Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-06-03
2001-10-23
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S144000
Reexamination Certificate
active
06307878
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to cellular telephony and, more particularly, to a searcher for a DSSS cellular telephony system.
In a DSSS cellular telephony system, the base stations identify themselves by transmitting pilot signals. Each pilot signal is a sequence of zero bits, modulated, according to the principles of DSSS encoding, by a pseudonoise (PN) sequence, or an extended pseudonoise sequence.
For example, under the IS-95 interim standard, the PN sequence is 2
15
chips long, with the n-th chip including an in-phase component i(n) and a quadrature component q(n). The initial values of i and q are i(1)=q(1)=1 and i(n)=q(n)=0 for 2≦n≦15. Subsequent values of i and q, up to n=2
15
−1, are obtained recursively as follows:
i(n)=i(n−15)+i(n−10)+i(n−8)+i(n−7)+i(n−6)+i(n−2) (1)
q(n)=q(n−15)+q(n−12)+q(n−11)+q(n−10)+q(n−9)+q(n−5)+q(n−4)+q(n−3) (2)
where the additions are modulo 2. Finally, i(2
15
)=q(2
15
)=0.
The same PN sequence is used by each of the base stations. The base stations are synchronized; and each base station uses the PN sequence with a different delay (also called “PN offset”) to produce the pilot signal. This enables the mobile units of the cellular telephony network to distinguish one base station from another.
The total signal received by a mobile station, as a function of time t, is:
RX
(
t
)
=
∑
b
=
1
B
∑
m
=
1
M
b
C
(
b
,
m
,
t
)
·
PN
(
t
+
offset
(
b
)
+
τ
(
b
,
m
)
)
·
[
1
+
∑
i
=
1
I
b
α
i
·
D
(
i
,
b
,
t
)
·
W
(
i
,
b
)
]
+
N
(
3
)
Here, b indexes the B base stations; m indexes the M
b
transmission paths (multipath channels) from base station b to the mobile station; C is the channel gain of multipath channel m; &tgr; is the additional delay introduced to the PN sequence by multipath channel m; the “1” inside the brackets represents the sequence of zeros that is modulated by the base stations to produce the pilot signals; i indexes the I
b
other users that are transmitting via base station b at time t; &agr; is the power of user i relative to the pilot signal; D is the data transmitted by user i; W is a code sequence (for example, a Hadamard code sequence) that is used in addition to the PN sequence to modulate data D and allow simultaneous transmission on the same physical channel by all the users in addition to the pilot signals; and N is additive noise.
Each mobile unit of the cellular telephony network determines which base station to communicate with (typically, the nearest base station) by correlating this signal with the PN sequence at a set of trial delays. Because data D are modulated by sequences W, the correlation of the part of the signal that comes from other users is negligible. The correlation with the pilot signals also is negligible, except at trial delays that are equal to the PN offsets used by the base stations, as modified by multipath delays &tgr;. Specifically, a pilot signal that arrives at a delay, that is equal to the sum of a base station offset and one of the multipath delays &tgr; associated with transmissions from that base station, gives a significant contribution to the correlation at a matching trial delay; and all other pilot signals contribute negligibly to the correlation at that trial delay. This correlating is performed when the mobile station powers up, and continuously thereafter, to allow hand over from one base station to another when the mobile station crosses a cell boundary. The delays of the various base stations are well separated, by more than the largest anticipated multipath delay, so in the absence of additive noise and in the absence of multipath delays, only a small number of correlations, equal to the number of potential nearest base stations, would have to be performed, to identify the base station whose delay gives the highest correlation as the nearest base station. According to the IS-95 standard, this separation is at least 256 chip durations T
c
. Because the pilot signals and data D are received by the mobile station from each base station via several paths at different delays (PN offset+&tgr;), the various replicas of the signals thus received are combined to suppress the deterministic noise represented by the various multipath delays &tgr;. For example, maximal ratio combining is the optimal combination method in a bit error rate and frame error rate sense. In order to do this combining, the multipath delays must be determined. Therefore, the correlation is performed at a series of delays in a window centered on the nominal delay. The size of this window depends on the local topography, and is provided to the mobile unit by the base station. One typical window size, according to the IS-95 standard, is 60 chip durations.
FIG. 3
is a schematic block diagram of a mobile station receiver
30
. RF signals are received by an antenna
60
, down converted to an intermediate frequency (IF) by a down converter
62
, filtered by a bandpass filter
64
(typically a surface acoustic wave filter) to eliminate signals outside the required bandwidth, and amplified by an automatic gain control
66
. The amplified IF signals are multiplied by an IF sinusoid
65
, without (block
68
i
) and with (block
68
q
) a 90° phase shift 67, to produce an in-phase signal I and a quadrature signal Q. In-phase signal I is filtered by a low-pass filter
70
i
and digitized by an A/D converter
72
i
. Similarly, quadrature signal Q is filtered by a low-pass filter
70
q
and digitized by an A/D converter
72
q
. A searcher
80
receives the digitized signals and performs the correlations needed to determine the various multipath delays &tgr; inside the target window. The digitized signals are again correlated, at the delays determined by searcher
80
, by the correlators of a correlator bank
74
, and the outputs of correlator bank
74
are combined, in a maximal ratio sense, in a rake combiner
76
to produce the final output signal.
In order to ensure uninterrupted communication as a mobile station crosses from one cell to another, the correlations performed by searcher
80
must be performed rapidly. In fact, it is not necessary to perform the full correlation at each delay in the window. It suffices to perform a correlation that is only long enough to ensure a high detection probability at the right delay and a low false alarm probability at the wrong delay. Typically, the length of the correlation, measured as a multiple N of the chip duration T
c
, is between 500T
c
and 2000T
c
.
To make the correlations even more efficient, the dual dwell algorithm is used. At each delay in the window, the correlation is performed for a number M of chip durations that is less than N. Only if the correlation value after M chip durations exceeds a certain threshold is the correlation performed for the full N chip durations. The threshold, and the parameters N and M, are chosen to maximize the detection probability while minimizing both the false alarm probability and the time spent correlating. See, for example, M. K. Simon, J. K. Omura, R. A. Scholtz and B. K.
Levitt,
Spread Spectrum Communication, Vol. III
, Computer Science Press, 1989, chapter 1, particularly section 1.3, and D. M. Dicarlo and C. L. Weber, “Multiple dwell serial search: performance and application to direct sequence code acquisition”,
IEEE Transactions on Communications
vol. COM-31 no. 5 pp. 650-659, May 1983. In the prior art implementation of this algorithm, several correlators are used by searcher
80
to correlate the received pilot signal with the PN sequence at several adjacent delays in the window. If none of the correlation values exceeds the threshold after M chip durations, then the correlators are used to correlate the received pil
Ben-Eli David
Sokolov Dotan
DSPC Technologies LTD
Friedman Mark M.
Pham Chi
Tran Khai
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