Detection of digital signals transmitted over a known time-varying channel

Detection process of a 9600 bit/s serial modem for HF radio links

“…Clearly, d k is obtained from \U k+f \ 2 by setting to zero all {x,} for / > k, while still considering all {r,} up to and including r k+f . The detector now selects from the vectors {P k } a set of m vectors {Q k } according to some appropriate criterion and then stores the selected vectors together with their costs [2,25]. A vector Q k is, of course, here given by the last n components of the corresponding vector P k and it has the same cost d k .…”

“…The model assumes that the transmitted HF radio signal travels via two independent Rayleigh fading sky waves, each introducing the same average attenuation and the same frequency spread into the data signal. The relative delay in transmission for the two sky waves may be 1, 2 or 3 ms and the frequency spread introduced into the data signal may be 0.5, 1 or 2 Hz [1][2][3]. Extensive computersimulation tests have shown that, at a transmission rate of 2400 bit/s and at the higher signal/noise ratios, an adaptive serial modem using near-maximum-likelihood detection has a much better tolerance to additive white Gaussian noise than the corresponding conventional parallel modem [3].…”

“…Unfortunately there are difficulties in holding the adaptive noise-whitening matched filter correctly adjusted for the given time-varying channel, and these can perhaps best be avoided by replacing the adaptive filter with a conventional simple fixed lowpass filter. Fundamental changes in the method of operation of the detector are, however, now required to compensate for the absence of the adaptive filter [25], and the appropriate development of the resulting system has led to a basic detection process suitable for use over HF radio links, at a transmission rate of 2400 bit/s [2,3]. For correct detection to be achieved here the receiver only has to estimate the sampled impulse response of the channel, and this can be achieved more rapidly and more accurately than can the adjustment of the adaptive linear filter [1,26,27].…”

“…The detection process, used successfully at a transmission rate of 2400 bit/s with a four-level quadrature amplitude modulated (QAM) signal over the given HF radio link [1][2][3], has been tested at the much higher transmission rate of 9600 bit/s, with a 16-level QAM signal, but satisfactory operation is no longer obtained regardless of the adjustment of the system parameters. Tests by S.M.…”

“…The characteristics of the filters in the HF radio link are therefore taken as their typical values. The model of the HF radio link assumed here has a given constant value of the relative transmission delay for the two sky waves and a given constant value for the frequency spread introduced into the data signal by each sky wave, and it is fully described elsewhere [1][2][3]. The important parameters of the model are shown in Table 1.…”