Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-11-12
2001-03-13
Le, Amanda T. (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C370S342000
Reexamination Certificate
active
06201828
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to spread-spectrum radio communication systems such as CDMA systems, and particularly to synchronization of a receiver's despreading codes with the spreading codes used at a transmitter to spread the received signal, and most particularly to refining and maintaining the accuracy of synchronization of a receiver's despreading codes with a received signal.
BACKGROUND OF THE INVENTION
CDMA (code-division multiple access) systems are well known. See, generally,
CDMA Cellular Mobile Communications and Network Security
, Dr. Man Young Rhee, Prentice Hall 1998, ISBN 0-13-598418-1, and standard TIA/EIA/IS-95, hereinafter “IS-95”.
In CDMA transmission between a base station and a plurality of mobile stations, all transmissions share all the bandwidth all the time. (Typically, all the transmissions from mobile stations to base share one band, and all transmissions from base to mobiles share another band.) Each mobile station's data stream is multiplied by binary codes (pseudonoise codes, or PN codes) in order to spread its spectrum, and typically is orthogonally modulated (as by Walsh-code modulation) to transform it from a series of bits into a series of PN “chips”. Broadband signal received at the base station is converted from radio frequency back to baseband. At the base station signals from a plurality of mobile stations are, in effect, summed.
To decode the data stream from a particular mobile station, it is necessary to “correlate” the spread-spectrum signal with the same codes that the particular mobile station used for spreading its data stream. For this purpose, the base station is equipped with the same kinds of code generators as the mobile station and is able to produce “local replicas” of each base station's PN codes.
It remains to get the local replicas synchronized with the codes in a received signal. The received signal is subject to transmission delays, and the transmission delays may vary, especially if the mobile user is in motion. Furthermore, the received signal may include several multipath components from a mobile user. State-of-the-art base stations employ “rake” receivers which correlate each of the multipath components and recombine them, requiring that the transmission delay of each one be determined.
A base station typically includes an apparatus known as a “searcher” which can determine the delay of a signal from a mobile user, or under multipath conditions the delays of several multipath components of a signal from a mobile user, by attempting to correlate through a range of various amounts of delay and determining the amount of delay at which the best correlation occurs. The range of delay over which attempts are made is known as the “uncertainty region” and is chosen to include the range of greatest expected delay.
A typical state-of-the-art searcher can determine the transmission delay of a component within one-half of a PN chip duration. The searcher depicted in
FIG. 1
, for example, employs a plurality of correlators to correlate the signal against various delay amounts of the local replica PN codes against the input signal, each sequence being delayed from the previous one by an amount “delta”. Most of the correlations will be “noise” except for those that correspond to the signal (including a multipath component) coded with that PN code which will produce a value above the noise. The transmission delay of each component can then be determined with a granularity of “delta” taken as one-half a PN chip duration for the present discussion.
A signal correlated against a perfectly synchronized local replica would yield the maximum signal-to-noise ratio (SNR) attainable from that signal, and SNR would decrease as synchronization decreases. For example, for a discrepancy of one-half a PN chip in determining delay, SNR could decrease up to 2.5 db. To maximize performance, particularly regarding SNR of received signals, there is a need to determine transmission delay with accuracy significantly greater than one-half of a PN chip duration.
It is also necessary to “track” the transmission delay of the components, as it may vary during a user's call, particularly if he is in motion. With reference to pages 901 et. seq. of
Spread Spectrum Communications Handbook
, M. K. Simon, J. K. Omura, R. A. Scholz and B. K. Levitt, Second Edition, 1994, one finds that there are two PN tracking loop configurations in predominant use, namely, the delay-locked loop (DLL) and tau-dither loop (TDL). Both of them fall within the class of early-late gate type loops in that the received PN code is correlated either simultaneously or alternately with delayed and advanced version of the receiver local code PN generator output to produce the error correcting characteristic. Both are complex and calculation-intensive.
The tracking problem is compounded by frequency-selective fading, which degrades the performance of the prior-art tracking loops. In addition, new requirements of emerging systems with intensive data traffic are adding considerable complexity, and therefore more system cost. There is thus a need for a delay tracking loop that introduces little complexity and cost.
It is thus an object of the present invention to determine the transmission delay of a spread-spectrum signal to an accuracy of at least within one-eighth of a PN chip duration at low cost and with low complexity.
It is a further object of the present invention to track the transmission delay of an ongoing spread-spectrum signal at low cost and with low complexity.
These and other objects of the invention will become apparent to those skilled in the art from the following description thereof.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, these and other objects may be accomplished by the present systems and methods of generating despreading codes with refined estimates transmission delays in CDMA systems. An embodiment of the present invention includes correlating a CDMA signal with PN codes at an estimated delay, an increment earlier than the estimated delay, and the same increment later than the estimated delay, and interpolating the early and late correlations normalized by the estimated delay and the increment to obtain a refined estimate of delay. A practice of the invention makes these correlations with values available in a conventional searcher for refining an initial estimate of transmission delay. Another practice of the invention provides a delay-locked loop for inclusion in a rake receiver to dynamically track the transmission delay.
The invention will next be described in connection with certain exemplary embodiments; however, it should be clear to those skilled in the art that various modifications, additions and subtractions can be made without departing from the spirit or scope of the claims.
REFERENCES:
patent: 5796775 (1998-08-01), Aoyama
patent: 5867525 (1999-02-01), Giallorenzi et al.
patent: 5926502 (1999-07-01), Schilling
Spread Spectrum Communications Handbook, M.K. Simon, J.K. Omura, R.A. Scholz, and B.K. Levitt, Second Edition, 1994. pp. 901-903, 915.
El-Tarhuni Mohamed G.
Kotov Anatoli V.
Cobrin & Gittes
Le Amanda T.
Nortel Networks Limited
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