Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1999-02-09
2003-04-01
Liu, Shuwang (Department: 2734)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S148000, C375S229000, C375S231000, C375S316000
Reexamination Certificate
active
06542562
ABSTRACT:
BACKGROUND
The present invention relates to channel estimation in mobile communication systems, and more particularly to methods and apparatuses that perform channel estimation with lower computational intensity.
Mobile radio telephony is characterized, among other things, by multipath propagation of the radio signal that is transmitted between base stations (BSs) and mobile stations (MSs). Because different rays of the transmitted signal may take different paths before arriving at a receiver's antenna, some rays are received later than others by the receiver. The resulting received signal, then, includes one or more echoes of the transmitted signal. When the information transmitted in the signal consists of digital symbols, these echoes are referred to as Inter-Symbol Interference (ISI). ISI detrimentally affects a receivers ability to determine the informational content of the received signal.
In order to reduce or eliminate ISI in a received signal, it is known to use equalizers in the receiver. This will be further described with reference to systems that utilize Code Division Multiple Access (CDMA) techniques to distinguish between the signals associated with different users. It will be recognized, however, that CDMA is but one of many possible examples (e.g., Time Division Multiple Access, or “TDMA” being another such example) of radio systems that employ a RAKE receiver or equalizers to address the problem of multipath propagation.
The basic idea in a CDMA system is to separate different users, base stations, and services by means of unique spreading sequences/codes. In one type of CDMA system, the informational data stream to be transmitted is impressed upon a much higher rate data stream known as a signature or spreading sequence. Typically, the signature sequence data are binary, thereby providing a bit stream. One way to generate this signature sequence is with a pseudo-noise (PN) process that appears random, but can be replicated by an authorized receiver. The informational data stream and the high bit rate signature sequence stream are combined by multiplying the two bit streams together, assuming the binary values of the two bit streams are represented by +1 or −1. This combination of the higher bit rate signal with the lower bit rate data stream is called spreading the informational data stream signal. Each informational data stream or channel is allocated a unique signature sequence.
A plurality of spread information signals modulate a radio frequency carrier, for example by binary phase shift keying (BPSK), and are jointly received as a composite signal at the receiver. Each of the spread signals overlaps all of the other spread signals, as well as noise-related signals, in both frequency and time. If the receiver is authorized, then the composite signal is correlated with one of the unique signature sequences, and the corresponding information signal can be isolated and despread. If quadrature phase shift keying (QPSK) modulation is used, then the signature sequence may consist of complex numbers (having real and imaginary parts), where the real and imaginary parts are used to modulate respective ones of two carriers at the same frequency, but ninety degrees out of phase with respect to one another.
Traditionally, a signature sequence is used to represent one bit of information. Receiving the transmitted sequence or its complement indicates whether the information bit is a +1 or −1, sometimes denoted “0” or “1”. The signature sequence usually comprises N bits, and each bit of the signature sequence is called a “chip”. The entire N-chip sequence, or its complement, is referred to as a transmitted symbol. The conventional receiver, such as a RAKE receiver, correlates the received signal with the complex conjugate of the known signature sequence to produce a correlation value. When a large positive correlation results, a “0” is detected; when a large negative correlation results, a “1” is detected.
It will be understood, then, that the rate of the spreading code (usually referred to as the chip rate) is larger than the information symbol rate. The code rate divided by the information symbol rate is referred to as the spreading factor (S
f
). In a system with the transmission of several users being separated by different spreading codes, the code that separates these users is referred to as the long code. By correlating the composite signal with the conjugate of one of the used codes in a receiver, the corresponding user information is recreated while signals related to other users are experienced as noise.
In order to overcome the multipath characteristics in a mobile radio channel, the RAKE receiver and the ray searcher are two essential units for the Wideband Code Division Multiple Access (W-CDMA) technology being standardized under the name IMT2000. (See, e.g., IMT-2000 Study Committee Air-interface WG, SWG2, “Volume 3 Specifications of Air-interface for 3G Mobile System”, Ver. 0-3.1, December 1997.) An exemplary RAKE receiver is illustrated in FIG.
1
. Briefly, the fundamental idea with the RAKE is to synchronize each of the relevant multipath components of the input radio signal to a rather simple receiver. (See, e.g., J. G. Proakis,
Digital Communications
, McGraw-Hill, 1983). The simple receiver is often referred to as an arrangement of RAKE fingers. Six RAKE fingers
101
are depicted in the exemplary receiver of FIG.
1
. The different multipath components are assumed to be reasonably uncorrelated. When the assumption is valid and a sufficient number of fingers are used, maximum ratio combining of the fingers results in a quite simple receiver technology with good performance.
Channel Estimation Overview
The overall frame structure for the physical channels of the exemplary W-CDMA scheme are depicted in FIG.
2
. The transmitted base band signal, s
i,j,k
, is given by
s
i,j,k
=c
i,j,k
·u
j,k
(1)
where c
i,j,k
is the complex spreading sequence and u
j,k
represents the jth complex symbol in slot k. The notation above gives the signals of interest for chip i in symbol j and slot k, where i=0,1, . . . S
i
−1 and j=0,1, . . . N
s
−1. The spreading factor is given by S
f
and N
s
is the number of symbols per slot. For the W-CDMA system, the chip rate is 4.096e6 chip per second (cbps) and
N
s
=
2560
S
f
.
(
2
)
The long code is cyclically repeated every frame. In order to get a coherent receiver, the channel corruption ĥ
j,k−n
B
(of amplitude and phase) for each symbol j in slot k−n
B
needs to be estimated. Due to the different arrival times of the multipath components, the channel corruptions are correspondingly different for each multipath component. In order to perform Maximum Ratio Combining (MRC) the channel corruption needs to be estimated for each of the multipath components that is synchronized to a RAKE finger
101
. The first step in the channel estimation procedure is to obtain a primary channel estimate {overscore (h)}
k
for each slot. A channel estimate, ĥ
j,k−
B
for each symbol j in slot k−n
B
is then obtained, based on m consecutive primary channel estimates, where m≧n
B
. The parameter n
B
is the number of slots that are buffered. The distribution of the channel characteristics in the multipath components is dependent on the environment and can, for example, be Rayleigh distributed. The amplitude and phase variation of consecutive primary channel estimates depend on the one hand on the Rayleigh distribution and on the other hand on the Doppler frequency.
The principle blocks related to one exemplary finger
101
of the RAKE receiver are depicted in FIG.
3
. As an overview to its operation, the RAKE finger
101
performs spreading code correlation, integration over a symbol, and estimation of the channel using a priori known pilot symbols. The channel estimate is then used to compensate for the channel distortion. The operation of the RAKE finger
101
will now be described in greater detail.
The received signal in a W-CDM
Lindoff Bengt
Östberg Christer
Burns Doane Swecker & Mathis L.L.P.
Liu Shuwang
Telefonaktiebolaget LM Ericsson (publ)
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