mohatta2015.pdf

THE LITERATURE 145
EDGE is an evolution of GSM that provides higher data rates through 8-PSK
and partial response signaling. To maintain reasonable complexity, reduced state
forms of MLSD (RSSE and DFSE) along with channel shortening prefiltering have
been developed for EDGE [AriOOa, SchOl, Dha02, Ger02a].
MLSD is also an option for the second-generation (2G) cellular system known as
US TDMA (see Chapter 5). The symbol rate is 24.3 kbaud, giving a large symbol
period (41.2 //s) relative to typical delay spreads. It is reasonable to address ISI
from at least one previous symbol, giving rise to at least a 4-state Viterbi algorithm.
In [Cho96], use of 4 and 16 states are considered, and 16 states is found useful with
symbol-spaced MLSD when there are two T/2-spaced paths and sampling is aligned
with the first path. In [Jam97], fractionally spaced MLSD is considered using a 4-
state Viterbi algorithm. In [SunOO], 4-state MLSD is only used when needed. Joint
detection of cochannel interference is considered in [Haf04].
6.6 THE LITERATURE
MLSD and ML state detection were proposed in [Chan66]. We saw that MF gives
sufficient statistics for MLSD, leading to the Ungerboeck form [Ung74]. Using a
WMF leads to the Forney form [For72]. The two have been shown to be mathematically
equivalent [Bot98]. An early history of MLSD can be found in [Bel79].
Other front ends include a brick-wall bandlimited filter [Vac81] and a zero-forcing
linear equalizer [Bar89]. In general, all MLSD receivers with access to the same
data should be equivalent [Bar89].
With multiple receive antennas, the Ungerboeck metric has the same form, except
now there is a multichannel matched filter [Mod86]. The Forney metric becomes
the sum of metrics from different antennas, sometimes called metric combining.
When cochannel interference is modeled as spatially colored noise, the term
interference rejection combining (IRC) is sometimes used [Bot99]. Multichannel
MLSD has been applied to underwater acoustic channels [Sto93].
Treating cochannel interference as noise (suppressed linearly) can also be used
with DFSE [AriOOa]. In [Che94], ISI due to partial response signaling is handled
with MLSD whereas ISI due to time dispersion is handled with DFE. In the GSM
system, cochannel interference can be treated as improper or noncircular noise in
formulating an MLSD solution [Hoe06].
Extension of MLSD to time-varying channels can be found in [Bot98, Har97[.
The Ungerboeck form requires channel prediction, motivating a partial Ungerboeck
form that avoids this [Bot98].
MLSD decision can be made on some symbols based on thresholding [OdlOO].
This property can be integrated into the Viterbi algorithm to reduce the number
of states at a particular iteration without loss of performance [Luo07[.
Early work on MLSD for joint detection of multiple signals can be found in
work on crosstalk in wireline communications [Ett76] and CDMA [Sch79, Ver86[.
MLSD has been considered for joint detection of cochannel interference in TDMA
[Wal95, MÍ195, Gra98] systems as well as in underwater acoustic communications
[Sto96j. It has been combined with multiple receive antennas in [Mil95[.