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A high speed data modem for digital cellular radio
Abstract
This paper describes an adaptive rate modem ideally suited for
fading mobile radio channels. The modem adapts its information rate to
the time-varying channel conditions by changing the code rate of a
special trellis code. The modem exploits the unused capacity in the
cellular radio channel and can quadruple the mean data rate of wireless
modems. Because trellis coded modulation is used, the bandwidth and
symbol rate are constant. A novel estimator and rate changing hardware
ensure the modem can track and adjust to rapid multipath fading. The
modem can be made backward compatible with existing TDMA digital
cellular standards like IS-136 and GSM
The symbol error probability (SEP) of a digital signal processor
implemented π/M M-ary differentially coherent phase-shift keying
(MDPSK) modem, using quasi-analytical (QA) simulation techniques, is the
subject of this investigation. We study M values of 4, 8, 16, 32, and 64
for use in a fast adaptive data rate mobile radio communication system.
The proposed modulation scheme π/M-MDPSK, is a generalization of
π/4 differentially coherent quadrature phase-shift keying, which is
used in the North American time-division multiple-access wireless
standard. A QA simulation approach is developed so that real system
impairments can be studied without having to resort to lengthy Monte
Carlo simulation. In particular, it is found that implementation losses,
most notable at M=32 and 64, which result from practical transmit and
receive filtering, and symbol timing error, can be largely overcome by
using a fixed equalization filter and increased accuracy of symbol
timing recovery. We focus on an additive white Gaussian noise channel
since, in a fast adaptive rate system, the Doppler spread mobile channel
is approximately Gaussian on short time intervals. However, the
quasi-analytic technique developed here is directly extendable to a fast
Rayleigh fading channel. Specifically, we find that for M=64, the
inclusion of a fixed seven-tap zero forcing equalizer at the matched
filter output, decreases the SEP degradation at Pe=10<sup>-4
</sup> from 7.5 dB down to 0.24 dB. The symbol timing error using a
finite-precision interpolator is held to within 1/64 symbol period
A modulation system is proposed that continuously adjusts its data rate in response to signal strength variations in a fading channel. The optimum variation of data rate with channel conditions is determined, and includes the effects of feedback channel time delay, the interval between rate changes, and restriction of the number of allowable rates. Application of these results to a full duplex situation allows determination of the optimum fraction of the data stream to be devoted to service information. Comparison of this scheme with diversity transmission on the basis of error probability and bandwidth utilization reveals a reduction on the order of 14 dB in transmitter power for a typical duplex link.
Because voice cellular systems must maintain low blocking
probabilities, almost 20% of cellular channel capacity is unused, even
during the busy hour. In an attempt to capture some of this unused
capacity, CDPD hops between idle channels, working independently of the
voice base station and the cellular switch. On a 54 channel base station
with 2% blocking probability, a CDPD data channel can achieve almost 98%
availability without increasing the blocking probability to voice users.
For the popular “least idle” voice channel assignment
algorithm, the hopping channel stream will dwell on a channel for an
average of 14 seconds and will hop about 290 times per busy hour. A
hopping CDPD channel stream adds average co-channel interference that
would not be present otherwise. For a typical base station, the
co-channel interference introduced by a single hopping channel stream is
only 0.1 dB and results in a loss of geographical coverage of about
0.3%. Because the CDPD base station must monitor the frequency spectrum
in real time, the demands on the radio frequency front-end and the
signal processing firmware are severe. A wideband receiver offers
performance improvements over a narrowband receiver
Theoretical study of on/off transmission over Gaussian multiplicative circuits
- P A Bello
- W M Cowan