Asynchronous CDMA decorrelating detector

• US 5,917,829 A
• Filed: 12/01/1997
• Issued: 06/29/1999
• Est. Priority Date: 12/04/1996
• Status: Expired due to Fees

• Current Assignee: Rafael - Armament Development Authority Ltd.
• Original Assignee: State Of Israel-Ministry Of Defense Rafael - Armament Development Authority
• Inventors: Hertz, David, Greenberger, Harvey J.
• Primary Examiner(s): Vu, Huy D.
• Assistant Examiner(s): BOAKYE, ALEXANDER O
• Application Number:
  US08/980,766
• Time in Patent Office: 575 Days
• Field of Search: 370/320, 370/335, 370/342, 370/441, 370/479, 370/515
• US Class Current: 370/479
• CPC Class Codes: H04B 1/707 using direct sequence modulation

Abstract

Method and apparatus for optimally decoding messages from CDMA signal sent by a plurality of users and received asynchronously at one receiver, using a minimal number of computations and minimal memory and processing resources. It is based on a novel approach to signal correlation, as well as to subsequent decoding by decorrelation, that requires a correlation period of only one symbol length, by providing for each user a pair of partial signatures sequences, with which the signal is correlated, deccorelating the results with the inverse of the cross-correlation matrix of all partial sequences, and combining the resultant partial symbol estimates--to obtain final estimated symbol values. The partial sequences are formed by separating each original sequence at a point corresponding to the estimated symbol boundary time, relative to an arbitrary correlation window, of one symbol length. The method can be modified to also apply to the case that any of the signals is received over multiple paths, any path possibly undergoing Doppler shift.

Claims

1. A method for detecting transmitted messages in signals received in one receiver from a plurality of users within a CDMA transmission system,

each user being characterized by a unique one of a plurality of signature sequences, all the signature sequences having an equal and constant duration T,

each received signal consisting of consecutive symbols, each being of duration T and carrying a message value, the points of time of the beginning and end of each symbol constituting symbol boundaries,

each received signal being associated with a complex amplitude and a delay, the delay being the time span between a common arbitrary point of time and a symbol boundary, and

the received signals, in addition to reception noise, collectively forming a total received signal,

the method comprising the steps of:

(a) defining a temporal sequence of consecutive correlation windows of duration T each, the beginning and end of each correlation window constituting correlation window boundaries;

(b) detecting, within the total received signal, signals sent by users, each detected signal being identified in terms of its signature sequences and associated with a corresponding user, this user being an active user;

(c) for each of said detected signals, deriving from the total received signal an estimated symbol delay, <

τ

>

, associated with the corresponding active user, said estimated symbol delay being the estimated duration of time from any one of said correlation window boundaries to the next occurring symbol boundary;

(d) partitioning each signature sequence that corresponds to one of said active users exactly into a head portion and a tail portion, the head portion preceding the tail portion and the duration of the tail portion being essentially equal to the corresponding one of said estimated symbol delays;

(e) constructing for each of said active users a pair of partial sequences of duration T each, one of said partial sequences being a tail sequence and the other one being a head sequence, each of said partial sequences consisting of a leading portion and a succeeding trailing portion, whereby the leading portion of said tail sequence is essentially identical to said tail portion of the corresponding signature sequence, the trailing portion of said head sequence is essentially identical to said head portion of the corresponding signature sequence and the other portion of each of said partial sequences is uniformly zero;

(f) for each of said active users, correlating the total received signal, over the span of each of said correlation windows, with one or both of the corresponding pair of said partial sequences, the result being one or two corresponding series of correlation values; and

(g) calculating from said correlation values, obtained for a plurality of users, estimated message values of symbols in received signals corresponding to one or more of said active users.

2. The method of claim 1, further comprising the step of comparing, for each of said active users, said estimated symbol delay, <

τ

>

, and its complement, T-<

τ

>

with a threshold value, τ

_th, and wherein in step `f`, said one or both of the corresponding pair of said partial sequences consists of

said head sequence only, if <

τ

>

<

τ

_th,

said tail sequence only, if <

τ

>

>

T-τ

_th, or

both partial sequences, otherwise.

3. The method of claim 1, wherein in step (g), for each active user for whom in step `f` said one or both is both, said calculating includes, for each symbol in the corresponding received signal, the substeps of

(i) calculating two partially estimated symbol values, one of said partially estimated symbol values, it being a tail estimated value, being calculated from those of said correlation values that are obtained from correlation with tail sequences and the other one of said partially estimated symbol values, it being a head estimated value, being calculated from those of said correlation values that are obtained from correlation with head sequences;

and

(ii) weight-averaging said two partially estimated values.

4. The method of claim 3, wherein in substep `ii`, weight-averaging includes giving to said tail estimated value a weight essentially equal to <

τ

>

/T and to said head estimated value--a weight essentially equal to 1-<

τ

>

/T.

5. The method of claim 3, further comprising the step of constructing a decorrelation matrix based on said partial sequences and wherein in substep (i) of step (g) said calculating includes multiplying said correlation values by appropriate elements of said decorrelation matrix.

6. The method of claim 1, further comprising the step of constructing a decorrelation matrix based on said partial sequences and wherein in step (g) said calculating includes multiplying said correlation values by appropriate elements of said decorrelation matrix.

7. The method of claim 6, wherein said step of constructing a decorrelation matrix includes the sub step of estimating the level of the reception noise, calculating therefrom a noise decorrelation element and adding said noise decorrelation element to elements of said decorrelation matrix.

8. The method of claim 1, further comprising the step of deriving from the total received signal, for any of said active users, an estimated amplitude, associated with the corresponding received signal, and wherein in step (g) said calculating includes dividing by the corresponding one of the thus estimated amplitudes.

9. The method of claim 8, wherein said amplitude is complex amplitude.

10. The method of claim 1, wherein in step `c` said deriving includes the substeps of

(i) obtaining consecutive samples of the total received signal over a plurality of symbol durations, the spacing of said samples being a sufficiently high sub-multiple of T;

(ii) obtaining samples of the corresponding signature sequence at a spacing equal to that of the samples of substep `i`;

(iii) deriving a sampled correlation function between the samples of substep `i` and the samples of substep `ii` for all possible time relations therebetween;

(iv) summing all samples of said correlation function that are spaced exactly T apart, for all possible time relations between said samples and any of said correlation windows; and

(v) determining the particular one of the time relations of substep `iv` for which said sum is greatest.

11. The method of claim 2, wherein the absolute time of any of said correlation window boundaries is chosen so that for each of a substantial number of said active users, the corresponding value of <

τ

>

is either between 0 and τ

_th or between T-τ

_th and T.

12. A method for detecting transmitted messages in signals received in one receiver from a plurality of users within a CDMA transmission system, the signal received from at least one user arriving with significant strength over a plurality of paths of unequal lengths, the signal arriving from any one user over any one path constituting a subsignal,

each user being characterized by a unique one of a plurality of signature sequences, all the signature sequences having an equal and constant duration T,

each received subsignal consisting of consecutive symbols, each being of duration T and carrying a message value, the points of time of the beginning and end of each symbol constituting symbol boundaries,

each received subsignal being associated with a complex amplitude and a delay, the delay being the time span between a common arbitrary point of time and a symbol boundary, and

the received subsignals collectively forming a total received signal,

the method comprising the steps of

a) determining a temporal sequence of consecutive correlation windows of duration T each, the beginning and end of each correlation window constituting correlation window boundaries;

b) identifying users in terms of their signature sequences, each thus identified user being an active user;

c) for each of said active users, deriving from the total received signal an estimated complex amplitude and an estimated symbol delay, associated with each corresponding received subsignal, said estimated symbol delay being the estimated duration of time from any one of said correlation window boundaries to the next occurring symbol boundary;

d) for each of said active users and each of the corresponding ones of said subsignals, dividing the corresponding signature sequence exactly into a head portion and a tail portion, the head portion preceding the tail portion and the duration of the tail portion being essentially equal to the corresponding one of said estimated symbol delays;

e) for each of said active users and each of the corresponding ones of said subsignals, constructing a pair of component partial sequences of duration T each, one of said component partial sequences being a component tail sequence and the other one being a component head sequence, each of said component partial sequences consisting of a leading portion and a succeeding trailing portion, whereby the leading portion of said component tail sequence is essentially identical to said tail portion of the corresponding signature sequence, the trailing portion of said component head sequence is essentially identical to said head portion of the corresponding signature sequence and the other portion of each of said component partial sequences is uniformly zero;

f) for each of said active users, constructing a pair of partial sequences of duration T each, one of said partial sequences being a tail sequence and the other one being a head sequence, by appropriately combining all corresponding ones of said component partial sequences;

g) for each of said active users, correlating the total received signal, over the span of each of said correlation windows, with one or both of the corresponding pair of said partial sequences, the result being one or two corresponding series of correlation values;

h) calculating from said correlation values, obtained for a plurality of users, estimated message values of symbols in received signals corresponding to one or more of said active users.

13. The method of claim 12, wherein at least one of the paths has a continuously varying length and wherein step `c` also includes deriving an estimated Doppler frequency shift.

14. Apparatus for detecting transmitted messages in signals received in one receiver from a plurality of users within a CDMA transmission system,

each user being characterized by a unique one of a plurality of signature sequences, all the signature sequences having an equal and constant duration T,

each received signal consisting of consecutive symbols, each being of duration T and carrying a message value, the points of time of the beginning and end of each symbol constituting symbol boundaries,

each received signal being associated with a complex amplitude and a delay, the delay being the time span between a common arbitrary point of time and a symbol boundary, the received signals collectively forming a total received signal,

the apparatus comprising:

a) a preprocessor operative to

determine a temporal sequence of consecutive correlation windows of duration T each, the beginning and end of each correlation window constituting correlation window boundaries;

identify users in terms of their signature sequences, each thus identified user being an active user;

derive from the total received signal, for each of said active users, an estimated symbol delay, <

τ

>

, associated with the corresponding received signal, said estimated symbol delay being the estimated duration of time from any one of said correlation window boundaries to the next occurring symbol boundary;

partition each signature sequence that corresponds to one of said active users exactly into a head portion and a tail portion, the head portion preceding the tail portion and the duration of the tail portion being essentially equal to the corresponding one of said estimated symbol delays; and

construct, for each of said active users, a pair of partial sequences of duration T each, one of said partial sequences being a tail sequence and the other one being a head sequence, each of said partial sequences consisting of a leading portion and a succeeding trailing portion, whereby the leading portion of said tail sequence is essentially identical to said tail portion of the corresponding signature sequence, the trailing portion of said head sequence is essentially identical to said head portion of the corresponding signature sequence and the other portion of each of said partial sequences is uniformly zero;

b) a correlation processor operative, for each active user whose symbols are to be detected, to inner-vector multiply the total received signal, over the span of each of said correlation windows, by one or both of the corresponding pair of said partial sequences, and to sum the products of each such multiplication, to yield a respective pair of correlation values;

c) a detection processor operative on a plurality of said correlation values to calculate therefrom estimated message values of symbols in signals received from users.

15. The apparatus of claim 14, wherein said detection processor is further operative to calculate, for any of said active users and for each symbol in the corresponding received signal, two partially estimated symbol values and to weight-average them.

16. The apparatus of claim 14, wherein said preprocessor is further operative to construct a decorrelation matrix based on said partial sequences, and wherein said detection processor is further operative to multiply said correlation values by appropriate elements of said decorrelation matrix.