Pulse or digital communications – Spread spectrum – Hybrid form
Reexamination Certificate
1999-01-15
2001-04-10
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Spread spectrum
Hybrid form
C375S135000
Reexamination Certificate
active
06215810
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to code division multiple access (CDMA) systems, and more particularly, it relates to a parallel hopping hybrid direct sequence (DS)/slow frequency hopping (SFH) code division multiple access (CDMA) system.
DESCRIPTION OF THE RELATED ART DESCRIPTION OF THE RELATED ART
Over the last decade, deployment of wireless communications has been significant. The wireless communication market may be a $60 billion dollar industry by the turn of the century. Expanded radio channel capacity is the key objective of a wireless network to meet demand for the indefinite future.
At present there are two basic strategies whereby a fixed spectrum resource can be allocated to different users: narrowband channelized systems and wideband systems. Two narrowband systems are Frequency-division multiple access (FDMA), where each user is assigned to a different frequency. Guard bands are maintained between adjacent signals to minimize crosstalk between channels. The second being Time-division multiple access (TDMA), where each user is assigned to a different time slot. By contrast, in wideband systems, the entire system bandwidth is made available to each user and is many times larger than the bandwidth required to transmit information. Such systems are referred to as Spread Spectrum (SS) systems. In recent years, spread spectrum has moved from military to commercial communications, culminating in the introduction of International Standard IS-95 code division multiple access (CDMA) technology as an alternative standard for commercial digital cellular and personal communication system (PCS) networks. The IS-95 CDMA is now being used in numerous cellular and PCS markets around the world. Multiple access systems share a fixed resource (i.e. frequency spectrum) to provide voice channels on demand to users. The IS-95 standard employs a type of spread spectrum technology called direct-sequence spread spectrum (DS-SS). In a direct sequence (DS) code-division multiple access (CDMA) system, a signal to be transmitted occupies a bandwidth in excess of the minimum necessary to send the information; the band spread is accomplished by means of a code that is independent of the data, and a synchronized reception with the code at the receiver is used for despreading the subsequent data recovery. In operation, a data signal is multiplied by a pseudo-random noise (PN) sequence having a faster data rate than the data signal to be transmitted.
Another form of spread spectrum is called frequency-hopped spread spectrum (FH-SS) where the carrier frequency signal is moved (hopped) around in the band in a pseudorandom fashion. The result is an increase in effective bandwidth over time. Specifically, in a frequency hopped spread spectrum system (FH-SS) the carrier frequency is shifted in discrete increments in a pattern generated by a code sequence. In a (FH-SS) system, the signal frequency remains constant for a specified time duration, referred to as a hopping time T
b
. The (FH-SS) system can be either a fast-hop system or a slow hop system. In a fast-hop system, the frequency hopping occurs at the rate that is greater than the message bit rate. In a slow-hop system, the hop rate is less than the message bit rate. There is also an intermediate situation in which the hop rate and message bit rate are of the same order of magnitude.
FH-SS radio systems experience occasional strong bursty errors, while DS-SS radio systems experience continuous but lower-level random errors. With DS-SS radio systems, single errors are dispersed randomly over time; with FH-SS radio systems, errors are distributed in clusters. Bursty errors are attributable to fading or single-frequency interference, which is time and frequency dependent. The DS-SS spreads the information in both the time and frequency domains, thus providing time and frequency diversity thereby minimizing the effects of fading and interference.
Hybrid DS/FH CDMA radio systems combine the advantages of the DS and FH radio systems. The hybrid DS/FH CDMA system spreads the bandwidth partially with direct sequence spreading method and partially with frequency hopping spreading method over the entire available spreading bandwidth.
FIG. 1
is a prior art illustration of a conventional transmitter unit in accordance with a hybrid DS/FH CDMA system. As illustrated in
FIG. 1
, data is first spread by a pseudo-noise sequence at multiplier
6
and the resulting signal is then multiplied at multiplier
8
by a carrier frequency, ƒ
h
(t) generated in frequency synthesizer
10
.
Recently, multi-carrier CDMA systems have separated the entire available frequency bandwidth into a plurality of sub-bands to transmit data. The multi-carrier method is advantageous in that it is possible to obtain frequency diversity as a result.
FIG. 2
illustrates a transmitting unit of a conventional multi-carrier DS CDMA system. It is shown at
FIG. 2
that the signal data is multiplied by a plurality of carrier frequencies, ƒ
1
, ƒ
2
, . . . ƒ
M
14
and then combined at a combiner unit
16
prior to transmission.
Bandwidth spreading utilizing only the direct sequence (DS-SS) method, is disadvantageous for a number of reasons including; first, the PN sequence chip rate requires complex hardware; second, a very fast signal processing algorithm is required to perform demodulation. The frequency synthesizer becomes complicated in the case of a spreading bandwidth that uses the entire available bandwidth with only the frequency hopping method.
Additionally, in the case of using a hybrid CDMA system in combination with a DS system and a FH system, it is possible to decrease the chip rate of the PN sequence and thereby make the frequency synthesizer less complicated. A tradeoff occurs, however, in that the combined system limits the number of users as compared to the number of users that may simultaneously access a DS CDMA system.
Another well known technique in the art is a multi-carrier DS CDMA system that transmits a narrow band DS spread spectrum signal using multiple carriers in contrast to the broadband single carrier DS CDMA system. The multi-carrier DS CDMA system makes it possible to decrease the system complexity while maintaining the same efficiency found in the single-cannier DS CDMA by substituting the advantages of the path diversity effect of the conventional RAKE receiver of a single carrier DS CDMA system for the advantages of a frequency diversity effect that is characteristic of a multi-carrier DS CDMA system.
However, in the multi-carrier DS CDMA system, each user is utilizing the entire frequency band, which is disadvantageous in that interference occurs in the whole frequency band due to other users.
SUMMARY OF THE INVENTION SUMMARY OF THE INVENTION
The present invention provides an apparatus for a parallel hopping hybrid DS/SFH CDMA system. Specifically, the present invention is a radio system that combines the characteristics of a conventional CDMA system with characteristics of a DS/CDMA system, an FH/CDMA system, and a multi-carrier CDMA system. The present invention advantageously supports more users than can be supported in a conventional CDMA system that supports multiple users.
REFERENCES:
patent: 4355399 (1982-10-01), Timor
patent: 5657343 (1997-08-01), Schilling
patent: 5812522 (1998-09-01), Lee et al.
Bernard Sklar “Digital Communications Fundamental and Application,” Prentice Hall, pp 555-570 1988.
Bocure Tesfaldet
Dilworth & Barrese LLP
Samsung Electronics Co., Inc.
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