ ⎢ 0 0⎥ ⎢ 0 0⎥ ⎢ 0 0⎥ ⎢ 1 1⎥ ⎢ 0 1⎥ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ 5 3 1 5 4 3 2 1 0 = x , x , x ( x + x ), ( x + x ), ( x + x )
5 The last equation says that symbol 1 consists of bits 5, 3 and 1 and symbol consists of sums of all the bits. This is same as the constellation mapper shown in Figure below. We can see this function as a kind of reordering or interleaving.
Y1 x5 x4 x3 x2 x1 x0 x5
+ Y2 x1+ x0 x3+ x2 x5+ x4 x3
+ x1
+ Figure 27  What the constellation mapper is doing for 2 x 8PSK TCM to the 6 incoming bits to produce the output symbols. As we increase L, the constellation mapping gets more complex. The free distance of a multiD TCM can be increased by both increasing the number of states or by increasing the number of dimensions. We determine the performance the same way as before by determining dfree of the code trellis. Extensive list of Asymptotic Coding Gain (ACG) for a L x 8PSK for various L can be found in Ref 1.. 3 x 8PSK constellation mapper In this 3 x 8PSK TCM there are 8 input bits and it puts out 3 8PSK symbols, its efficiency is 8/3 bits per symbol and its code rate is 2/3.
Z1 x9 x8 x7 x6 x5 x4 x3 x2 x1 x0 Z2 Z1 Constellation Mapper Z3 Carrier Z2 Z3 Code R = 2/3 Figure 28  3 x 8PSK TCM, producing three symbols at a time. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 28 A 3 x 8PSK generates three independent 8PSK symbols to pick from. The total number of input symbols, three at a time, are 8 x 8 x 8 = 512. This partition will require 9 levels, since (29 = 512) This is too complex to draw so I will just give the list of set coset generators, of which there are 9. g 0 = (0, 0,1) g1 = (0,1,1) g 2 = (1,1,1) g 3 = (0, 0, 2) g 4 = (0, 2, 2) g 5 = (4, 4, 4) g 6 = (2, 2, 2) g 7 = (4, 4, 0) g 8 = (0, 4, 4)
The three symbols are given by expressions
2 ⎡Y1 ⎤ ⎡ y1 ⎢2 Y = ⎢Y2 ⎥ = ⎢ y2 ⎢⎥ 2 ⎢Y3 ⎥ ⎢ y3 ⎣⎦⎣ 1 y1 1 y2 1 y3 y10 ⎤ 0⎥ y2 ⎥ 0 y3 ⎥ ⎦ ⎡ Y1 ⎤ ⎡0⎤ ⎡4⎤ ⎡ 2⎤ ⎡ 4⎤ ⎡0⎤ ⎢Y ⎥ = x8 ⎢ 4 ⎥ + x 7 ⎢ 4 ⎥ + x 6 ⎢ 2⎥ + x5 ⎢ 4⎥ + x 4 ⎢ 2⎥ ⎢ 2⎥ ⎢⎥ ⎢⎥ ⎢⎥ ⎢⎥ ⎢⎥ ⎢Y3 ⎥ ⎢4⎥ ⎢0⎥ ⎢ 2⎥ ⎢ 4⎥ ⎢ 2⎥ ⎣⎦ ⎣⎦ ⎣⎦ ⎣⎦ ⎣⎦ ⎣⎦ ⎡0⎤ ⎡1⎤ ⎡0⎤ ⎡0⎤ ⎢ 0 ⎥ + x 2 ⎢1⎥ + x1 ⎢1 ⎥ + x 0 ⎢ 0⎥ +x ⎢ ⎥ ⎢⎥ ⎢⎥ ⎢⎥ ⎢2⎥ ⎢1⎥ ⎢1 ⎥ ⎢1 ⎥ ⎣⎦ ⎣⎦ ⎣⎦ ⎣⎦
3 ⎡1⎤ ⎡0⎤ ⎡1⎤ ⎡ 0⎤ = ( 4 x 5 + 2 x 6 + x 2 ) ⎢1⎥ + ( 4 x 8 + x 4 + x1 ) ⎢1 ⎥ + ( 4 x 7 ) ⎢1⎥ + ( 2 x 3 + x 0 ) ⎢ 0⎥ ⎢⎥ ⎢⎥ ⎢⎥ ⎢⎥ ⎢1⎥ ⎢1 ⎥ ⎢ 0⎥ ⎢1⎥ ⎣⎦ ⎣⎦ ⎣⎦ ⎣⎦
or alternately in mod(8) math ⎤ 4 x7 + 2 x6 + 4 x5 + x 2 ⎡Y1 ⎤ ⎡ ⎢Y ⎥ = ⎢ 4 x 8 + 4 x 7 + 2 x 6 + 4 x 5 + 2 x 4 + x 2 + x1 ⎥ ⎥ ⎢ 2⎥ ⎢ ⎢Y3 ⎥ ⎢ 4 x 8 + 2 x 6 + 4 x 5 + 2 x 4 + 2 x 3 + x 2 + x1 + x 0 ⎥ ⎣⎦⎣ ⎦
If input bits are 101011001, then we get Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 29 4 ⎡Y1 ⎤ ⎡ ⎤ ⎡ 4⎤ ⎢Y ⎥ = ⎢ 4 + 4 + 2 ⎥ = ⎢ 2⎥ mod(8) ⎢ 2⎥ ⎢ ⎥ ⎢⎥ ⎢Y3 ⎥ ⎢ 4 + 2 + 2 + 1⎥ ⎢ 1 ⎥ ⎣⎦⎣ ⎦ ⎣⎦ ⎡100 ⎤ = ⎢ 010⎥ mod( 2) ⎢ ⎥ ⎢ 001⎥ ⎣ ⎦
Note that there can be more than one way to subdivide so the above list is not unique. 4 x 8PSK or 8D TCM NASA uses a 4 x TCM (also called 8D) for some deep space links. A 4 x 8PSK would have 4 8PSK constellation and as such would have 64 x 64 = 4096 total possible signals. This will require 12 levels of partitions since (212 = 4096) Four symbols are generates in one period. By adjusting the number of bits coming out of the encoder, we can create fairly large bit efficiencies and consequently reduce the overhead rate. The following Figure shows using a code of rate ¾ , so for this case, the bit efficiency would be 8/4 = 2.0 bits per symbol. b8 b7 b6 b5 b4 b3 b2 b1 x8 x7 x6 x5 x4 x3 x2 x1 Code R = 3/4 x0 Y1 Y2 Y1 Y2 Y3 Y4 Y3 Carrier Y4 Constellation Mapper Figure 29  4 x 9PSK TCM To determine the constellation mapping, again we look at the coset generators. Since there are total of 12 levels in the 4 x MPSK partition, we have a much deeper partition. The coset generator which we need to map the input bits to out bits are given here for one case. Others are also possible. g 0 = (0, 0, 0,1) g 3 = (0, 0, 0, 2) g 6 = (0, 2, 0, 2) g 9 = (0, 0, 4, 4) g1 = (0, 0,1,1) g 2 = (0,1, 0,1) g 4 = (1,1,1,1) g 5 = (0, 0, 2, 2) g 7 = (0, 0, 0, 4) g 8 = (2, 2, 2, 2) g10 = (0, 4, 0, 4) g11 = (4, 4, 4, 4) The SED at the lowest level is 16 and the transformation matrix is given by Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 30 ⎡Y1 ⎤ ⎡ ⎛ 1⎞ ⎜ 1⎟ ⎢Y ⎥ ⎢ ⎢ 2 ⎥ = ⎢ (4 x 8 + 2 x 5 + x1 ) ⎜ ⎟ ⎜ 1⎟ ⎢Y3 ⎥ ⎢ ⎜⎟ ⎢⎥⎢ ⎝ 1⎠ ⎣Y4 ⎦ ⎣ 0 0 ⎛ ⎞ ⎛ ⎞⎤ ⎜ ⎟ ⎜ ⎟⎥ 7 3 x x ⎜ ⎟ + 2⎜ ⎟⎥ +4 ⎜ ⎟ ⎜ ⎟⎥ x6 x2 ⎜7 ⎜3 6 4⎟ 2 0 ⎟⎥ ⎝x + x + x ⎠ ⎝ x + x + x ⎠⎦ ⎞ 4 x 8 + 2 x 5 + x1 ⎡Y1 ⎤ ⎛ ⎟ ⎢Y ⎥ ⎜ 8 7 5 3 1 4x + 4x + 2x + 2x + x 2⎥ ⎜ ⎟ ⎢ = 8 6 5 2 1 ⎟ Y3 ⎥ ⎜ ⎢ 4x + 4x + 2x + 2x + x ⎢⎥⎜8 7 6 5 4 3 2 1 0⎟ ⎣Y4 ⎦ ⎝ 4 x + 4 x + 4 x + 2 x + 4 x + 2 x + 2 x + x + 2 x ⎠
If input bits are 110011001, then output bits are 4 ⎡Y1 ⎤ ⎛ ⎞ ⎛ 4⎞ ⎟ ⎜ 2⎟ ⎢Y ⎥ ⎜ 4+4+2 ⎟ = ⎜ ⎟ mod(8) ⎢ 2⎥ = ⎜ ⎟ ⎜ 0⎟ ⎢Y3 ⎥ ⎜ 4+4 ⎟ ⎜⎟ ⎢⎥⎜ ⎣Y4 ⎦ ⎝ 4 + 4 + 4 + 2 + 2 ⎠ ⎝ 0 ⎠ ⎛ 100 ⎞ ⎜ 010 ⎟ ⎟ mod( 2) =⎜ ⎜ 000 ⎟ ⎜ ⎟ ⎝ 000 ⎠
Phase Invariance and TCM TCM is always used with differential encoding. Differential encoding allows recovering from phase ambiguity. Take a BPSK signal. Let us say that a phase shift of 180 degrees occurs, the received bit is now 0 instead of 1. How is the receiver to know that it was a 1? There is no way, unless differential encoding/decoding is used. Phase lock is a critical problem in demodulation. A 8PSK signal has 45 degrees phase shifts. When the signal is acquired, it is best if any phase shift near one of the allowed phases is locked on instead of having to worry about if it is the correct phase. Once a phase lock is obtained, then if the lock was incorrect then all other decoding would be valid but wrong. For example, if symbol 0 is shifted by 45 degrees, then symbol 7 is now demodulated as 000. Take a gray coded QPSK signal. If one symbol is locked on the loop and the resulting bits are 11 instead of 00, the differential encoder would be able to correct this error. The code in this case would be considered 180 degrees phase invariant. If a phase error of 90 degrees occur and bits 01 are confused for 00, then differential decoder which can only correct complements error is unable to correct this. In this case, this code is not phase invariant for 90 degrees. Similarly 8PSKs signal are 90 degrees phase invariant when numbered in natural order but not 180 or 45 degrees phase invariant. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 31 Certain codes when combined with differential encoding, allow recovering from phase shift errors. The sequence demodulated is automatically corrected for the given phase invariance. TCM sequences are phase lock sensitive. MultiD TCM codes offer an advantage that they offer phase invariance. Many different phase shifts error can be tolerated and are quickly taken out. One of the most comprehensive coverage of this material is in Ref, the book by Lin and Costello References In doing this tutorial, I relied mostly on the following four references. The first is a book, which I recommend highly. I didn’t like its pervious edition but this last edition is great. If you have to buy just one book on coding, this is the one. The 2nd and third are the seminal papers on this by Ungerboeck and the last is an excellent paper on multidimension TCM by Steven S. Pietrobon. The last paper is by Andrew Viterbi et all, an excellent paper on pragmatic TCM. 1.”Error Control Coding” Second Edition Shu Lin and Daniel J. Costello, Pearson, Prentice Hall 2. Gottfried Ungerboeck, "TrellisCoded Modulation with Redundant Signal Sets Part I: Introduction", IEEE Communications Magazine, vol. 25, no. 2, February 1987, pp. 511. 3. Gottfried Ungerboeck, "TrellisCoded Modulation with Redundant Signal Sets Part II: State of the Art", IEEE Communications Magazine, vol. 25, no. 2, February 1987, pp. 1221.
4. TrellisCoded Multidimensional Phase Modulation, Steven S. Pietrobon, Robert H. Deng, Alain Lafanechere, Gottfried Ungerboeck, Daniel Costello, 00189448. IEEE Transactions on Information Theory, Vol 36, No. 1, January 1990 5. Andrew J. Viterbi, Jack K. Wolf, Ephraim Zehavi, and Roberto Padovani, "A Pragmatic Approach to TrellisCoded Modulation", IEEE Communications Magazine, (probably vol. 27, no. 7), July 1989, pp. 1221. Other references Saud A. AlSemari and Thomas E. Fuja, (abstract of) "IQ TCM: Reliable Communication Over the Rayleigh Fading Channel Close to the Cutoff Rate", IEEE Trans, on Information Theory, vol. IT30, no. 6, November 1984, pp. 784791. John B. Anderson and Desmond P. Taylor, "A BandwidthEfficient Class of SignalSpace Codes", IEEE Trans, on Information Theory, vol. IT24, no. 6, November 1978, pp. 703712. Sergio Benedetto, M. Ajmone Marsan, Guido Masera, Gabriella Olmo, and Z. Zhang, "Encoded 16PSK: A Study for the Receiver Design", IEEE Journal on Selected Areas in Communications, vol. 7, no. 9, December 1989, pp. 13811391. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 32 Ezio Biglieri, "HighLevel Modulation and Coding for Nonlinear Satellite Channels", IEEE Trans, on Communications, vol. COM32, no. 5, May 1984, pp. 10911096. A.R. Calderbank and N.J.A Sloane, "An EightDimensional Trellis Code", Proc. IEEE, vol. 74, no. 5, May 1986, pp. 757759. A.R. Calderbank and N.J.A Sloane, "New Trellis Codes Based on Lattices and Cosets", IEEE Trans, on Information Theory, vol. IT33, no. 2, March 1987, pp. 177195. Robert Calderbank and James E. Mazo, "A New Description of Trellis Codes", IEEE Trans, on Information Theory, vol. 43, no. I, January 1997, pages not given. Pierre R. Chevillat and Evangelos Eleftheriou, "Decoding of TrellisEncoded Signals in the Presence of Intersymbol Interference and Noise", IEEE Trans, on Communications, vol. 37, no. 7, July 1989, pp. 669676. Robert H. Deng and Daniel J. Costello, Jr., "High Rate Concatenated Coding Systems Using Multidimensional BandwidthEfficient Trellis Inner Codes", IEEE Trans, on Communications, vol. 37, no. 10, October 1989, pp. 10911096. Dariush Divsalar, "Multiple Trellis Coded Modulation (MTCM)", IEEE Trans, on Communications, vol. 36, no. 4, April 1988, pp. 410419. Dariush Divsalar, Marvin K. Simon, and Joseph H. Yuen, "Trellis Coding with Asymmetric Modulations", IEEE Trans, on Communications, vol. COM35, no. 2, February 1987, pp. 130141. Karin Engdahl and Kamil Sh. Zigangirov, "On the Calculation of the Error Probability for a Multilevel Modulation Scheme Using QAMSignaling", IEEE Trans, on Information Theory, vol. 44, no. 4, July 1998, pp. 16121620. G. David Forney, Jr., Robert G. Gallager, Gordon R. Lang, Fred M. Longstaff, and Shahid U. Qureshi, "Efficient Modulation for BandLimited Channels", IEEE Journal on Selected Areas in Communications, vol. SAC2, no. 5, September 1984, pp. 632647. G. David Forney, Jr., and LeeFang Wei, "Multidimensional Constellations  Part I: Introduction, Figures of Merit, and Generalized Cross Constellations", IEEE Journal on Selected Areas in Communications, vol. 7, no. 6, August 1989, pp. 877892. Roger L. Freeman, Reference Manual for Telecommunications Engineering. Second Edition, Wiley Intescience, New York, 1994, pp. 326328, 13871424. Allen Gersho and Victor B. Lawrence, "Multidimensional Signal Constellations for Voiceband Data Transmission", IEEE Journal on Selected Areas in Communications, vol. SAC2, no. 5, September 1984, pp. 687702. Michael L. Honig, "On Constructing Embedded Multilevel Trellis Codes". IEEE Trans. on Communications, vol. 36, no. 2, February 1988, pp. 218221. Rob Howald, "Desperately Seeking Shannon", Communications Systems Design. March 1999, pp. 1216. Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 33 Kamal Jain, Ion Mandoiu, and Vijay V. Vazirani, "The 'Art of Trellis Decoding' Is Computationlly Hard  For Large Fields", IEEE Trans, on Information Theory, vol. 44, no. 3, May 1998, pp. 12111214. Rolf Johannesson and Emma Wittenmark, "Two 16State, Rate R  2/4 Trellis Codes Whose Free Distances Meet the Heller Bound", IEEE Trans, on Information Theory, vol. 44, no. 4, July 1998, pp. 16021604. Donald A. Johnston and Stephen K. Jones, "Spectrally Efficient Communication via Fading Channels Using Coded Multilevel DPSK", IEEE Trans, on Communications, vol. COM29, no. 3, March 1981, pp. 276284. James M. Kroll and Nam C. Phamdo, "Analysis and Design of Trellis Codes Optimized for a Binary Symmetric Markov Source with MAP Detection", IEEE Trans, on Information Theory, vol. 44, no. 7, November 1998, pp. 29772987. L. H. Charles Lee, "Combined Convolutional Coding and Modulation", Chapter 7 of Convolutional Coding: Fundamentals and Applications. Artech House, Boston, 1997, pp. 149167. Also see Chapter 2 on Convolutional Code Structures, pp. 1055, in the folder TCMMy Notes. M. Ajmone Marsan, G. Albertengo, and S. Benedetto, IEEE Trans, on Communications, vol. COM35, no. 9, September 1987, pp. 969972. Steven V. Pizzi and Stephen G. Wilson, "Convolutional Coding Combined with Continuous Phase Modulation", IEEE Trans, on Communications, vol. COM33, no. 1, January 1985, pp. 2029. Mischa Schwartz, 'TrellisCoded Modulation", Section 710 of Information Transmission. Modulation, and Noise". 4th edition, McGrawHill, New York, 1990. Desmond P. Taylor and Hing C. Chan, "A Simulation Study of Two BandwidthEfficient Modulation Techniques", IEEE Trans, on Communications, vol. COM29, no. 3, January 1981, pp. 267275. Anthony Tehan and John O. Scanlan, "A Simulation Study of TrellisCoded Modulation for a Satellite Link", IEEE Trans, on Communications, vol. 45, no. 11, November 1997, pp. 13711374. Hemant K. Thapar, "RealTime Application of Trellis Coding to HighSpeed Voiceband Data Transmission", IEEE Journal on Selected Areas in Communications, vol. SAC2, no. 5, September 1984, pp. 648658; Gottfried Ungerboeck, "Channel Coding with Multilevel/Phase Signals", IEEE Trans, on Information Theory, vol. IT28, no. 1, January 1982, pp. 5567. LeeFang Wei, "Rotationally Invariant Convolutional Channel Coding with Expanded Signal Space  Part I: 180 degrees", IEEE Journal on Selected Areas in Communications, vol. SAC2, no. 5, September 1984, pp. 659671 Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 34 Error 001 001 001 001 001 001 001 001 010 010 010 010 010 010 010 010 011 011 011 011 011 011 011 011 100 100 100 100 100 100 100 100 101 101 101 101 101 101 101 101 110 110 110 110 110 110 110 110 111 111 111 111 111 111 111 111 Symbol 000 001 010 011 100 101 110 111 000 001 010 011 100 101 110 111 000 001 010 011 100 101 110 111 000 001 010 011 100 101 110 111 000 001 010 011 100 101 110 111 000 001 010 011 100 101 110 111 000 001 010 011 100 101 110 111 Errored Symbol 001 000 011 010 101 100 111 110 010 011 000 001 110 111 100 101 011 010 001 000 111 110 101 100 100 101 110 111 000 001 010 011 101 100 111 110 001 000 011 010 110 111 100 101 010 011 000 001 111 110 101 100 011 010 001 000 Sq. Distance Const1 Const2 0.586 0.586 0.586 0.586 0.586 0.586 0.586 0.586 0.586 0.586 0.586 0.586 0.586 0.586 0.586 0.586 2 2 2 2 2 2 2 2 3.414 0.586 0.586 3.414 3.414 0.586 0.586 3.414 4 4 4 4 4 4 4 4 3.414 3.414 3.414 3.414 3.414 3.414 3.414 3.414 2 2 2 2 2 2 2 2 0.586 0.586 0.586 0.586 0.586 0.586 0.586 0.586 3.414 0.586 3.414 0.586 3.414 0.586 3.414 0.586 2 2 2 2 2 2 2 2 0.586 3.414 0.586 3.414 0.586 3.414 0.586 3.414 2 2 2 2 2 2 2 2 4 4 4 4 4 4 4 4 3.414 3.414 3.414 3.414 3.414 3.414 3.414 3.414 Finding the Error Vector 000 001 010 011 100 MSED 101 Min. sq distance Const 1 Const 2 0 0 0.586 0.586 2 0.586 0.586 2 4 0.586 3.414 0.586 Copyright 2004 Charan Langton www.complextoreal.com Trellis Coded Modulation (TCM) 35 Copyright, 2004 Charan Langton All Rights reserved Other tutorials at www.complextoreal.com Copyright 2004 Charan Langton www.complextoreal.com ...
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