279 research outputs found
A co-designed equalization, modulation, and coding scheme
The commercial impact and technical success of Trellis Coded Modulation seems to illustrate that, if Shannon's capacity is going to be neared, the modulation and coding of an analogue signal ought to be viewed as an integrated process. More recent work has focused on going beyond the gains obtained for Average White Gaussian Noise and has tried to combine the coding/modulation with adaptive equalization. The motive is to gain similar advances on less perfect or idealized channels
High speed data transmission over HF radio links
The thesis describes the results of research work on techniques
for high speed data transmission (2.4 kbit/s) over voice-band HF
radio channels. This work has been carried out using extensive computer
simulation of the various transmission techniques and the HF radio
channels.
Firstly, the characteristics of HF radio channels are discussed in
detail and an HF channel model, suitable for computer simulation, is
developed. The first of two techniques for high data rate transmission
over HF links is then introduced, namely, multi-channel (or parallel) DPSK
transmission. Parallel transmission is a well known technique in this
application but it has been studied and simulated, in order to compare its
performance with that of the second, more novel, transmission technique.
This is a single channel system employing 4 point QAM signalling at the
transmitter and maximum likelihood detection at the receiver. Initially,
the parallel system is compared with an idealised serial system
employing optimum Viterbi detection at the receiver with all other functions
of the serial function assumed perfect. However, having shown the vastly
superior performance of this serial system, a more practical serial modem
is gradually developed, with further performance comparisons at each
stage in this development. The final comparison is made with a very
practical form of serial modem in which all practical receiver functions are
simulated. Theseinclude a simpler, adaptive near maximum likelihood
detector, receiver filtering, channel estimator, carrier phase tracking,
timing synchronisation and automatic gain control.
Finally, the design and implementation of the serial modem is
studied and details of the complexity of a digital, processor-based,
realisation are given
An investigation into a DSP implementation of partial response signaling for 4800 bits per second full-duplex data communications over M.1020 telephone lines
Includes bibliographical references.This thesis investigates high-speed digital transmission over a conditioned, voice-grade telephone circuit (M.1020), using a technique known as partial response signaling, or PRS. In particular, the case where 4800 bps, full-duplex transmission is required in a CCI'PT V. 22 type format is investigated. The main v.22 criterion to be adhered to, is that frequency-division multiplexing (FDM) is to be used as the means of separating thetransmit and receive channels. The carrier frequencies should be 1200 Hz and 2400 Hz respectively. The investigation concerns the modulation and demodulation sections only
A study of multilevel partial response signalling for transmission in a basic supergroup bandwidth
Includes bibliographical references.The work in this thesis is primarily directed toward the design, construction and testing of an experimental multilevel partial response signalling baseband system. The system will find practical application in existing frequency division multiplexed-frequency modulated microwave links. The basic supergroup bandwidth of these links is 240 kHz. The design requires a transmission rate of 1.024 Mb/s in this bandwidth. Class-4 15 partial response signalling is the coding technique suitable to achieve this. A pilot tone scheme is used to facilitate symbol timing recovery at the demodulator. A sixth order Butterworth low pass filter approximates the ideal raised-cosine Nyquist channel. A theoretical discussion on impairments caused by deviation from this channel is given. Since the experimental system was non-ideal, it produced a degradation in the channel signal to noise ratio. This degradation, coupled with other factors, showed that further development was necessary for the system to be suitable for connection into an existing microwave link
Techniques of detection, estimation and coding for fading channels
The thesis describes techniques of detection, coding and estimation, for use in
high speed serial modems operating over fading channels such as HF radio and land mobile
radio links. The performance of the various systems that employ the above techniques are
obtained via computer simulation tests.
A review of the characteristics of HF radio channels is first presented, leading
to the development of an appropriate channel model which imposes Rayleigh fading on the
transmitted signal. Detection processes for a 4.8 kbit/s HF radio modem are then
discussed, the emphasis, here, being on variants of the maximum likelihood detector that is
implemented by the Viterbi algorithm. The performance of these detectors are compared
with that of a nonlinear equalizer operating under the same conditions, and the detector
which offers the best compromise between performance and complexity is chosen for
further tests.
Forward error correction, in the form of trellis coded modulation, is next
introduced. An appropriate 8-PSK coded modulation scheme is discussed, and its
operation over the above mentioned HF radio modem is evaluated. Performance
comparisons are made of the coded and uncoded systems.
Channel estimation techniques for fast fading channels akin to cellular land
mobile radio links, are next discussed. A suitable model for a fast fading channel is
developed, and some novel estimators are tested over this channel. Computer simulation
tests are also used to study the feasibility of the simultaneous transmission of two 4-level
QAM signals occupying the same frequency band, when each of these signals are
transmitted at 24 kbit/s over two independently fading channels, to a single receiver. A
novel combined detector/estimator is developed for this purpose.
Finally, the performance of the complete 4.8 kbit/s HF radio modem is
obtained, when all the functions of detection, estimation and prefiltering are present, where
the prefilter and associated processor use a recently developed technique for the adjustment
of its tap gains and for the estimation of the minimum phase sampled impulse response
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