Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-09-07
2001-12-11
Vo, Don N. (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S343000
Reexamination Certificate
active
06330274
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains generally to structures and methods for correlating signals, and more particularly to a structure and method for performing continuous-time analog correlation of signals in direct sequence spread-spectrum communication systems.
BACKGROUND
As the information super-highway continues its frantic expansion, an increasing portion of the computer and communications systems being deployed and in development utilize wireless technologies. One of the key signaling techniques being used in many of these systems is direct sequence code-division multiple access (DS-CDMA) which is one method of spread-spectrum. (See references [1]-[4], hereby incorporated by reference.) Monolithic implementations of receivers for spread spectrum systems typically implement much of the signal processing in the digital domain (See references [1]-[3]). Two important parameters of such implementations are chip cost and power consumption; power consumption being of particular importance in portable applications in order to prolong battery life. For low chip cost, minimum die size in digital CMOS technologies is desirable.
Many systems used in wireless technologies utilize a correlator for correlating the spread-spectrum data signal. One conventional approach for analog implementation of the correlator is to utilize Surface Acoustic Wave (SAW) filters. However, these are not amenable to integrated circuit implementation and are not economically attractive. Charge Coupled Devices (CCD) are another analog approach used to correlate signals. CCDs can be integrated into standard CMOS technology. However, CCDs: i) require additional steps of manufacturing, increasing overall cost; ii) require high-voltage clocking which results in increased power consumption; and iii) require high-voltages are undesirable in battery and portable applications. Another approach to the correlation function is what is referred to as Ad Hoc Mixed-Signal Implementations. These ad hoc approaches greatly compromise the signal processing by implementing gross approximations of the required signal processing, resulting in large performance losses. These ad hoc approaches are less attractive than digital implementations which perform better and can be more easily integrated. Another approach for analog implementation of the correlator is described in K. Onodera and P. Gray (See reference [5]), which uses Switched-Capacitor (SC) techniques. The incoming signal is sampled at twice the chip rate onto capacitors and the sampled voltages are subsequently summed in the charge domain. The SC technique described by K. Onodera and P. Gray underscore some low-power advantages of analog processing by placing the analog-to-digital converter after the correlation, allowing a lower-sampling rate and thus lower-power analog-to-digital converter. However, conventional SC sampling arrays require large areas on the chip die which translates to higher manufacturing cost.
By far the most widespread correlation technique used today is through digital implementation. Analog-to-digital (A/D) converters are used to convert an analog input signal to the digital domain where correlation and all other processing is performed. These techniques are effective for low bit rate systems. But as data rates increase, the sampling rate of the AID converter increases much faster. Thus, digital implementations for future higher rate systems will required an increased power consumption which is undesirable for battery operated portable systems. The performance of the digital correlators is usually compromised by the use of a smaller number of bits in the A/D conversion and processing which affects the size and power of the circuit implementation. The use of a smaller number of bits also compromises the attainable signal processing performance of DS-CDMA signaling. AID converters use smaller number of bits prior to the correlation of the signal that reduces the robustness to continuous-wave interferers. The small number of bits in the A/D conversion makes the system very sensitive to the input dynamic range, requiring complex transmitting power control mechanisms along with the need for a high performance gain control mechanism in the receivers. Other descriptions of spread spectrum techniques are available in the technical literature, including for example, a review article provided in R. Dixon,
Spread Spectrum Systems with Commercial Applications
, Third Edition, John Wiley & Sons, Inc., 1994, which is hereby incorporated by reference.
Partial List of Relevant Literature
[1] C. Chien, P. Yang, et. al., “A 12.7Mchip/s all-digital BPSK direct sequence spread-spectrum IF transceiver in 1.2 &mgr;m CMOS”,
ISSCC
1994
Digest of Tech. Papers
, vol. 39, pp. 30-31, February 1994.
[2] S. Sheng, L. Lynn, et. al., “A low-power CMOS chip set for spread-spectrum communications,”
ISSCC
1996
Digest of Tech. Papers
, vol. 39, pp. 346-347, February 1996.
[3] B. Chung, et. al., “Performance analysis of an all digital BPSK direct sequence spread spectrum IF receiver architecture,”
IEEE J. Selected Areas in Communication
, vol. 11, pp. 1096-1107, September 1993.
[4] R. Dixon,
Spread Spectrum Systems with Commercial Applications
, Third Edition, John Wiley & Sons, Inc., 1994.
[5] K. Onodera and P. Gray, “A 75 mW 128 MHz DS-CDMA baseband correlator for high-speed wireless applications,” 1997 VLSI
Circuit Symposium Dig. of Tech. Papers
, Kyoto, Japan, June 1997.
[6] B. Gilbert, “A precise four quadrant multiplier with sub-nanosecond response,”
IEEE J. Solid-State Circuits
, pp. 365-373, December 1968.
[7]
Understanding GPS Principles and Applications
, Editor: Elliott D. Kaplan; Chapter 6, “Effects of RF Interference on GPS Satellite Signal Receiver Tracking,” pp. 227-231, Chapter by Phillip Ward, Artech House Publishers, 1996.
SUMMARY OF THE INVENTION
This invention provides a novel integrated circuit structure, method of correlating, and method for the design of a correlator including embodiments for a correlator for DS-CDMA spread-spectrum systems in the analog domain. The inventive structure and method provide both for significantly lower power consumption than known conventional digital correlator implementations and a relatively small required die area. The inventive structure and method uses a different approach and implements the correlation using continuous-time processing. The result is an overall simpler implementation with a lower power consumption and minimal die area, as compared to heretofore known techniques.
While there are many functions involved in the implementation and use of a spread spectrum receiver, this invention disclosure focuses on implementation of important aspects of the inventive correlator structure and method, particularly those aspects involving the multiplication and integrate-and-dump functions.
In one aspect the invention includes a correlator structure which receives two input signals, the receive baseband signal which is the demodulated receive signal after the RF carrier has been removed and a PN code signal. The correlator includes a multiplier coupled to an integrate-and-dump circuit. The multiplier multiplies the two input signals and produces a multiplier output current that is integrated by an integrate-and-dump function which produces a voltage which is proportional to the correlation between the two input signals. This voltage is the correlator output voltage.
In another aspect the invention includes a correlator method of multiplying the receive baseband signal with a locally generated PN code signal producing a multiplied output current. Then integrating the multiplied output current onto a continuous-time and switched capacitor circuit thus producing a correlator output voltage. Other aspects are shown in the drawings and described in the detailed description.
REFERENCES:
patent: 4346475 (1982-08-01), Alexis
patent: 4400790 (1983-08-01), Chambers et al.
patent: 4475208 (1984-1
Flehr Hohbach Test Albritton & Herbert LLP
University of Hawai'i
Vo Don N.
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