This format works well when link conditions are good, but poorly under suboptimal conditions. A particular problem appears when the spacecraft antennas are off-pointed from the earth, as they must be at certain times to ensure sufficient solar panel illumination and/or to control spacecraft temperature. Under these conditions, the spacecraft's antenna patterns and its spin give rise to periodic fades that can be very deep. Overall signal levels are also reduced.
The intent of the present project is to devise a new format for AO-40 telemetry that uses strong error control coding and interleaving to resist the effects of spin fading. A secondary goal is to reduce average signal-to-noise ratio requirements to permit the use of poorer (i.e., lower G/T) ground stations.
The format described here has been implemented in software and tested on a simulated channel. The channel simulator applies a sinusoidal envelope to simulate spin fading (with two nulls per cycle) and adds Gaussian noise to simulate receiver and cosmic background noise. The improvement over the old uncoded format is, quite simply, dramatic: solid copy is easily attained with fade rates comparable to those observed in practice, and with average Eb/No ratios down to about 7 dB. Because of the rate 0.4 (-4dB) coding, such signals "sound" like uncoded BPSK at an average Eb/No of only 3 dB. This is barely detectable to the human ear even at the peaks between nulls. In these conditions, approximately 15% (or about 780 out of 5200) of the raw demodulated channel symbols are in error, but are completely corrected by the error control coding. In the present uncoded format, just one channel error will ruin an entire frame.
A separate reference implementation of the encoder has been written in C. It is intended to be translated into IPS for integration into the operational software running in the IHU.
GCC on the Pentium compiles the reference encoder into about 900 bytes of program space plus about 1300 bytes for variables, lookup tables and the output buffer. Several space-time tradeoffs are possible; e.g., if additional RAM is available, it can be used for extra lookup tables to reduce CPU loading. Prime candidates for additional lookup tables are byte parity computation (256 bytes) and scrambler sequence generation (320 bytes).
The prototype demodulator/decoder is a UNIX/Linux filter that accepts 16-bit samples from a generic sound card and emits decoded, corrected data to standard output, with optional informational messages to standard error. The software is primarily in GNU C and can port to any environment supported by that compiler; it is intended to be integrated into popular telemetry reception programs such as AO40RCV.
Optional assembler code is provided that uses the various Intel/AMD IA32 SIMD instruction sets, if available, to reduce CPU utilization in filtering and Viterbi decoding. All three generations of IA32 SIMD instructions are supported: MMX, SSE and SSE2. Demodulating and decoding in real time uses less than 1% of a 500 MHz Pentium III CPU with SSE.
The demodulator currently uses a purely noncoherent DBPSK technique well suited for fading channels with Doppler shift and other carrier phase disturbances. A future enhancement is to add a coherent demodulator that is expected to reduce Eb/No requirements by another 4 dB or so (i.e., to about 3dB) on non fading channels, and to automatically select whichever technique works best for the current conditions.
The AO-40 telemetry hardware follows its heritage to the original Phase 3 project begun in the mid 1970s and first flown on the ill-fated Phase 3A in May 1980. Software running in the IHU formats a telemetry frame in IHU memory, and hardware then takes over: the data is converted to a bit serial format, differentially encoded, Manchester encoded, low-pass filtered to limit bandwidth, and used to phase-shift-key an IF carrier with a balanced modulator. This modulated signal is then injected into the transponder where it is up-converted to RF, amplified and transmitted along with repeated uplink signals in the spacecraft downlink. Any new AO-40 telemetry format must therefore use the existing differential encoder, Manchester encoder, filter and 400 bps BPSK modulator without change.
However, many elements of the existing telemetry format are implemented in software and can be changed or removed. These include the sync vector, the CRC and the exact length of a frame.
Rationale: Cutting the user payload in half from the previous uncoded format creates room for coding overhead while keeping approximately the same frame duration as the old format: 13 s for the new vs 10.36 s for the present. When the 2.6 s of idle inter-frame padding in the old format (but not the new) is considered, the two frame formats are nearly the same length.
The most common telemetry frame (A-frame) in current use consists of two halves: 256 bytes of relatively static ASCII text followed by 256 bytes of binary-encoded telemetry data. A 256-byte frame size still allows these two blocks of information to be sent separately, possibly at different intervals.
The (160,128) Reed-Solomon code is shortened from the standard (255,223) Reed-Solomon code specified by the CCSDS. (See CCSDS 101.0-B-5, section 3). The shortening is accomplished by zero-padding the first 95 symbols of the data field of each Reed-Solomon codeword.
The field generator for GF(256) as specified by CCSDS is the polynomial
F(x) = x^8 + x^7 + x^2 + x + 1
The Reed-Solomon code generator polynomial as specified by CCSDS is
g(x) = Product(j=112,j<=143,(x-α^11j))i.e., the repeated product of the 32 terms (x-α^11j) for values of j from 112 to 143 inclusive. The factor α^11 is the primitive root specified by CCSDS.
This is another way of saying that the roots of the code generator polynomial are α^11j for values of j from 112 to 143 inclusive.
Multiplied out, the 33 coefficients of the generator polynomial g(x) are expressed as the following powers of α:
G0 = G32 = 0 G1 = G31 = 249 G2 = G30 = 59 G3 = G29 = 66 G4 = G28 = 4 G5 = G27 = 43 G6 = G26 = 126 G7 = G25 = 251 G8 = G24 = 97 G9 = G23 = 30 G10 = G22 = 3 G11 = G21 = 213 G12 = G20 = 50 G13 = G19 = 66 G14 = G18 = 170 G15 = G17 = 5 G16 = 24where g(x) = G0 + G1*x + G2*(x^2) + G3*(x^3) + ... + G32*(x^32)
Note that while the CCSDS standard Reed-Solomon code specifies the use of Berlekamp's "dual basis" for each symbol, here the conventional polynomial basis is used.
Rationale: Large Reed-Solomon block codes are easy to generate and provide excellent burst-error detection and correcting ability. This particular code can correct up to 16 byte errors in each codeword. Reed-Solomon codes are widely used in conjunction with Viterbi-decoded convolutional codes (discussed below) to "clean up" the errors made by the Viterbi decoder. Because of the nonlinear nature of Viterbi decoding, these errors occur in bursts even when the channel errors are random, as with Gaussian noise. In this application the burst error correction capability provides a second defense against channel fading, although this is the primary responsibility of the block interleaver discussed below.
Interleaving the user data in even/odd fashion into the two RS codewords allows the parity bytes in the two RS codewords to be computed "on the fly" as each data byte is presented, avoiding the need to buffer the entire user data frame in the encoder.
Shortening the code allows the data portion of the codeword to be set to 128, one half the desired user payload of 256 bytes. Two codewords are thus necessary. To handle 256 bytes of user data in a single Reed-Solomon codeword, a code over GF(512) could have been chosen, but this would imply 9-bit symbols that would be clumsy to handle on byte-oriented CPUs.
Shortening the code by padding with zeros at the start makes it easy to implement an encoder that simply "skips over" the padded data, spending CPU cycles only on actual user data. The syndrome computation in the decoder can be similarly optimized.
CCSDS chose this particular field generator and code generator polynomial and specified the dual basis representation to minimize the gate complexity of the Galois field multipliers in a custom hardware encoder.
Because the coefficients of the generator polynomial are palindromic, there are only half as many distinct coefficients as there are terms in the polynomial. This halves the number of distinct Galois multiplications that must be performed. Furthermore, two pairs are the same (G3 = G29 = G13 = G19 = 4), and another pair (G0 and G32) are 0, implying multiplication by unity.
The choice of field generator has no effect on the complexity of a software implementation that uses log/anti-log lookup tables and ordinary addition to perform Galois field multiplication, while the palindromic code generator polynomial does potentially halve the required number of distinct Galois field multiplications per data byte. So there is no particular reason not to use this aspect of the CCSDS standard RS code.
However, the "dual basis" representation actually complicates a software RS codec. And since we are implementing this code in software the dual-basis representation is not used.
h(x) = x^8 + x^7 + x^5 + x^3 + 1This PN sequence starts with 8 1's, i.e., the shift register is initialized to all 1's, and repeats after 255 bits. The first bit of the sequence is XORed with the high order bit of the first data byte, the 8th bit of the PN sequence covers the high order bit of the second data byte (the first in the second Reed-Solomon codeword), and so on. The PN sequence covers the Reed-Solomon parity bytes as well as the user data. (See CCSDS 101.0-B-5 section 6.5.)
Rationale: Experimentation showed that DBPSK demodulator performance was significantly worse on an all-0's data frame due to impaired symbol time tracking caused by insufficient transition density in the encoded and interleaved data stream, aggravated by the inherent ambiguities of Manchester encoding in this situation. I first tried to overcome this by inverting the output of every other symbol from the convolutional encoder as called for by the CCSDS spec, taking care that these became channel reversals after block interleaving, but this still proved insufficient. Covering the Reed-Solomon codewords with a PN sequence introduces a sufficient transition density to overcome this problem.
In the event that the user data exactly matches the PN sequence bits covering the data, the scrambled Reed-Solomon parity symbols would still be non-zero. Even though such a frame is clearly unlikely, it would still contain a greater number of channel symbol transitions than would an unscrambled frame consisting of all zeros.
The output of the G2 taps is inverted, as specified by the CCSDS. See CCSDS 101.0-B-5 section 2 for encoder block diagrams and connection vectors.
Each 8-bit Reed-Solomon symbol is sent into the convolutional encoder in "big endian" order, i.e., most significant bit first. The first 8 bits into the convolutional encoder come from the first data byte of the "even" Reed-Solomon codeword. The next 8 bits come from the first byte of the "odd" Reed-Solomon codeword, the third 8 bits come from the second data byte of the "even" RS codeword, and so on. After all the data bytes have been sent, the RS parity bytes are sent in identical fashion. When all parity bytes have been encoded, six zero bits are encoded (without scrambling) to form a "tail" for the convolutional encoder, returning the encoder shift register to the all-0's state.
Rationale: The use of byte interleaving between concatenated Reed-Solomon and convolutional codes is a well established technique. This helps distribute individual error bursts from the Viterbi decoder across the two Reed-Solomon codewords to reduce the chances of either having an uncorrectable number of byte errors (more than 16). The choice of interleaving pattern, along with the ordering of the user data into the Reed-Solomon codewords, allows the concatenated RS encoder/byte interleaver/convolutional encoder to process user data one byte at a time, without having to explicitly store the user data. Aside from a few variables, the only internal encoder state required are the two 32-byte Reed-Solomon shift registers and the 5200-bit (650 byte) output interleaver.
The choice of bit order is arbitrary, so there was no particular reason to depart from CCSDS practice.
This particular convolutional code has a long and distinguished history; it exists in many hardware and software implementations. My own Viterbi decoders for the Intel MMX/SSE/SSE2 instruction sets achieve speeds well in excess of what is required here (about 9 megabits/sec on a 1 GHz Pentium-III with SSE).
There have been several conventions for the ordering and polarity of the two encoded symbols. We follow the CCSDS standard, including the inversion of the second symbol to help guarantee a minimum channel bit transition density. This isn't really necessary since we also have a scrambler, but nor does it hurt.
Encoding this code is extremely simple, and it works well on deep space channels with reasonable Viterbi decoder complexity (which is exponential with constraint length). However, advances in technology now make longer constraint codes entirely practical, and such codes do provide greater coding gain on the non-fading Gaussian noise channel. However, my references claim that these increased gains are not generally realized on fading channels. There is room in the encoded frame to go to a r=1/2 k=9 code, which I have already implemented, and it should be tried experimentally at some point.
s(x) = x^7 + x^3 + 1The 7-bit shift register is set to all 1's at the start of the frame; the entire sequence is:
The sync bits are the same in each frame and need be written into the interleaver only once if they are not disturbed.
When filled, the block interleaver contains 65 sync vector bits and 5132 symbols from the convolutional encoder. These 5132 symbols in turn come from the 2048 bits (256 bytes) of user data, the 512 bits of Reed-Solomon parity, and the 6 tail bits, or 2566 in all, times two for the rate 1/2 convolutional code:
(2048 + 512 + 6) * 2 = 5132 encoder output symbolsThe 5132 symbols from the convolutional encoder plus the 65 sync bits make 5197 bits in all, leaving the last 3 bits unused in the block interleaver. This space is available for the additional tail bits of a longer constraint length convolutional code, if desired.
Once the block interleaver has been filled with sync and encoded data, it is read out by columns and sent to the differential encoder, Manchester encoder, filter and BPSK modulator. The first symbol transmitted is therefore the first bit of the sync vector. This is followed by the first symbol from the convolutional encoder, then the 66th and 131st, and so on until all 80 rows of the first column have been sent. Then the second bit of the sync vector is transmitted followed by the 2nd, 67th and 132nd symbols from the convolutional encoder, and so on.
Rationale: Here we add something to CCSDS practice; this block interleaver and sync vector is wholly new for the Phase 3 format. This is because the CCSDS standards are designed for non-fading Gaussian noise channels, while our primary concern is spin fading. Because this fading can be quite slow, we want to interleave over the longest possible time span, i.e., one frame time. A block interleaver is the classic way to do this when the data is packetized, as it is here.
This leaves the choice of exact interleaver size and dimensions. The number 5200 is large enough to accommodate all the encoded bits plus the sync vector with only 3 bits left over, and it factors into a roughly square (65 x 80) array.
The length of the sync vector was chosen to provide reasonable energy for reliable detection in the presence of noise and fading without consuming too much overhead. Since the FEC is strong enough to operate at an Eb/No of 3 dB (on a non-fading channel, with coherent detection), we want the sync vector to have enough energy to permit reliable detection under these conditions. The overall code rate is (256/320) * (1/2) = 0.4, so the Es/No is 10log10(0.4) = -4 dB relative to the Eb/No. So an Eb/No of 3 dB is an Es/No of -1 dB.
A correlator looking for a 65-bit sync vector will have an output signal-to-noise ratio 10log10(65) = 16 dB above the input SNR (Es/No). So for Es/No = -1dB, the correlator output SNR will be +15 dB. And 15 dB is the usual rule of thumb for reasonably reliable detection.
Interleaving the sync vector into the encoded data is an essential anti-fading measure. If only the encoded data were interleaved, the sync vector would remain as an Achilles heel; a fade that removed the sync vector would kill the entire frame, as it does now in the present uncoded format.
While lower code rates provide asymptotically greater coding gain with coherent modulation, this is not the case for noncoherent modulation because of a "threshold" effect much like the one in FM (which is also noncoherent). Below the threshold, demodulator output SNR falls much more quickly than the input SNR, and this overpowers the additional coding gain. There is a minimum Es/No below which the system simply won't work very well, regardless of coding, placing a lower practical bound on the code rate and implying the existence of an "optimum" code rate. The optimum rate is different for fading and nonfading channels; it is around 0.5 (1/2) for a nonfading channel and about 0.2 for a Rayleigh fading channel (our "spin faded" channel is probably somewhere in between.) Fortunately the optimum is fairly broad, so rate 0.4 works reasonably well in both cases.
Another lower bound on code rate comes from the increased difficulty of carrier and symbol time tracking at the lower Es/No ratios implied by these codes. The trend in signal design for terrestrial fading channels (e.g., terrestrial digital cellular) is to use spread spectrum to provide time tracking and other benefits and to add coherent carrier references ("pilots") to assist in carrier phase tracking without the "squaring losses" associated with carrier recovery from suppressed carrier signals, e.g., with Costas or squaring loops. However, this would require hardware that is unavailable on the AO-40 IHU.
Some readers may ask: why not turbo codes? Why use FEC technology that has already been around for 25 years?
The answer is simple: while turbo codes do outperform by 1-2 dB the classic concatenated codes proposed here, they were only discovered in the early 1990s and as such are surrounded by a minefield of patents that will not begin to expire for another decade. The Viterbi decoder was invented in the late 1960s, and the concatenated code used here has been around, almost in the present form, since before Voyager 1 and 2 were launched in 1977, so all patents on this technology have long since expired. Block interleaving and noncoherent DBPSK demodulation are likewise established techniques that have been around for decades.
Technology unencumbered by intellectual property rights seems a better match for an organization like AMSAT with limited financial resources that depends on volunteer labor and emphasizes the open and unrestricted dissemination of its technology to further its educational and recreational goals.