Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1998-10-14
2002-10-22
Marcelo, Melvin (Department: 2663)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S441000, C375S150000
Reexamination Certificate
active
06470000
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to code division, multiple access (CDMA) communication systems, and, more particularly, to a shared, vector correlator of a CDMA receiver.
2. Description of the Related Art
Several code division, multiple-access (CDMA) standards have been proposed, and one such standard is the IS-95 standard adopted for cellular telephony. As with many CDMA systems, IS-95 employs both a pilot channel for a base station and data, or message, channels for communication by the base station and users. The base station and users communicating with the base station each employ assigned, pseudo-random sequences, also known as pseudo-noise (PN) code sequences, for spread-spectrum “spreading” of the channels. The assigned PN code sequence is a sequence of a predetermined number of bits. For each user transceiver, the PN code sequence is used to spread data transmitted by the transceiver and to despread data received by the transceiver. The PN code sequence is used for both In-phase (I) and Quadrature-phase (Q) channels, is a sequence with a known number of bits, and is transmitted at a predetermined clock rate.
Each bit-period, or phase transition, of the PN code sequence is defined as a chip, which is a fraction of the bit-period of each data symbol. Consequently, the PN code sequence is combined with the data sequence so as to “spread” in frequency the frequency spectrum of the data. In IS-95, for example, 64 chips represent one data symbol. The pilot channel and each user are also assigned a different Walsh code that is combined with the spread channel to make each spread channel signal orthogonal. The pilot channel is assigned the all zeros Walsh code. An exclusive-OR combination of the zero Walsh code with the PN code sequence of the I and Q channels, respectively, leaves the PN code sequence of the pilot channel unaltered. No data symbols are spread or transmitted on the pilot channel.
To determine when a signal is transmitted, and to synchronize the reception and processing of a transmitted signal, IS-95 specifies a search finger correlating a known portion of the PN code sequence, for example, an IS-95 pilot epoch, with the sampled received signal. The pilot epoch is the time interval over which a pseudo-noise (PN) code sequence of a pilot signal repeats. The known portion of the IS-95 pilot epoch is the first 64 chips output from I-phase and Q-phase PN sequence generators subsequent to a rollover state. The beginning of the pilot epoch is the rollover state, and is the state at which the I-phase sequence and Q-phase sequence in respective PN generators have the same logic value in all register stages of the PN sequence is a multiple of 2. Additional logic may be required to insert the extra value into each sequence following 14 consecutive 1's or 0's. The extra value renders a 2
15
chip period PN sequence. Consequently, for systems such as IS-95, at the beginning of the pilot epoch the value in the first register stage is forced to a logic “1” prior to the next state transition from the all zero register state.
Demodulation of a spread signal received from a communication channel requires synchronization of a locally generated version of the PN code sequence with the embedded PN code sequence in the spread signal. Then, the synchronized, locally generated PN code sequence is correlated against the received signal and the cross-correlation extracted between the two. For a user channel, the signal of the extracted cross-correlation is the despread data signal. For IS-95 systems, demodulation begins by first synchronizing a local code sequence pair, one for the I-phase spread data channel (I-channel) and one for the Q-phase spread data channel (Q-channel), with an identical pair of PN code sequences embedded in the signal received from the communication channel.
Communication systems are often subject to transmission path distortion in which portions, or paths, of a transmitted signal arrive at a receiver, each portion having different time offsets and/or carrier phase rotation. Consequently, the transmitted signal appears as a multiplicity of received signals, each having variations in parameters relative to the transmitted signal, such as different delays, gains and phases. Relative motion between a transmitter and receiver further contribute to variations in the received signal. The receiver desirably reconstructs the transmitted signal from the multiplicity of received signals.
A type of receiver particularly well suited for reception of multipath, spread spectrum signals is a RAKE receiver. The RAKE receiver comprises several search fingers to cross correlate each multipath signal with an offset version of the local PN code sequence. The RAKE receiver optimally combines the multipath signals received from the various paths to provide an extracted cross-correlated signal with high signal-to-noise ratio (SNR). The RAKE receiver may be analogized to a matched-filter where the path gains of each search finger, like the taps of a matched-filter, may be estimated to accurately detect a received multipath, spread spectrum signal. Since a transmitted signal is subject to many types of distortion as it passes through a communication channel to a receiver (i.e., multipath effects, Rayleigh fading, and Doppler shifts), the receiver must estimate the path gains utilizing the transmitted signal as distorted at the receiver. Thus, the detected received signal will only be as good as the path gain estimation of each search finger in the RAKE receiver.
U.S. Pat. Nos. 5,448,600; 5,442,661; 5,442,627; 5,361,276; 5,327,455; 5,305,349; and 5,237,586, the disclosures of which are hereby incorporated by reference, each describe a RAKE receiver. In RAKE receivers, for each fractional chip increment, a correlation with the pilot epoch is performed, which may be represented using the complex conjugate of the expected sequence, x
r
(n)+x
i
(n), as
cc
r
(
n
)
=
∑
m
=
0
63
x
r
(
m
)
·
y
r
(
m
+
n
τ
)
+
∑
m
=
0
63
x
i
(
m
)
·
y
i
(
m
+
n
τ
)
and
(
1
)
cc
i
(
n
)
=
∑
m
=
0
63
x
r
(
m
)
·
y
i
(
m
+
n
τ
)
-
∑
m
=
0
63
x
i
(
m
)
·
y
r
(
m
+
n
τ
)
(
2
)
where:
n and m are integer counters
cc
r
(n) are the real components of the cross-correlation
cc
i
(n) are the imaginary components of the cross-correlation
y is the sampled received signals
x is the reference sequence (matched-filter PN vector sequence)
&tgr; is a fractional chip
Thus, as can be seen from equations (1) and (2), four real correlations are performed in the process of performing one complex correlation.
The locally generated PN code sequence (the “local PN code sequence” or “reference PN code sequence”) provides the basic elements for generating reference PN sequences, or matched-filter PN vectors, for matched-filter correlation against the received signal. Each PN code sequence is deterministic with a period of 2
N−1
chips (PN values), N an integer greater than 1. The PN code sequence is identical between base-stations in an IS-95 system, and maybe augmented by one chip to provide a sequence with a period of 2
15
chips. This PN code sequence is also known as the “short” code in IS-95 systems. The PN code sequence of each base-station is used for forward channel spreading, and in IS-95-based CDMA communication systems the code-phase offset of the PN code sequence is unique to a base-station. Therefore, to differentiate between base-stations, each base-station is assigned a unique time offset in the PN code sequence.
A PN code generator of an exemplary IS-95 system provides the code sequence for each of the I and Q channels recursively using a 15
th
order polynomial, resulting in a period of, for example, 2
15
−1 chips. The hardware realization for such a PN code generator is a shift register having 15 stages and with se
Burns Geoffrey F.
Kolagotla Ravi K.
Agere Systems Guardian Corp.
Hughes Ian M.
Marcelo Melvin
Mendelsohn Steve
LandOfFree
Shared correlator system and method for direct-sequence CDMA... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Shared correlator system and method for direct-sequence CDMA..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Shared correlator system and method for direct-sequence CDMA... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2945450