Abstract
Some radio channels are fast fading in the sense that
they are randomly timevariant and the Doppler spread is
in the order of or larger than the symbol rate of the
digital signal. Such channels can be found in
landmobile, aircraft, and satellite communications,
especially when the symbol rate is small, the velocity of
the mobile station is large, and the carrier frequency is
large. With small symbol rates, a Doppler barrier exists,
below which the complexity of the optimal receiver is
increased. The Doppler barrier is the timefrequency dual
of the ISI barrier with large symbol rates. In the
thesis, the history of digital communications in fading
multipath channels is first summarized. The ML sequence
detector, consisting of a correlator and an estimator, is
derived in a WSSUS fast Rayleigh fading channel. In the
system model, each symbol is divided into several short
segments and ideal symbol synchronization is assumed.
Finally, suboptimal receivers based on linear prediction
and smoothing are analyzed in a fast fading channel. The
authors main contribution is the extension of Kams ideas
to the segmented waveform concept, so that the channel
may be considered timeinvariant over a segment duration.
The system model is very useful, since it allows for a
very tidy theoretical development concerning receiver
structures; this bridges the gap between the classical
oneshot receivers and the modern sequence estimation
approach. Of almost equal importance is that the results
of the thesis also bridge the gap between receivers for
slow and for fast fading channel models. In a fast fading
frequencynonselective channel, an unexpected diversity
effect was observed. The performance of the receiver is
improved since the fading in different parts of a symbol
is partially uncorrelated. In addition, the RAKE
principle is extended to a fast fading channel, and this
gives an additional improvement in performance. To
simplify the receiver, data modulation must somehow be
avoided or removed before the estimator, and either a
predictor or a smoother can be used in the estimator.
Performance analysis includes MMSE and bit error
probability analysis. Many numerical results are given,
and their validity is demonstrated with Monte Carlo
simulations. The results can be applied in both
conventional narrowband and spreadspectrum (CDMA)
systems. An extensive bibliography consisting of some 250
references is included in the thesis.
Original language  English 

Qualification  Doctor Degree 
Awarding Institution 

Award date  19 Nov 2026 
Place of Publication  Espoo 
Publisher  
Print ISBNs  9513847918 
Publication status  Published  1995 
MoE publication type  G4 Doctoral dissertation (monograph) 
Keywords
 electronics
 digital communications
 radio communications
 fading
 receivers
 aircraft communications
 satellite communications
 land mobile communications
 transmitters
 design
 models
 diversity reception
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Cite this
Mämmelä, A. (1995). Diversity receivers in a fast fading multipath channel: Dissertation. VTT Technical Research Centre of Finland.