---------- Original Message ---------------------------------- From: KV9U <[EMAIL PROTECTED]> Reply-To: [email protected] Date: Mon, 22 Jan 2007 14:47:12 -0600
>Some of us did try Chip modes when Nino first came out with them, but >they did not seem to perform as well as existing modes. So we coincide...it is a pity...but using the ionosphere, you have to play by its rules. >I really implore to our treasured programmers to see if they can come up >with some modes that can compete with Pactor modes. Especially some ARQ >modes that can work on MS OS. > >We know from Pactor 2, that a raised cosine shaped pulse is likely a >very good basic waveform. That is for saving bandwidth, mostly. It might allow better decoding, as well. >Then for the most robust mode, a two tone >DBPSK modulation is used and as the conditions improve, the modulation >changes to DQPSK and then with further improvements to 8-DPSK and even >16-DPSK for maximum throughput when conditions are very good. This is >what enables Pactor 2 to send about 700 bits per second at the peak >speed and do it in only a 500 Hz wide span. The steepest loss of performance in PSK constellations occurs from QPSK onwards as the distances between the constellation points diminish. It is well treated in "Communications Systems" by Carlson et al. >We know this can be done at the higher speeds under good conditions with >sound card modes since SCAMP was even faster than P2, although a much >wider signal. The problem with SCAMP was that it had no fallback position. So, fallback is important on HF. >Pactor 3 is runs an occupied bandwidth of about 2.4 kHz, but raw speed >is over 2700 bps. Instead of 2 tones, P3 uses up to 18, separated by 120 >Hz and modulated at 100 baud DBPSK or DQPSK. > >SCS has some fairly detailed data on Pactor 3 at: > >http://www.scs-ptc.com/download/PACTOR-III-Protocol.pdf > >I wish someone could explain why we can not have a sound card mode that >is roughly the same as Pactor 2 at least. Even if there was no ARQ at first. I don't know if the least complex of it all is ARQ...most likely, the rest is harder to implement. >And how different is Pactor 3, than what the SSTV hams are using >everyday? Aren't they using OFDM with QAM? If you recall what Tom Rink >said back in 1995 on the TAPR HF SIG: That it is not adaptive as pactor is. >"As mentioned in the introduction, PACTOR-II uses a two-tone DPSK modulation >system. Due to the raised cosine pulse shaping, the maximum required >bandwidth is only around 450 Hz at minus 50 dB. ASK, which was also tested in >the >early stage, provided poorer results in weak conditions compared with a higher >DPSK modulation, as different amplitude levels are more difficult to >distinguish in noisy channels than more phase levels. >Additionally, ASK increases the Crest >Factor of the signal. For these reasons, it is not used in the final >PACTOR-II protocol. Basic information on these items can also be found in the >first part of this series." > >Although not ASK, doesn't QAM employ amplitude changes as part of the >modulation scheme? Yes. A key requirement is having the highest distance between constellation points to have an edge against the noise (or QRM). That's why, in DRM, the FAC uses 4QAM, as it allows to send the reduced but very important info it conveys. But the MSC must use 64QAM, because the amount of data to be sent does not allow otherwise in the least bandwidth. >What happens if you use a multitone DPSK? It seems to a non-engineering >person like myself, that a lot of what P2 and P3 are made up of are >really a series of PSK100 or PSK200 tones (carriers). >Isn't Q15X25 a similar modulation scheme? It even runs at 83.33 baud >rather than a minimum of 100 baud such as P2. > >Why did it not work as well as P modes? > >Or is it because it has no coding such as Reed-Solomon block coding or >Viterbi convolutional coding? Certainly...all those tricks add up, and most likely, in a non proportional way...I cannot assure it by heart, but is very likely. One of the gains of the code used in pactor modes is using convolutional encoding with Viterbi decoding. The Viterbi decoder, knowing the history of what has been sent, as the convolutionally coded stream depends on what has been sent previously, makes a soft decode of what is the most likely symbol transmitted. RS coding, after deinterleaving, on the other side, may allow to recover erors WITHOUT retransmission, which may save more "bandwidth" than what is wasted on the FEC overhead. Also, P2 and P3 avoid the edges of the channel to have the least amplitude and delay differences between carriers. That's why a "reduced" version of Q15X25 is being more succesful in holding the link. >73, > >Rick, KV9U 73, Jose, CO2JA ______________ ______________ ______________ ______________ Correo enviado por ElectroMAIL. Facultad Eléctrica. CUJAE Dominio: electrica.cujae.edu.cu
