Rake receiver in direct spreading CDMA transmission

Details Rake receiver in direct spreading CDMA transmission Rake receiver in direct spreading CDMA transmission

1999-08-24

2003-01-14

Vo, Don N. (Department: 2631)

Pulse or digital communications

Spread spectrum

Direct sequence

C375S148000

Reexamination Certificate

active

06507605

ABSTRACT:

This application is a national stage filing of PCT application no. PCT/JP98/05860, filed Dec. 24, 1998.
TECHNICAL FIELD
The present invention relates to a RAKE receiver in direct sequence CDMA (DS-CDMA) transmission system that carries out multiple access in a spread spectrum system in mobile communications.
BACKGROUND ART
The DS-CDMA transmission system, which transmits information data modulation signals by spreading them into wideband signals using spreading codes of a processing gain (the number of chips per symbol) pg, is a communication system that assigns different spreading codes to users to enable them to communicate using the same frequency band.
FIGS. 16A and 16B
show a configuration of a receiver employing sliding correlators in a conventional DS-CDMA transmission system. In the block diagrams below including
FIGS. 16A and 16B
, although suffixes -
1
, -
2
, . . . , -L are attached to the same reference numerals that each designate L circuits provided in correspondence to L signal passages, only the same reference numerals are used in the following description.
In the configuration as shown in
FIGS. 16A and 16B
, a spread modulation signal received by an antenna
101
is amplified by a low noise amplifier
103
after passing through a bandpath filter
102
, and then undergoes frequency conversion into an intermediate frequency (IF) signal using a mixer
104
, an oscillator
105
and a bandpass filter (BPF)
106
, followed by linear amplification by an automatic gain control amplifier (AGC amplifier)
107
. Subsequently, a square-law detector
108
detects the envelope of the amplitude of the received signal, and amplitude fluctuations are negatively fed back to the AGC amplifier
107
to compensate for the amplitude fluctuations caused by fading. The linearly amplified signal by the AGC amplifier
107
undergoes quadrature detection by a quadrature detector
109
, resulting in a pair of baseband signals. The in-phase (I) and quadrature (Q) components are converted into digital values by A/D converters
112
and
113
. Replica generators
115
generate spreading code replicas synchronized with delay times of multipath signals to be RAKE combined. Sliding correlators
114
despread using the spreading code replicas the spread modulation signal converted into the digital values. Channel estimators
116
and multipliers
117
carry out differentially coherent detection or coherent detection of the despread signals to demodulate the data. An adder
118
carries out RAKE combining of the demodulation outputs, and a deinterleaver
122
deinterleaves the output of the adder
118
. A Viterbi decoder
123
decodes the output of the deinterleaver, and a data decision section or data restoring section
124
carries out hard decision to restore the received data.
In the conventional example as shown in
FIGS. 16A and 16B
, the absolute coherent demodulation scheme will be explained which is carried out by inserting pilot symbols into information symbols at fixed intervals. In land mobile communications, the received signal undergoes amplitude and phase fluctuations called fading because of changes in relative locations of a base station and a mobile station. In view of this, the channel estimators
116
estimate the complex envelopes, that is, the amplitude and phase fluctuations (or channels) caused by fading when the receiver carries out the coherent detection modulation. The channel estimators obtain received complex fading envelopes associated with the pilot symbols inserted into the transmitted information symbols at the fixed intervals, and then obtain the complex fading envelopes at individual information symbols between the pilot symbols. The multipliers
117
compensates for the fluctuations of the complex fading envelopes (channel fluctuations) of the individual information symbols using the values associated with the pilot symbols. The adder
118
carries out in-phase combining (RAKE combining) of multipath signals whose channel fluctuations are compensated for, thereby improving the ratio of the signal power to interference signals or thermal noise.
Selection of the multipath signals to be RAKE combined is carried out by the sliding correlators
114
which are called search fingers, in which average received signal powers of despread signals are measured at U timings in a multipath search range, and multipaths with great average received signal powers are selected. For example, when a single sliding correlator
119
is used, a correlation value (despread value) at one timing per symbol is selected so that the received signal power of the despread signal is selected at this timing. Sliding the timing of the spreading code one by one enables the power measurement to be achieved for the total of U timings.
Thus, selection of the paths to be RAKE combined requires to choose the multipath signals with great average signal power (after undergoing the fluctuations due to shadowing and relative location changes between the base station and the mobile station). On the other hand, under a land mobile communication environment, there are instantaneous fluctuations caused by Rayleigh fading. Accordingly, some multipaths may be missing from the paths to be RAKE combined because their received signal power may happen to be dropped due to the Rayleigh fading and has small signal power.
To circumvent the effect of such instantaneous fluctuations of the received power, the received signal power of the signals must be measured after averaging the Rayleigh fading fluctuations. To achieve this, the signal power measurement of the despread signals is iterated V times at U timings in the multipath search range, and delay profiles are formed from the average signal powers to select W greatest multipaths to be RAKE combined. Forming each delay profile by a single sliding correlator requires U×V symbol time, and forming an average delay profile by f sliding correlators (search fingers) require (U×V)/f symbol time. The timings of the spread code replicas used in the RAKE combining fingers are updated every time the delay profile is generated. When the mobile station moves fast with respect to the base station, the delay profiles fluctuate quickly. Accordingly, the multipath search using the sliding correlators, which take a rather long time, cannot follow the fluctuations of the delay profiles sometimes. Although fast multipath search may be possible by reducing the multipath search range and the number of times of the averaging, the reduction in the multipath search range will reduce the time diversity effect of the RAKE combining, and the reduction in the number of times of averaging the signal power will impair the accuracy of the RAKE combined multipath selection by the search fingers.
FIGS. 17A and 17B
show as a related art (not as a prior art) a configuration of a receiver employing a matched filter in the DS-CDMA transmission system, which the assignee of the present invention proposed in Japanese patent application laid-open No. 10-190522 (laid open on Jul. 21, 1998) (that is, Japanese patent application No. 8-346025 filed on Dec. 25, 1996). In the configuration as shown in
FIGS. 17A and 17B
, a spread modulation signal received is amplified by the low noise amplifier
103
, and then undergoes frequency conversion into the IF signal. The IF signal is fed to the AGC amplifier
107
, and then to the square-law detector
108
that controls the amplifier
107
to compensate for the amplitude fluctuations caused by fading. Then, the amplifier output is fed to the quadrature detector
109
to undergo quadrature detection. The baseband signals output from the quadrature detector
109
are fed through the low-pass filters
110
and
111
to the A/D converters
112
and
113
to be converted into digital signals. A matched filter
131
, which has pg taps, despreads using the output of a spreading code replica generator
132
the spread modulation signal converted into digital values, thereby dividing it into L timing signals, where L=pg×s, where s is the number

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