Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1999-02-10
2003-04-29
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
Reexamination Certificate
active
06556634
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the area of code division, multiple access (CDMA) wireless communication systems, and, more specifically, to decoding signals by a novel maximum likelihood sequence estimation decoder.
BACKGROUND OF THE INVENTION
According to the IS 95 CDMA standard, a base station transmitter receives communication content and translates the content into symbols. The symbols are expanded by a spreading factor to a greater number of chips (the basic measurement of time in a CDMA system) and the chips are scrambled using an access code. The signal is then transmitted along with many other signals on the same frequency. The signal propagates through a dispersive medium and arrives at a receiver via multiple paths (also known as channels), each having a different propagation delay. As a result, each signal is received at the receiver overlapping with copies of itself, and each copy is delayed by one or more chips.
In most radio communications systems, such multi-path signals are out of phase with each other, which causes the signal to interfere with itself, resulting in a drop in received energy and concomitant loss of signal quality. In a CDMA system, in theory, these multi-path signals are added together to increase the signal strength, thus raising the quality of the received signal. To this end, several receivers are used simultaneously in a configuration known as a “rake” receiver, where each receiver.is a “finger” of the “rake.” Each finger is timed to receive a different multi-path signal. The signal received at each finger of the rake receiver is delayed to align with the last arriving signal. All of the signals are then added together to increase signal strength. Thus, a rake receiver is designed to receive signals that have propagated through multi-path channels and rely on the so-called processing gain of the combined multi-path signals to suppress interference between the multiple paths.
While the signal strength may be improved by processing gain, symbols may still be incorrectly interpreted because the frequency of the signal may have changed due to multi-path and other well known effects (i.e., Doppler shift, etc.). In some systems, previously-decoded data symbols are used to establish what the channel error (also called “frequency shift” or “frequency error”) must have been immediately after decoding the last data symbol and immediately prior to decoding the next data symbol, thus forming a coherent reference. However, coherent references formed in this manner are inaccurate if a previously decoded symbol was in error, leading to error propagation. Therefore, IS 95 transmits a pilot code that is modulated only with known symbols to permit receivers to obtain a coherent reference that is independent of the unknown data symbols being decoded. The pilot code method is only suitable if the pilot code and all information bearing coded signals are transmitted from the same antenna.
Future CDMA systems may wish to exploit smart antenna beam forming by creating a directive beam specific to each signal. Then each signal must carry known symbols with which to establish a per-beam coherent reference. If the proportion of known symbols to unknown symbols is too small, then the coherent reference will be noisy; on the other hand, if the proportion of known to unknown symbols is too large, then the overhead creates inefficiency. This deficiency is resolved if an improved method of using unknown symbols is used to refine the coherent reference or channel estimates.
Prior art rake receivers for Code Division Multiple Access signals are described, for example, in U.S. Pat. No. 5,305,349 to Applicant entitled “Quantized-Coherent Rake Receiver” and in U.S. Pat. No. 5,572,552 to Dent and Bottomley, entitled “Method and System for Demodulation of Downlink CDMA Signals,” both of which are incorporated herein by reference. Prior art rake receivers, while being designed to receive signals that have propagated through multipath channels, assume that the processing gain upon despreading the signal is sufficient to suppress interference between the multiple paths. Moreover, prior art coherent rake receivers assume a means to obtain a coherent reference that is independent of the unknown data symbols being decoded. For example, in the U.S. CDMA cellular system based upon the IS 95 standard, base stations transmit a special coded signal known as the pilot, specifically intended to provide a coherent reference for the mobile rake receivers. In U.S. Pat. No. 5,187,619 to Applicant entitled “CDMA Subtractive Demodulation” which is herein incorporated by reference, and in the already incorporated '349 patent, previously decoded data symbols are used to establish what the received signal phase must have been immediately after decoding the last data symbol and immediately prior to decoding the next data symbol, thus forming a coherent reference. However, coherent references formed in the latter manner are inaccurate if a previously decoded symbol was in error, leading again to error propagation.
Other data symbol-assisted methods to provide coherent references exist in the prior art. In particular, for non-CDMA signals, Gudmundson describes, in U.S. Pat. No. 5,164,961, how to determine coherent references for all possible sequences of a limited number of successive symbols and how to use those references in deciding which sequences to retain as decoded sequences. In Gudmundson, later symbol decisions are not used to improve the measure of likelihood for the decoded symbol sequence. However, in U.S. Pat. Nos. 5,557,645 and 5,619,533 to Dent, entitled “Channel Independent Equalizer Device,” a method is disclosed whereby the likelihood measure for the decoded symbol sequence is periodically updated to be the value it would have been had the most recent symbol decisions been available from the beginning. Neither the Dent or Gudmundson patents, however, disclose how to apply those techniques to CDMA signals, which is an objective of the invention described below.
When a successive decoder such as described in the above '961, '645 and '533 patents are decoding fading signals, there can be advantages in dynamically selecting between forward and reverse time order for decoding, as disclosed in U.S. Pat. No. 5,335,250 to Applicant, which is herein incorporated by reference.
Pilot symbol-assisted methods to provide coherent references exist in the prior art. In non-CDMA systems, coherent references are determined for all possible sequences of a limited number of successive symbols, which are then used in deciding which sequence to retain as the decoded sequence. In one such system, the likelihood measure for the decoded symbol sequence is periodically updated to the value it would have been had the most recent symbol decisions been available from the beginning.
This invention is directed toward overcoming one or more of the problems set forth above.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, a method for decoding symbols in a received signal is disclosed for use in a receiver that receives signals carrying symbols over a plurality of channels, wherein the symbols are selected from a predefined set of symbols. The receiver has a memory storing a plurality of previously hypothesized sequences of symbols and a plurality of cumulative vectors, each describing one of the sequences of symbols. The method comprises the steps of generating a plurality of correlated vectors by correlating the received signal over a plurality of time offsets equal in number to the plurality of correlated vectors, the plurality of time offsets corresponding to selected ones of the plurality of channels, and generating a set of new symbol vectors by combining each of the plurality of correlated vectors with each one of the set of symbols. The method further includes hypothesizing a plurality of extended sequences of symbols by combining each of the plurality of new symbol vectors with each of the plurality of cumulative vectors and determining the most li
Burd Kevin M
Ericsson Inc.
Moore & Van Allen PLLC
Pham Chi
Phillips Steven B.
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