No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
An advanced base station receiver concept for DECT
Abstract
The concept of a simple non-coherent equaliser receiver for DECT,
as a means to obtain robust performance in outdoor and large indoor
applications is discussed. This simple structure renders the receiver
insensitive to dispersion but cannot substitute diversity in flat fading
channels. The authors describe practical approaches leading to a high
performance base station receiver for DECT-type systems, based on the
above concept. It is shown that a significant portion of the DECT
preamble is not required by the present equaliser receiver, and that
this interval can be used to emulate ideal selection diversity (without
the need for two parallel receivers). In addition, the authors also
introduce an approach to further extend the receiver's frequency offset
tolerance. They therefore propose a novel integrated structure that
performs both equalisation and selection diversity in a single base
station receiver. It is shown through simulation that this receiver
provides robust radio performance
PCS and RLL (radio in the local loop) are applications which set new demands on the DECT receiver, due to the fact that the RMS delay spread often exceeds 200 ns for outdoor environments. For comparison a typical low-cost DECT receiver based on a limiter-discriminator detector is not able to cope with more than 100 ns delay spread. A well-known solution which increases the receiver resistance to time dispersion is antenna diversity. The optimum realizable selection diversity scheme is based on the status of the R-CRC (cyclic redundancy check) check and it requires two parallel receiver chains. The simpler switching diversity scheme, normally used in commercial equipment, requires only one receiver chain, but it suffers from a strong velocity dependency. In this paper a diversity scheme using a new signal quality measure is presented. The signal quality is extracted successively in the two antenna branches during the DECT preamble which makes the scheme velocity independent as no significant delay is introduced. With the new quality measure it is possible to closely emulate genuine R-CRC selection diversity by use of only one receiver chain. For an Eb /N0 of 25 dB, the maximum allowable RMS delay spread is increased from 180 ns for RSSI driven diversity to 220 ns with the new scheme. This is only 10 ns less than R-CRC selection diversity. For realistic hardware conditions, the performance is only degraded 10 ns which makes the new diversity scheme applicable for a practical implementation
Diversity measurements have been performed in microcellular environments where DECT typically might be implemented. The intention has been to investigate how well different implementations of antenna space diversity can combat the problems DECT experiences in multipath environments. Diversity with 2 antennas and selection/switching based on signal level and on delay spread has been investigated, and the effect on the delay spread statistics has been studied. Measurements have been performed in three different types of scenarios; both radio base station (RBS) and portable part (PP) indoors (indoor case), RBS outdoors and PP indoors (outdoor to indoor case) and both RBS and PP outdoors (outdoor case). For the indoor case the delay spread values were small. The most severe time dispersion was observed in the outdoor to indoor case, where delay spread values exceeded 100 ns in almost 50% of the cases. In the outdoor scenario delay spread values above 100 ns were observed for approximately 10% of the cases. It seemed that for both the outdoor, and outdoor to indoor cases, where the time dispersion may cause problems for DECT performance, the diversity algorithm should be based on some other selection/switching criterion than signal level, preferably BER or some other quality criterion. In the indoor environment, signal level as a selection/switching criterion should be sufficient. The work presented in this paper is part of an investigation of DECT performance in environments which DECT has not been originally designed for, to possibly extend the usage of DECT for RLL applications and as a public access system
An advanced DECT receiver concept that is based upon a
non-coherent differential detection with equalization, and includes
switching diversity with performance of true RSSI selection diversity,
has been previously described (Safavi et al., 1995). This paper deals
with a hardware testbed evaluation of the concept. The analysis includes
practical implementation aspects, performance results obtained from a
wideband channel simulator, and measurements conducted in a typical DECT
telepoint environment
The digital enhanced cordless telecommunications (DECT) is a
candidate for radio in the local loop applications. Telenor has
established a DECT field trial. DECT most often gives satisfactory
quality of service (QoS), but in some areas the system shows poor
performance despite adequate signal strengths. To investigate the
influence of multipath propagation on DECT performance, we have
conducted combined wideband propagation measurements and DECT
performance measurements in areas where poor DECT QoS has been
experienced. Some simple means to cope with multipath propagation would
considerably improve DECT performance in an outdoor environment
Previously, we introduced, the concept of an advanced DECT-type
receiver based on a hybrid combination of equalisation and diversity.
Such a structure is capable of coping with most hostile microcell
outdoor environments. This paper considers other possible solutions
which attempt to achieve a similar performance level whilst allowing a
completely nonlinear front-end. We first introduce an improved clock
recovery technique applicable to standard DECT receivers. When using a
differential phase detector, this technique can extend the maximum delay
spread range to about 140-160 ns from 80-100 ns for a standard set-up
with a discriminator receiver. In addition, a new diversity scheme based
on emulation of a quality-based selection diversity in a single receiver
is introduced. It is shown that the performance in dispersive channels
is significantly improved with respect to RSSI schemes (up to 270 ns).
Flat-fading gains of over 10 dB are also observed. Finally the
combination of the two techniques introduced produces excellent
performance (≈350 ns delay spread range) closely approximating an
ideal CRC-driven selection diversity (speed independent), but using only
a single receiver chain. This structure could be a strong candidate for
practical implementation, particularly in handsets for DECT PCS
applications
The DECT system is targeted at a large range of different
applications, which have widely differing radio conditions. These can
vary from short range line of sight indoor systems to relatively hostile
outdoor environments. The authors present and discuss a range of
matching options for the radio receiver, with particular emphasis on
possible structures for space diversity operation and various options
for equalisation. Illustrations of performance obtained by both software
simulation and hardware testing are included. It is concluded that the
DECT standard allows manufacturers to make technology choices suitable
to the applications of interest. This includes even demanding outdoor
environments where advanced processing can be used to obtain robust
performance with relatively low complexity
For pt. I see ibid., vol.5, no.5, p.1040-54 (2002). In Part I, we introduced a robust noncoherent maximum likelihood sequence estimation (MLSE) equalizer receiver structure applicable to radio channels with impulse responses spanning less than two bit intervals. The distinct characteristic of this receiver was its robustness to carrier frequency offsets. However, due to the differential operation prior to the MLSE equalization, we observed some performance degradation, resulting in a delay spread range significantly smaller than an equivalent coherent MLSE equalizer. We propose techniques to significantly improve the performance of the noncoherent equalizer by using a second, complementary differential processor. The performance assessment of the new receiver is presented. In particular, using the Digital Enhanced Cordless Telecommunications system as an example, it is shown that the modified receiver's dispersive channel operation range is almost twice as much as the basic structure, with a multipath diversity gain comparable to a coherent equalizer receiver. On the other hand, unlike coherent structures, it retains low sensitivity to both frequency offsets and modulation index drifts. Finally, we introduce an approach to further extend the receiver's frequency offset tolerance to that of a standard differential detector receiver.
In time-dispersive mobile radio channels, determination of the
proper sampling instants of the receiver is one of the most difficult
problems. A promising way is to oversample the received signal while a
training sequence is transmitted, and to use this information to extract
the optimum sampling instant; this approach has been termed
“adaptive sampling.” We analyze the influence of various
metrics for the evaluation of the training sequence and find that even
very simple implementations can achieve near-optimum performance. The
non-idealities of the training sequence used in the (DECT) system have
no significant impact on the performance if proper evaluation methods
are used
A new structure for a non-coherent equaliser receiver applicable
to the DECT-type systems is derived, where the equaliser algorithm takes
into account the receiver non-linearities. It employs a modified channel
estimation procedure suitable for differential detection. The outputs of
the differential detector itself are then fed to a simple 2-state
Viterbi algorithm which uses the modified channel estimates. Performance
results are given for the new receiver which show that it can yield
significant improvements with respect to standard non-coherent receivers
in dispersive channels. At the same time, however, it will operate with
receiver frequency offsets two orders of magnitude greater than a
standard coherent detector equaliser structure (similar to that found in
GSM) and is much less sensitive to the variations in the modulation
index. In addition, performance limits of the basic structure are
discussed and techniques for further improvement of the performance are
considered
High bit rate cordless transmission systems (e.g., DECT and
emerging radio LANs) tend to use simple non-coherent receivers, and
their performance can be constrained by channel dispersion. Equalisation
can be used (supplementing other techniques such as diversity), but
requires high frequency stability. In this contribution we use the DECT
system as a case study and introduce a robust non-coherent receiver
which performs equalisation at the output of a differential detector.
The limitations of this basic structure are overcome by using a second,
complementary differential processor. Simple algorithms are utilised to
switch between the basic and complementary structures. The performance
assessment of the new receiver is presented. The results suggest that
the receiver achieves a significant extension of the dispersive channel
operation range for DECT (maximum tolerable delay spreads of around
460-500 ns) with a multipath diversity gain comparable to a coherent
equaliser receiver. On the other hand, unlike coherent structures, it
remains largely unaffected by both frequency offsets and modulation
index drifts
The DECT (Digital European Cordless Telephone) test bed is a
collaborative DTI/SERC LINK project involving eight UK companies and two
UK universities. The purpose of the test bed program is to validate the
DECT specifications and investigate certain aspects of system
performance through hardware trials. Two work packages were carried out
at Leeds University for the program. The first was a channel modelling
study, to characterise the DECT propagation channel and provide typical
and worst case channel models for the hardware channel emulator to be
used in the test bed laboratory trials. The second was a link simulation
study to investigate the effect of the channel on system performance by
software simulation. The work of these two studies is described
The authors looks at the application of the DECT standard to
outdoor public access systems (e.g. cordless access and telepoint) and
the technical issues raised by its deployment in outdoor environments.
They aim to fully characterise propagation conditions in a typical
telepoint area and then to accurately predict the DECT link performance
for such an environment. Conclusions are drawn as to the major factors
that affect performance indicating ways to configure a public access
system in order to provide the best possible performance
A non-coherent equaliser structure suitable for DECT-type systems
is introduced which performs equalisation at the output of a
differential detector. It employs a two-state Viterbi algorithm using a
simple channel estimation process. Performance assessments are presented
which suggest that the new receiver can yield significant improvements
with respect to standard non-coherent receivers in dispersive channels.
At the same time, unlike coherent equaliser structures, it remains
largely unaffected by both modulation index drifts and frequency offsets
The performance of DECT (digital European cordless telecommunications) radio links under dispersive channel conditions is studied as a function of clock recovery strategies. It is shown that due to the non-optimal sampling, analytical and practical results based on fixed timing will not be acceptable above delay spreads of approximately 40ns. On the other hand, a burst-adaptive clock recovery scheme can more than double this range.
Performance of the DECT system with a 2-state Viterbi equaliser
- G Kadel
G. Kadel, "Performance of the DECT system with a 2-state Viterbi equaliser," COST 231 T0(93)78, Grimstad, Norway, May 1993.
A Preamble based switching diversity implementation for DECT
- P E Mogensen
- F Frederiksen
- P K Thomsen
P. E. Mogensen, F. Frederiksen and P. K. Thomsen, "A Preamble based switching diversity implementation for DECT,"COST 231 TD(94)122, Darmstadt, Germany, Sep. 1994.