Re: [Fwd: Re: Re: Please check these calculations]
I think that we said the data rate was 30-50 bps in the meeting. The bandwidth will be determined by the rate of the error-correcting code. Phil needs to determine that. More coding makes for a wider signal which reduces the effect of spin modulation. However, it also means more processing in the SDX.
The signal bandwidth has no effect on spin modulation as the fades are pretty much flat over frequency. So there's no frequency diversity and no benefit to spectral spreading here. You need either space or time diversity to get you through a spin fade, and that means either 1) another pair of physically separate TX and RX antennas, and/or 2) interleaving in combination with FEC coding.
As for optimal code rates, it depends on the modulation and especially the type of demodulator and the channel. On a fully coherent channel with fixed power you get steadily more capacity as bandwidth goes to infinity. Most of the gain comes with relatively little excess bandwidth and then you reach diminishing returns. Between 1 bit/sec/Hz and zero bps/Hz (i.e., infinite bandwidth) the increase is only 1.6 dB.
Fully coherent means the receiver has perfect knowledge of carrier phase. This doesn't come out of thin air. You can derive carrier phase from the data itself, or you can dedicate some fraction of the transmitter power to a reference pilot (carrier).
Deriving carrier phase from data is a non-linear squaring process. It works well at high SNR, but breaks down at low SNRs. That means you can't use it with strong, low-rate FEC codes, especially if the carrier phase is changing rapidly and unpredictably as it is on a fading channel. You have to put some power into a reference pilot that can be tracked linearly, and that takes power away from the data.
The extreme form of deriving carrier phase from the data is non coherent demodulation, e.g., the differentially coherent BPSK I used on AO-40. It uses each symbol as the phase reference for the next. This responds very quickly to the carrier phase changes that occur during a spin null, but the squaring loss is very high.
The non coherent demodulator essentially has a threshold effect just like a FM demodulator. Below some SNR, the output SNR falls much more rapidly than the input SNR. Because a very low rate code puts very little energy into each channel symbol, you fall below the demodulator threshold and you lose more than you gain from the code.
So on a non coherent channel there's an optimum code rate. It depends on the order of the modulation (binary, M-ary, etc) and the nature of the fading, if any, but rate 1/3 to rate 1/2 is the usual rule of thumb. My FEC scheme for AO-40, which I designed to be as strong as possible against spin fading, was rate 0.4, i.e., rate 1/2 convolutional * (160,128) Reed Solomon, i.e. 0.5 * (128/160) = 0.5 * 0.8 = 0.4.
In my AO-40 system I figure I lost about 3-4 dB because of my use of non coherent BPSK, but that was the price I paid to deal with deep, rapid spin fading. It's a trade off, so I really need to know whether fading is going to be a serious problem here or not. If it will be some of the time but not others, then perhaps we need two modes.
In practice, nobody seems to go lower than rate 1/6 on deep space channels even when they don't fade and coherent demodulation is used. The Cassini convolutional code is rate 1/6, and the lowest CCSDS standard Turbo code is also rate 1/6. I wouldn't go any lower than this.
--Phil
I was thinking of phase modulation due to the spinning off-center antenna. A wider bandwidth would reduce the effects. The idea has been to have a 90-120 degree beamwidth antenna at 70 cm so that it works well over 75% of the orbit. The last antenna proposal that I saw was a single patch so there shouldn't be much fading on the class 1 uplinks due to the antenna pattern.
Since the access method for class 1 hasn't been decided, another approach to widening the bandwidth would be to use TDMA on one or more uplinks and a higher data rate. Ground station costs proabably don't change much up to 10 W PEP. It would be nice to keep class 1 uplink and downlink bandwidths below 2.5 kHz so that a sound-card modem could be written that works with existing transceivers.
73,
John KD6OZH
----- Original Message ----- From: "Phil Karn" karn@ka9q.net To: "Jim Sanford" wb4gcs@amsat.org Cc: "eagle list" eagle@amsat.org; "John B. Stephensen" kd6ozh@comcast.net Sent: Sunday, November 05, 2006 06:52 UTC Subject: Re: [Fwd: Re: [eagle] Re: Please check these calculations]
I think that we said the data rate was 30-50 bps in the meeting. The bandwidth will be determined by the rate of the error-correcting code. Phil needs to determine that. More coding makes for a wider signal which reduces the effect of spin modulation. However, it also means more processing in the SDX.
The signal bandwidth has no effect on spin modulation as the fades are pretty much flat over frequency. So there's no frequency diversity and no benefit to spectral spreading here. You need either space or time diversity to get you through a spin fade, and that means either 1) another pair of physically separate TX and RX antennas, and/or 2) interleaving in combination with FEC coding.
As for optimal code rates, it depends on the modulation and especially the type of demodulator and the channel. On a fully coherent channel with fixed power you get steadily more capacity as bandwidth goes to infinity. Most of the gain comes with relatively little excess bandwidth and then you reach diminishing returns. Between 1 bit/sec/Hz and zero bps/Hz (i.e., infinite bandwidth) the increase is only 1.6 dB.
Fully coherent means the receiver has perfect knowledge of carrier phase. This doesn't come out of thin air. You can derive carrier phase from the data itself, or you can dedicate some fraction of the transmitter power to a reference pilot (carrier).
Deriving carrier phase from data is a non-linear squaring process. It works well at high SNR, but breaks down at low SNRs. That means you can't use it with strong, low-rate FEC codes, especially if the carrier phase is changing rapidly and unpredictably as it is on a fading channel. You have to put some power into a reference pilot that can be tracked linearly, and that takes power away from the data.
The extreme form of deriving carrier phase from the data is non coherent demodulation, e.g., the differentially coherent BPSK I used on AO-40. It uses each symbol as the phase reference for the next. This responds very quickly to the carrier phase changes that occur during a spin null, but the squaring loss is very high.
The non coherent demodulator essentially has a threshold effect just like a FM demodulator. Below some SNR, the output SNR falls much more rapidly than the input SNR. Because a very low rate code puts very little energy into each channel symbol, you fall below the demodulator threshold and you lose more than you gain from the code.
So on a non coherent channel there's an optimum code rate. It depends on the order of the modulation (binary, M-ary, etc) and the nature of the fading, if any, but rate 1/3 to rate 1/2 is the usual rule of thumb. My FEC scheme for AO-40, which I designed to be as strong as possible against spin fading, was rate 0.4, i.e., rate 1/2 convolutional * (160,128) Reed Solomon, i.e. 0.5 * (128/160) = 0.5 * 0.8 = 0.4.
In my AO-40 system I figure I lost about 3-4 dB because of my use of non coherent BPSK, but that was the price I paid to deal with deep, rapid spin fading. It's a trade off, so I really need to know whether fading is going to be a serious problem here or not. If it will be some of the time but not others, then perhaps we need two modes.
In practice, nobody seems to go lower than rate 1/6 on deep space channels even when they don't fade and coherent demodulation is used. The Cassini convolutional code is rate 1/6, and the lowest CCSDS standard Turbo code is also rate 1/6. I wouldn't go any lower than this.
--Phil
I am loathe to see a 10W PEP transmitter be needed to mitigate poor phase noise in the RX LO's or to mitigate spin modulation induced fades. If we have a spin rate of 6-10 RPM and we use low power and FEC providing time diversity by encoding in blocks, surely the better solution is improve the phase noise performance of the LO's in the receiver and require less peak power on the uplink?
John B. Stephensen wrote:
I was thinking of phase modulation due to the spinning off-center antenna. A wider bandwidth would reduce the effects. The idea has been to have a 90-120 degree beamwidth antenna at 70 cm so that it works well over 75% of the orbit. The last antenna proposal that I saw was a single patch so there shouldn't be much fading on the class 1 uplinks due to the antenna pattern.
Since the access method for class 1 hasn't been decided, another approach to widening the bandwidth would be to use TDMA on one or more uplinks and a higher data rate. Ground station costs proabably don't change much up to 10 W PEP. It would be nice to keep class 1 uplink and downlink bandwidths below 2.5 kHz so that a sound-card modem could be written that works with existing transceivers.
73,
John KD6OZH
I don't see fading as a big issue. With the 70 cm atenna mounted off-center, I was worried that induced phase modulation may be an issue. If the antenna is 30 cm from the spin axis, the path length will vary by cos(offpointing angle) * 60 cm. In the worst case (MA 32 and MA 224), the arrival angle will vary between 30 and 60 degrees, depending on the ground station's location on Earth. The distance then varies by 0.43-0.74 wavelengths, resulting in phase modulation at the spin rate. If the maximum spin rate is 0.17 Hz, and the carrier tracking loop can handle it, it's no longer an issue.
The remaining issue is then interference from adjacent uplink channels which will depend on uplink channel spacing.
73,
John KD6OZH
----- Original Message ----- From: "Robert McGwier" rwmcgwier@comcast.net To: "John B. Stephensen" kd6ozh@comcast.net Cc: "Phil Karn" karn@ka9q.net; "Jim Sanford" wb4gcs@amsat.org; "eagle list" eagle@amsat.org Sent: Sunday, November 05, 2006 18:15 UTC Subject: Re: [eagle] Re: [Fwd: Re: Re: Please check these calculations]
I am loathe to see a 10W PEP transmitter be needed to mitigate poor phase noise in the RX LO's or to mitigate spin modulation induced fades. If we have a spin rate of 6-10 RPM and we use low power and FEC providing time diversity by encoding in blocks, surely the better solution is improve the phase noise performance of the LO's in the receiver and require less peak power on the uplink?
John B. Stephensen wrote:
I was thinking of phase modulation due to the spinning off-center antenna. A wider bandwidth would reduce the effects. The idea has been to have a 90-120 degree beamwidth antenna at 70 cm so that it works well over 75% of the orbit. The last antenna proposal that I saw was a single patch so there shouldn't be much fading on the class 1 uplinks due to the antenna pattern.
Since the access method for class 1 hasn't been decided, another approach to widening the bandwidth would be to use TDMA on one or more uplinks and a higher data rate. Ground station costs proabably don't change much up to 10 W PEP. It would be nice to keep class 1 uplink and downlink bandwidths below 2.5 kHz so that a sound-card modem could be written that works with existing transceivers.
73,
John KD6OZH
-- AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair "You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat." - Einstein
participants (3)
-
John B. Stephensen -
Phil Karn -
Robert McGwier