Pulse or digital communications – Spread spectrum
Reexamination Certificate
1998-07-14
2001-04-17
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
C375S219000, C375S329000, C370S335000
Reexamination Certificate
active
06219374
ABSTRACT:
FIELD OF THE INVENTION
1. Field of the Invention
The present invention relates to a structure of a coherent dual channel QPSK (Quadrature Phase Shift Keying) transceiver using pilot symbols in a code division multiple access (hereinafter called CDMA) system. In particular, it relates to a structure of a coherent dual channel QPSK transceiver which can greatly reduce crosstalk by adding pilot symbols into a stream of data bits, separating the input information bits into two channels and then allocating different quadrature codes to respective channels.
2. Description of the Prior Art
In a CDMA system, the performance and capacity of the overall system are greatly influenced by noise and interference signals. Therefore, in order to improve the performance of the overall system, it is extremely important to minimize the interference signals. The modulation and demodulation scheme in this CDMA system may be classified into a coherent modulation/demodulation scheme and a noncoherent modulation/demodulation scheme depending on whether required carrier information is required or not upon receipt of the information. When a complete carrier information is provided, the coherent modulation/demodulation scheme has better performance than the noncoherent modulation/demodulation scheme by about 3 dB under additive white Gaussian noise (hereinafter called AWGN). In addition, even in the case of subtractive interference noise suppressor for increasing the capacity of the system, the coherent modulation/demodulation scheme is required because the coherent modulation/demodulation scheme has a channel estimation function which is adapted to variation of radio conditions, for embodying the interference noise suppressor.
The coherent modulation/demodulation schemes in the CDMA system includes the followings.
First, the most simplest BPSK modulation/demodulation scheme has problems that it causes twice inter-chip interference (ICI) compared to QPSK scheme and also it greatly reduces the performance according to non-linear characteristics of the amplifier since the shifts of transmit carrier is rapidly varied by 180° at the moment when data change. In a balanced QPSK scheme, same data bits are input to in-phase and quadrature channels, each of which is spreaded by different spreading sequences. Though this scheme has a similar characteristic to that of dual channel QPSK in that crosstalk between in-phase signals and quadrature signals is reduced, etc., it is inferior to the dual channel QPSK in view of performance. In addition, there is a scheme that the data bits of the scheme are input through separate in-phase and quadrature channels and then spreaded using a same Walsh code and spreading sequence. However, this scheme has problems that it causes crosstalk between the in-phase channel and the quadrature channel when exact carrier information is not received at the receiving terminal and it also reduces the performance according to non-linear characteristics of the amplifier since the shifts of transmit carriers are varied by 180°.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve the problems involved in the prior art, and to provide a structure of a coherent dual channel QPSK transceiver using pilot symbols in a code division multiple access system which can improve gains of the system through coherent modulation/demodulation and can also improve the overall system performance by reducing both crosstalk between in-phase signals and quadrature signals and the degree of variations in envelop of the entire signals.
To achieve the above object, the structure of a coherent dual-channel QPSK transceiver using pilot symbols in a code division multiple access system according to the present invention is characterized in that it comprises a serial-to-parallel converter for separating data bits into an in-phase channel signal and a quadrature channel signal; first and second pilot symbol adders for putting pilot symbols into the signals the channels of which are separated through said serial-to-parallel converters at each given period; first and second mixers for spreading the in-phase channel signals and the quadrature channel signals into which said pilot symbols are periodically added using different Walsh codes; third and fourth mixers for spreading the in-phase channel signals and the quadrature channel signals which are spreaded by said Walsh codes using PN codes; first and second pulse-shaping filters for pulse-shaping the outputs of said third and fourth mixers using them as inputs; fifth and sixth mixers for frequency-modulating said in-phase channel signals and said quadrature channel signals into carriers cos(2&pgr;f
o
t+&thgr;
k
) and sin(2&pgr;f
o
t+&thgr;
k
), respectively; and high-power amplifier for amplifying the outputs of said fifth and sixth mixers and then generating transmit signals to an antenna.
To achieve the above object, the structure of a coherent dual-channel QPSK receiver using pilot symbols in a code division multiple access system according to the present invention is characterized in that it comprises first and second mixers for frequency-modulating signals received through channels from a transceiver using carriers cos(2&pgr;f
o
t) for in-phase channels and sin(2&pgr;f
o
t) for quadrature channels, respectively; first and second matched filters for filtering said frequency-demodulated in-phase channels and quadrature channel signals; a sampler for multiple-sampling said in-phase and quadrature channels signals which passed through said first and second matched filters; third and fourth mixers for despreading said sampled signals using PN codes of a k-th user; a fifth mixer for combining said in-phase channel signals after said despreading with cosine-terms of channel estimation values of a first amplitude and phase; a sixth mixer for combining said quardrature channel signals after said despreading with sine-terms of channel estimation values of a first amplitude and phase; a first summer for summing the output of said fifth mixer with the output of said sixth mixer to correct channels; a seventh mixer for combining said in-phase channel signals after said despreading with sine-terms of channel estimation values of a second amplitude and phase; an eighth mixer for combining said quadrature channel signals after said despreading with cosine-terms of channel estimation values of a second amplitude and phase; a second summer for summing the output of said seventh mixer with the output of said eighth mixer to correct channels; ninth and tenth mixers for despreading the outputs of said first and second adders, respectively, using Walsh codes; first and second adders for adding signals values using the outputs of said ninth and tenth mixers as inputs during 2N chip periods; and first and second channel estimators for generating cosine-terms of channel estimation values and sine-terms of channel estimation values, respectively, which are used in demodulation channel estimation values, using the in-phase channels and quadrature channel pilot determination signals among the outputs of said first and second adders as inputs, respectively.
REFERENCES:
patent: 5412351 (1995-05-01), Nystrom et al.
patent: 5535247 (1996-07-01), Gailus et al.
patent: 5623485 (1997-04-01), Bi
patent: 5712869 (1998-01-01), Lee et al.
patent: 5717713 (1998-02-01), Natali
patent: 5953325 (1999-09-01), Willars
patent: 5960029 (1999-09-01), Kim et al.
Kang Sugbong
Kim Seong Rag
Lee Jeong Goo
Cohen & Pontani, Lieberman & Pavane
Electronics and Telecommunications Research Institute
Pham Chi
Tran Khai
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