James Miller G3RUH
In a previous Oscar News (ref 1,8) I described the error detection and correction (EDAC) of Oscar-13's computer memory. I showed how the simple addition of a few redundant "parity bits" to each data byte achieved this, and ended by noting that this technique is used to increase data transmission reliability. That is, you send more bits than you started with, so for a fixed transmitter power the intrinsic bit error rate worsens. However the error correction scheme more than makes up for this, a result which seems to defy intuition, but which is the justifiable substance of C E Shannon's famous coding theorems. (ref 2) We are not in the realms of academia here. When space communication costs are reckoned in millions of $$$ per db (ref 4), schemes that provide more bytes for your buck are of the greatest importance. Their use in amateur radio isn't far away. Intrigued? This article is a brief introduction. A BASELINE SYSTEM Lets explore; first of all we look at the standard baseline system, binary PSK transmission. That is to say, a 0 is sent by a carrier with phase 0 degrees, and a 1 by phase 180 degrees, i.e inverted. The optimum demodulator (PSK decoder) processes the received carrier to detect a 1 or 0. For our baseline assessment we need to draw a graph of the probability of a bit error as a function of signal-to-noise ratio (SNR) for binary PSK. Look at figure 1. The vertical axis is "Pe" for probability of error. For example Pe=0.01 means 1 error per 100 bits. Pe=10^-6 means one in a million bits, which is a pretty small chance. Horizontal is "Eb/No" which stands for Energy-per-bit per Noise-power-per-Hz-bandwidth. (This is the same as SNR when the bit rate is the same as the bandwidth). You can see in the various curves that a higher SNR results in a smaller error rate. Curve A shows the baseline PSK performance. At poor SNR the error rate is, not suprisingly, 50%. However by 7 db SNR it's down to 1 per 1000, and at 9.6 db about 1 in 100,000 bits. For reference, the mathematical equation for curve A is given by: Pe = 0.5*erfc(sqr(Eb/No)) where erfc(.) is the complementary error function, and sqr(.) is square root. Erfc(.) is related to the cumulative Gaussian distribution, and is tabulated in mathematical and statistical handbooks (ref 3). Typical values: erfc(0)=1.0, erfc(0.5)=0.48, erfc(1)= 0.14, erfc(1. 5)= 0.034, erfc(2)=0.005, etc, tending to zero.
Figure 1. Performance comparison of various channel coding schemes A. Baseline: Binary PSK B. (7,4) Hamming coding, hard decisions C. (7,4) Hamming coding, soft decisions D. Mariner '69 deep space probe, (32,6) bi-orthogonal coding E. Voyager probes, NASA deep space standard (ref 6) F. Theoretical Shannon limit on coding performance. [Added: I recently (2004) discovered this ; see page 4. I have no connection whatsoever ... ] HAMMING CODING Now what happens when we add parity bits? Lets examine a specific arrangement, the (7,4) Hamming code. In this, data is taken 4 bits at a time and 3 parity bits appended to make a "word" of 7 bits, hence the designation (7,4). I used this as my example in ref 1. Denoting the data bits D0, D1, D2, D3, the parity bits are formed from excusive-ORs of the data bits, viz: P0=D0+D1+D3, P1=D0+D2+D3, P2=D1+D2+D3. The 16 possible words are thus (with a little space for clarity): DDDD PPP DDDD PPP DDDD PPP DDDD PPP 3210 210 3210 210 3210 210 3210 210 -------------------------------------------------- 0000 000 0100 110 1000 111 1100 001 0001 011 0101 101 1001 100 1101 010 0010 101 0110 011 1010 010 1110 100 0011 110 0111 000 1011 001 1111 111 -------------------------------------------------- Table 1. 16 code words of the (7,4) Hamming code -------- So as an example, if the 4 data bits are 1100, P0=0,P1=1 and P2=1, so the bits actually transmitted are 1100011 - see top of 4th column. Consider transmission; we now send 7 bits in the time we would have sent 4 original ones. So the channel bit rate is 7/4 times what it was, implying a 7/4x wider bandwidth. Now reception; for the same transmitter power as used in the baseline system, the received Eb/No must be 4/7 worse, leading to a higher error rate for the 7 bits of the word. But the Hamming coding scheme allows correction of up to 1 error. So words with 0 or 1 error are OK, words with 2,3...7 errors are bad. Thus the probability of a word error is Pw = p2+p3+p4+p5+p6+p7 = 1-(p0+p1), where p0 is the probability of nil errors, p1 for 1 error and so on. When we get a word error, its data bits are essentially garbage, i.e 50% are in error. So the equivalent bit error rate for the system is approximately Pe=Pw/2 (actually 4/7*Pw). Curve B of figure 1 shows the performance of the (7,4) Hamming code using "Hard" decisions as described above, where the 7 elements of the word are demodulated individually and independently. At an SNR less than 7.5 db the baseline PSK system is a better performer, but above this SNR the Hamming code wins. The gain is equivalent to 0.4 db in SNR at an error rate of 1 per million. CODING GAIN Not dramatic, but it illustrates the point. We have achieved a "coding gain" of 0.4 db at the expense of a) increased bandwidth, b) increased decoder complexity and c) throughput delay. This gain can be used either to a) reduce transmitter power or antenna sizes by 0.4 db, or b) increase the data rate by 10%, or c) increase communication range by 5%. For reference, curve B is computed from the formulae: p=0.5*erfc(sqr(4/7*Eb/No)), Pw=1-((1-p)7 + 7*p*(1-p)6), and Pe=Pw*4/7. OPTIMUM DECODING There is a better way to decode the 7 bit words. Instead of demodulating them as 7 individual bits ("hard" decisions), we can demodulate the whole word using 16 detectors individually matched to the 16 patterns in table 1. Then the detector showing the highest output or "correlation" is chosen as correct and the associated 4 data bits are output. This is called "soft" decision decoding. These detectors would almost certainly not be mechanised in hardware, as there are so many of them. It is far simpler to do the equivalent operations in software - digital signal processing (DSP), though this need not concern us here; we assume the job done somehow. The performance of this new scheme, in which we appear to have forgotten Hamming's error detection scheme, is shown in curve C of figure 1. Compared with the baseline PSK, we have an improvement or coding gain of 2. 4 db at a bit error rate of 1 in a million. Now we're getting somewhere! It's interesting to note that the performance difference between hard and soft decision decoding, exemplified by this simple scheme, is just 2 db. This hard/soft penalty, 10*log(PI/2), shows up regularly in signal processing analysis, regardless of coding scheme, in the power-constrained situation. And no, we haven't really lost Mr Hamming. Remember there are 7 bits to the word; that means a potential 128 combinations of bits, from which we chose only 16. Not any 16, but a set in which each code-word is as different as possible from each other word. Hamming's code achieves this, as in fact do several other sub-sets. MARINER '69 MARS PROBE Mariner '69 Mars probe was one of the first missions to make use of coding. Why? Because at vast distances, with limited transmitter power, a finite antenna size on the spacecraft and on the ground, every fraction of a decibel counts. Mariner '69 used a (32,6) code, in which blocks of 6 bits were augmented to make a 32 bit word, a bandwidth expansion of 32/6 or 5. 3:1. Detection was by soft decisions using correlators. The performance is shown in curve D of figure 1. The coding gain is about 3.6 db at an error rate of 1 per million bits. SOURCE CODING Space probes typically send back images. However images are usually full of redundant information. For example a picture that includes a horizon might contain large areas of blackness. It would be a folly to send back such an image without pre-processing. The business of efficient representation of information is called "source coding", as distinct from channel coding discussed so far. In many applications, source coding is likely to have a far greater influence on overall communications efficiency than channel coding, yet it is often overlooked or simply ignored. A good example is amateur spacecraft telemetry, where data is sent down block after block, with little or no change in some of the data, but rapid changes in others. So it's efficient for some information, but not all. Another is plain language bulletins. When I checked a sample of 50 Oscar- 13 message blocks I found that based on the frequency of occurence of letters/digits etc, the average Information content was less than 5 bits/character; yet we actually transmit bulletins with 8 physical bits/character. The most common symbol was 'space'. Clearly it could be represented by one bit only, say 0. Next came the letter 'e', so it should be recoded as 10, and so on. This kind of scheme is Huffman coding, an algorithm for the construction of efficient signalling alphabets. In my example a saving of 5/8 in bits or 2 db would be achieved at the expense of a look-up table in the spacecraft, and a similar algorithm at the receiver. CONVOLUTIONAL CODING Returning to the subject of channel coding; the schemes outlined thus far are examples of "Block Coding", because data is transmitted and processed in discrete blocks. But there is another method called "Convolutional Coding". In this the parity bits and data bits are interwoven (often alternately); the parity bits are formed from the exclusive-OR of selected preceding data bits on a continuous basis. INTERLEAVING Bursts of noise are not uncommon on radio links, and coding schemes don't like error bursts. So in many applications the data is interleaved just prior to transmission. For example a 100 byte message could be sent in byte order 0,10,20...90, 1,11,21...91, 2,12,22 .... 89,99, and re-ordered upon reception. So a burst of channel errors is spread out in the re- ordered block, making the decoder's job less arduous. VOYAGER PROBES More advanced systems use a mixture of a block codes, a convolutional code and interleaving. Spectacular examples are the Voyager probes to the outer planets, which achieve a 1 per million error rate at an Eb/No of only 2.53 db, as sketched in curve E of figure 1. This represents a coding gain of 8 db, a very substantial figure. The bandwidth expansion factor is 2.3:1 (ref 6, page 430). SHANNON LIMIT It is natural to ask if these improvements have a limit. The extraordinary thing is that Shannon had already mathematically proved the fundamental limit theorems, long before anyone had thought of building coding hardware! (ref 2) For the systems I've been discussing, in the power limited/bandwidth unlimited situation, the ultimate is curve F in figure 1. When the SNR is less than -1.6 db the error rate is 50%; above -1.6 db the error rate is nil. (-1.6 db corrresponds to Ln2). Naturally to achieve the ultimate system requires signal processing using an infinite amount of time and an infinite amount of complexity! Nevertheless recent schemes have been proposed which approach the Shannon limit within 1-2 db or so. AMATEUR SATELLITE LINKS Why is all this of interest for amateur satellites? Well, we can start by looking at the performance limits of our current phase 3 type digital links. Assume the spacecraft is limited to 1 watt e.i.r.p, and that the range is 42000 km. Assume a realistic size of ground station antenna - say 18db gain on 70cm or a 1 metre dish on 13cm; assume also a realistic receive noise level. The maximum data rates achievable work out to around 600 b.p.s./watt on 70cm, and 350 b.p.s./watt at 13cm using PSK modulation and optimum demodulation, at an error rate of 1 per million bits. Sufficient at present for simple telemetry, but far short of our future needs. THE VALUE OF CODING Let's be ambitious; suppose we would like a throughput of 9600 bps on 70cm. This is an increase of 16:1, or 12 db link improvement. Where is this extra performance going to come from? The answer of course is from a combination of a minimum increased transmitter power and a maximum of coding gain. Coding gains of 5.5 db are probably realistic for amateurs, so the transmit power would have rise to just 4.5 watts (6.5 db), instead of a greedy 16 watts without coding. Studies such as these are currently being performed by AMSAT. Indeed the RUDAK-2 module carries additional experimental receivers and an RISC based computer for fast signal processing. These will allow us to evaluate the possibilities for new state of the art digital links for radio amateurs. (On AO-21). --------------------------------------------------------------------------- READING 1. Miller J.R.; "AO-13 Memories are made of this", AMSAT-UK Oscar News No. 80, December 1989. See also . 2. Shannon C. E.; "Communication in the Presence of Noise", Proc. IRE, vol 37, pp10-21, January 1949 3. Abramowitz M. and Stegun, I.; "Handbook of Mathematical Functions", Dover Publications, 1965. ISBN 0-486-61272-4 4. Forney D.G.; "Coding and its Applications in Space Communications", IEEE Spectrum, vol 7, pp 47-58, June 1970. (Excellent introduction). 5. Costas J.P.; "Poisson, Shannon, and the Radio Amateur", Proc. IRE, vol 47, pp. 2058-2068, Dec 1959. 6. Haykin, S.; "Digital Communications", John Wiley and Sons 1988. ISBN0-471-62947-2. 7. Posner E.C. and Stevens R.; "Deep Space Communication - Past Present and Future", IEEE Communications Magazine, vol 22, pp 8-21, May 1984 8. Miller J.R.; "Down Memory Lane", AMSAT-UK Oscar News No.108 August 1994 p16-19. Also: Amsat-VK Newsletter No. 112, 1994 Aug. Also: The Amsat Journal (USA) Vol 17 No. 5, Sep/Oct 1994.
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