Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
2001-05-04
2003-09-30
Chin, Stephen (Department: 2634)
Pulse or digital communications
Equalizers
Automatic
C375S350000, C708S304000, C708S305000, C708S323000
Reexamination Certificate
active
06628707
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an adaptive equalizer subsystem employed in fixed broadband wireless access (FBWA) applications operating in adaptive short-burst modems and multi-link hopping mesh radio networks over slow time-varying channels. The adaptive modem is capable of fast link-hopping from one link to another over such channels. That is, the channel is quasi-static from burst to burst for any given link.
The embodiments described herein may be used in conjunction with a wireless mesh topology network of the type described in U.S. patent application Ser. No. 09/187,665, entitled “Broadband Wireless Mesh Topology Networks” and filed Nov. 5, 1998 in the names of J. Berger and I. Aaronson, with carrier phase recovery system described in U.S. patent application Ser. No. 09/764,202, entitled “Carrier recovery System for Adaptive Modems and Link Hopping Radio Networks” and filed in the names of M. Rafie et al., and with network nodes including switched multi-beam antenna designs similar to the design described in U.S. patent application Ser. No. 09/433,542, entitled “Spatially Switched Router for Wireless Data Packets” and filed in the names of J. Berger, et al., as well as with the method and apparatus disclosed in U.S. patent application Ser. No. 09/699,582 entitled “Join Process Method For Admitting A Node To A Wireless Mesh Network, filed Oct. 30, 2000 in the names of Y. Kagan, et al. Each of these U.S. patent applications is incorporated in its entirety herein by reference. Other applications for the embodiments will be apparent from the description herein.
Burst transmission of digital data is employed in several applications such as satellite time-division multiple access, digital cellular radio, wide-band mobile systems and broadband wireless access systems. The design trade-offs and the resulting architectures are different in each of these applications.
In general, the receiver must filter the received burst waveforms in a way that will result in the best possible bit-error performance. In most cases, this means maximizing the ratio of signal power to power of noise, interference, and distortion. In modern systems, this implies using a matched filter or an adaptive equalizer.
In most of these applications, a preamble of known symbols is inserted in the beginning, middle, or at end of each burst of data packets for training purposes. Such an approach is not appropriate in applications involving transmission of short bursts. The insertion of a known data sequence greatly reduces the transmission efficiency for a short burst. As a result, preamble-based algorithms are not applicable in such systems.
Ideally, it is highly desirable to minimize the use of training sequences for initial acquisition or subsequent adaptation. This property is especially important for short-burst formats used in many existing wireless communication applications that utilize Time-Division Multiple Access (TDMA) such as IS-136, GSM, EDGE, and fixed broadband wireless access systems. Short burst formats are used to reduce end-to-end transmission delay and to limit the time variation of wireless channels over a burst. However, training overhead can be very significant for such short burst formats. This overhead ranges up to 30% in many systems. The overhead of these systems can be recovered by employing the adaptive equalization apparatus outlined in this invention. In cases where longer range or higher tolerance of delay spread is needed, adaptive equalization can be used for these systems without changing physical link formats.
A constant need for ever-increasing throughputs through fixed bandwidths, fueled by broadband Internet protocol (IP) applications, has pushed system designers toward more throughput-efficient modulation schemes. Because of their relatively good performance, large quadrature amplitude modulation (QAM) constellations are being used in many of these applications. One of the critical problems associated with the use of large QAM constellations is that of amplitude and delay distortion of the radio link, which for efficiency reasons, must often be done without the use of a preamble, particularly in burst modem systems.
There are several classes of approaches to adaptive equalization. Low complexity algorithms for adaptation, such as the least mean-square (LMS) error algorithms, are fairly common in adaptive equalizers. Faster approaches, such as the least-squares (LS), recursive least-squares (RLS), fast Kalman, and square-root RLS methods, require computationally-intensive matrix inversions and (in some cases) stability issues. Adaptive equalizers can be further classified into linear transversal and recursive structures.
In transversal (tap-delay-line) equalizers, the current and past values of the received symbols, r(t-nT), are linearly weighted by equalizer tap coefficients (tap gains) c(n) and summed to produce the equalized signal,
y
(
n
)
=
∑
k
c
(
k
)
r
(
t
0
+
nT
-
kT
)
.
A zero-forcing (ZF) equalizer minimizes the peak distortion of the worst case ISI (inter-symbol interference) only if the peak distortion before equalization is less than 100 percent. In an LMS equalizer, however, the equalizer tap coefficients are chosen to minimize the mean-square error—the sum of squares of all the ISI (inter-symbol interference) terms plus the noise power at the output of the equalizer.
Under the class of non-linear receiver structure, various optimality criteria related to error probability are considered. This culminated in the development of the maximum likelihood sequence estimator (MLSE) using the Viterbi algorithm (VA) and adaptive version of such a receiver. The computational complexity of the MLSE is proportional to m
L−1
, which grows exponentially with symbol alphabet size (m) and the number of terms in the discrete channel pulse response (L). Another branch of non-linear and sub-optimal receiver structure is the decision-feedback equalizer (DFE). A decision-feedback equalizer makes memoryless decisions and cancels all trailing inter-symbol interference (ISI) terms. DFE, however, suffers from a reduced effective signal-to-noise ratio (SNR) and error propagation, due to its inability to defer decisions.
Fast convergence is important for adaptive equalizers in receivers polling multi-point networks where each node in the network must adapt to receive typically short bursts of data from a number of transmitters over different radio links. Orthogonalized LMS algorithms are used to speed up equalizer convergence. In particular, a self-orthogonalization technique, such as RLS and adaptive lattice (AL) are used for rapidly tracking adaptive equalizers. Kalman (RLS) and fast Kalman algorithms obtain their fast convergence by orthogonalizing the adjustment made to the coefficients of an ordinary linear transversal equalizer. Adaptive lattice algorithms, on the other hand, use lattice filter structure to orthogonalize a set of received signal components. In some applications, use of fast converging equalizers are avoided due to computational complexity and stability issues.
If the impairments that the equalizer must resolve are small enough so that the modem can successfully track timing and carrier phase prior to equalization, then the equalizer can be made to train much more rapidly. For more severely distorted channels, an approach that trains the equalizer prior to recovery of timing and carrier may be needed.
The effect of carrier phase error, &phgr;
e
=&phgr;−{circumflex over (&phgr;)}, in high-level modulation schemes, such as M-QAM is to reduce the power of the desired signal component by a factor of cos
2
(&phgr;−{circumflex over (&phgr;)}) in addition to the cross-talk interference from the in-phase and quadrature components. Since the average power level of the in-phase and quadrature components is the same, a small phase error causes a large degradation in performance of the adaptive equalizer, particularly at higher modulation levels (i.e., M≧16). An accurate carrier phas
Fu Dengwei
Lu Jun
Rafie Manouchehr S.
Shah Tushar
Chin Stephen
Ha Dac V.
Pillsbury & Winthrop LLP
Radiant Networks PLC
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