Pulse or digital communications – Spread spectrum – Direct sequence
Patent
1997-12-04
2000-10-31
Tse, Young T.
Pulse or digital communications
Spread spectrum
Direct sequence
375150, 375350, H04B 1707
Patent
active
061413720
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention is generally directed to communications receivers of direct sequence spread spectrum signals, and in particular, to digitally sampling the spread spectrum signal at an IF frequency (approximately 200 MHZ and below) and simultaneously despreading and downconverting the signal to baseband.
BACKGROUND OF THE INVENTION
A block diagram of a typical direct sequence spread spectrum system 100 is shown in FIG. 1. A transmitter 102 consists of an MPSK modulator 104 which typically utilizes either Binary (BPSK) or Quaternary (QPSK) phase shift keying, followed by a spreader 106 which multiplies the modulated signal by a digital PN (pseudo noise) spreading code 108. The PN code 108 is typically generated by a PN Code Generator 110 at a rate (referred to as the chipping rate) at least an order of magnitude faster than a data symbol rate of the modulator 104, thus spreading the spectrum across a much greater bandwidth. For a multiple user system, each user has his own unique PN code and the bandwidth can be shared among different users using code division multiple access (CDMA) techniques.
A receiver 112 generates an exact replica 109 of the transmit PN sequence and multiplies it by the received signal to despread and hence recover the original modulated waveform. The receiver 112 must incorporate some means of synchronizing the timing of the locally generated PN sequence to that of the received signal. Both code acquisition circuitry 111 and tracking circuitry 113 must be included.
The receiver 112 structure typically uses one of three general structures as shown in FIGS. 2(a), (b) and (c). In FIG. 2(a), the receiver RF input 200 is first downconverted to a wideband intermediate IF frequency signal 214 in a wideband IF stage 208. The IF bandwidth must be greater than the spread bandwidth of the transmit signal. The IF signal 214 is then despread by a PN sequence 204 which has been upconverted from baseband to the IF frequency. The resulting despread signal 206 appears at baseband and is then filtered by a narrowband lowpass filter 202 with a bandwidth on the order of the data symbol rate.
A second scheme, shown in FIG. 2(b), also downconverts the RF signal 200 in the wideband IF stage 208. The despreading operation occurs at the IF frequency, although it is accomplished by multiplying the IF signal 214 by the baseband PN sequence 215. After despreading, the signal bandwidth is reduced, and the signal can then be filtered with a narrowband IF filter 210. The narrowband signal is then downconverted to baseband in the narrowband IF stage followed by narrowband baseband filtering.
The third scheme performs despreading at baseband as shown in FIG. 2(c). The wideband RF signal 200 is downconverted to a wideband baseband signal 216 and then filtered with a wideband baseband filter 212. The baseband signal is then despread by multiplying it by the baseband PN sequence 215 followed by narrowband baseband filtering.
One disadvantage of an all analog implementation of the IF and despreading circuits is the large number of components typically required. Each IF stage requires a local oscillator, mixer and filter. The despreading mixer must remain flat over a large bandwidth and accept a high slew-rate digital PN input. If pre-filtering is employed prior to despreading to improve noise performance, it typically exhibits a non-ideal frequency and time delay response, resulting in sub-optimum performance. The narrowband filter following the despreader should be reasonably sharp, often resulting in a physically large device. The baseband version of the despreader requires a complex downconverter where the local oscillator must be split into its in-phase and quadrature components. In addition, the phase noise of the local oscillators must be tightly controlled or there will be a performance loss in the subsequent coherent MPSK demodulator. DC offsets are also a concern and should be removed prior to demodulation. Analog circuits also suffer from component drift and aging and may be diff
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Comsat Corporation
Tse Young T.
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