Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-02-03
2001-10-09
Vo, Don N. (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S152000, C375S281000, C375S332000, C375S343000
Reexamination Certificate
active
06301291
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a code division multiple access (CDMA) communication system and, more particularly, to pilot symbol assisted modulation and demodulation in the forward and reverse links of such a CDMA communication system.
DESCRIPTION OF THE RELATED ART
Code Division Multiple Access (CDMA) modulation, which is known in the art, is a multi-user access transmission scheme in which the signals of different users overlap both in frequency and in time. This is in contrast to Frequency Division Multiple Access (FDMA), also known in the art, in which the signals of users overlap in time, but are assigned unique frequencies, and Time Division Multiple Access (TDMA) in which the signals of users overlap in frequency, but are assigned unique timeslots. According to CDMA, each user is assigned a unique code sequence that is used to modulate the user's signals. This allows the user to spread the information over the entire channel bandwidth, as opposed to particular sub-channel(s) in FDMA. In CDMA, the signal for each user at a mobile station (MS) is spread over a wide bandwidth, which is greater than the minimum bandwidth to transmit the signal. Each user's signal is spread by a different wideband code, each of which are orthogonal to each other. All the spread wideband signals for different users are added together to form a composite signal which is transmitted over the airwaves in the same frequency band. The receiver at a base station (BS) distinguishes among signals from different users by using a copy of the particular wideband code for a user, which is available to both the mobile stations and the base stations in the CDMA system. Such a process is called channelization. In an exemplary IS-95 CDMA system, which is a North American CDMA standard known in the art, channelization in the reverse link, i.e., when a mobile station (MS) is transmitting to a base station (BS) in the system, is accomplished using a wideband code called a pseudorandom noise (PN) code, also known in the art. The receiver at the base station (BS) sifts the desired signal from a particular user out of the composite signal by correlating the composite signal with the original wideband code. All other users having codes that do not match the code for the desired signal from that particular user are rejected.
A central facet of wireless communication systems is the reliability and integrity of the data which are being communicated. Ideally, the data which are being transmitted from a transmitter of a wireless system should be identical to the data which are being received at a receiver thereof. Practically, however, the data which are received at the receiver have often been corrupted with respect to the original data which were transmitted from the transmitter. Such data communication errors may be attributed to many factors, including multipath Rayleigh fading. There are times when a receiver is completely out of range of a transmitter in the CDMA system, i.e., there is no signal path traveling to the receiver. The received signals are made up of a group of reflections from objects, and none of the reflected signal paths is any more dominant than the other ones. The different reflected signal paths arrive at slightly different times, with different amplitudes, and with different phases. Because there are many different signal paths, constructive and destructive interference can result, namely, multipath Rayleigh fading. Furthermore, jitter may also result which prevents proper signal sampling and in turn negatively affects the bit error rate (BER), which is directly related to the signal quality of a transmission path assigned to a particular user.
There is therefore a general need in the art for a wireless system with improved data reception. A wireless system is particularly needed that enhances the signal quality at the receivers and minimizes the negative effects of multipath Rayleigh fading on data reception. There is a further need for a wireless system that prevents the occurrence of jitter with proper sampling and optimized bit error rate (BER).
SUMMARY OF THE INVENTION
The present invention is a method and system of data transmission using pilot symbol assisted modulation and demodulation. A preferred embodiment of the system according to the invention includes a transmitter having a QPSK (quadrature phase shift keying) modulator, and a receiver having a pilot correlation filter (PCF), a data matching filter (DMF), a timing recovery mechanism, a sampler, and a QPSK (quadrature phase shift keying) demodulator. The transmitter transmits a frame of data symbols and pilot symbols to a receiver in a wireless system. The pilot symbols are inserted in the frame at known time intervals. The QPSK modulator modulates the frame of data and pilot symbols using quadrature phase shift keying (QPSK) modulation. As the receiver receives the frame of data and pilot symbols from the transmitter, the PCF recovers the pilot symbols from the frame and produces a multipath response. The timing recovery mechanism tracks the timing of the pilot symbols in the frame. The DMF enhances the multipath response of the frame of data and pilot symbols at the known time intervals of the pilot symbols, and outputs an enhanced multipath response signal with a plurality of enhanced peaks. The sampler samples the enhanced multipath response of the frame of data and pilot symbols. The QPSK demodulator demodulates the sampled frame of data and pilot symbols and recovers the data symbols using quadrature phase shift keying (QPSK) demodulation based on the samples.
In accordance with a preferred embodiment of the method of the invention, a frame of data symbols and pilot symbols are transmitted, where the pilot symbols are inserted in the frame at known time intervals. The frame of data and pilot symbols are modulated using QPSK modulation. Once the frame is received, the pilot symbols are recovered and a multipath response is provided for the received frame at the known time intervals of the pilot symbols through the use of a finite impulse response (FIR) filter. At this point, the multipath response is hump-like in appearance in terms of its amplitude versus time. The multipath response of the received frame is enhanced by time reversal and complex conjugation to provide enhanced peaks of the multipath response. The enhanced multipath response is sampled at each of the enhanced peaks and the data symbols of the received frame are recovered using QPSK demodulation based on the samples.
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Hoffmann John E.
Nelson, Jr. George Rodney
Proctor, Jr. James A.
Rouphael Antoine J.
Darby & Darby
TANTIVY Communications, Inc.
Vo Don N.
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