Multistage PN code aquisition circuit and method

Details Multistage PN code aquisition circuit and method Multistage PN code aquisition circuit and method

2000-03-22

2004-12-14

Olms, Douglas (Department: 2661)

Multiplex communications

Communication techniques for information carried in plural...

Combining or distributing information via time channels

C375S152000

Reexamination Certificate

active

06831929

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to wideband code division multiple access (WCDMA) for a communication system and more particularly to detection of primary or secondary synchronization codes for WCDMA cell acquisition.
BACKGROUND OF THE INVENTION
Present code division multiple access (CDMA) systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected receiver to determine the proper recipient of a data signal. These different data signals arrive at the receiver via multiple paths due to ground clutter and unpredictable signal reflection. Additive effects of these multiple data signals at the receiver may result in significant fading or variation in received signal strength. In general, this fading due to multiple data paths may be diminished by spreading the transmitted energy over a wide bandwidth. This wide bandwidth results in greatly reduced fading compared to narrow band transmission modes such as frequency division multiple access (FDMA) or time division multiple access (TDMA).
New standards are continually emerging for next generation wideband code division multiple access (WCDMA) communication systems as described in U.S. patent application Ser. No. 90/217,759, entitled Simplified Cell Search Scheme for First and Second Stage, filed Dec. 21, 1998, and incorporated herein by reference. These WCDMA systems are coherent communications systems with pilot symbol assisted channel estimation schemes. These pilot symbols are transmitted as quadrature phase shift keyed (QPSK) known data in predetermined time frames to any receivers within the cell or within range. The frames may propagate in a discontinuous transmission (DTX) mode within the cell. For voice traffic, transmission of user data occurs when the user speaks, but no data symbol transmission occurs when the user is silent. Similarly for packet data, the user data may be transmitted only when packets are ready to be sent. The frames include pilot symbols as well as other control symbols such as transmit power control (TPC) symbols and rate information (RI) symbols. These control symbols include multiple bits otherwise known as chips to distinguish them from data bits. The chip transmission time (T
C
), therefore, is equal to the symbol time rate (T) divided by the number of chips in the symbol (N). This number of chips in the symbol is the spreading factor.
Previous WCDMA base stations have broadcast primary (PSC) and secondary (SSC) synchronization codes to properly establish communications with a mobile receiver. The PSC identifies the source as a base station within the cell. The SSC further identifies a group of synchronization codes that are selectively assigned to base stations that may transmit within the cell. Referring now to
FIG. 1
, there is a simplified block diagram of a circuit of the prior art for generating primary and secondary search codes. These search codes modulate or spread the transmitted signal so that a mobile receiver may identify it. Circuits
102
and
110
each produce a 256 cycle Hadamard sequence at leads
103
and
111
, respectively. Either a true or a complement of a 16-cycle pseudorandom noise (PN) sequence, however, selectively modulates both sequences. This 16-cycle PN sequence is preferably a binary Lindner sequence given by Z={
1
,
1
,−
1
,−
1
,−
1
,−
1
,−
1
,
1
,
1
,−
1
,
1
,
1
,
1
,−
1
,
1
}. Each element of the Lindner sequence is further designated z
1
-z
16
, respectively. Circuit
108
generates a 256-cycle code at lead
109
as a product of the Lindner sequence and each element of the sequence. The resulting PN sequence at lead
109
, therefore, has the form {Z,Z,−Z, −Z,−Z,−Z,Z,−Z,Z,Z,−Z,Z,Z,Z,−Z,Z}. Exclusive-OR circuit
112
modulates the Hadamard sequence on lead
111
with the PN sequence on lead
109
, thereby producing a PSC on lead
114
. Likewise, exclusive-OR circuit
104
modulates the Hadamard sequence on lead
103
with the PN sequence on lead
109
, thereby producing an SSC on lead
106
.
A WCDMA mobile communication system must initially acquire a signal from a remote base station to establish communications within a cell. This initial acquisition, however, is complicated by the presence of multiple unrelated signals from the base station that are intended for other mobile systems within the cell as well as signals from other base stations. The base station continually transmits a special signal at 16 KSPS on a perch channel, much like a beacon, to facilitate this initial acquisition. The perch channel format includes a frame with sixteen time slots, each having a duration of 0.625 milliseconds. Each time slot includes four common pilot symbols, four transport channel data symbols and two search code symbols. These search code symbols include the PSC and SSC symbols transmitted in parallel. These search code symbols are not modulated by the long code, so a mobile receiver need not decode these signals with a Viterbi decoder to properly identify the base station. Proper identification of the PSC and SSC by the mobile receiver, therefore, limits the final search to one of sixteen groups of thirty-two comma free codes each that specifically identify a base station within the cell to a mobile unit.
Referring to
FIG. 2
, there is a circuit of the prior art for detecting the PSC and SSC generated by the circuit of FIG.
1
. The circuit receives the PSC symbol from the transmitter as an input signal IN on lead
200
. The signal is periodically sampled in response to a clock signal by serial register
221
at an oversampling rate n. Serial register
221
, therefore, has 15*n stages for storing each successive sample of the input signal IN. Serial register
221
has
16
(N) taps
242
-
246
that produce 16 respective parallel tap signals. A logic circuit including 16 XOR circuits (
230
,
232
,
234
) receives the respective tap signals as well as sixteen respective PN signals to produce sixteen output signals (
231
,
233
,
235
). This PN sequence matches the transmitted sequence from circuit
108
and is preferably a Lindner sequence. Adder circuit
248
receives the sixteen output signals and adds them to produce a sequence of output signals at terminal
250
corresponding to the oversampling rate n. An oversampling rate of n=1, for example, corresponds to one sample per chip, 256 samples per symbol, 2560 samples per time slot, and 40,960 samples per frame.
A 16-symbol accumulator circuit
290
receives the sequence of output signals on lead
250
. The accumulator circuit
290
periodically samples the sequence on lead
250
in serial register
291
in response to the clock signal at the oversampling rate n. Serial register
291
, therefore, has 240*n stages for storing each successive sample. Serial register
291
has sixteen taps
250
-
284
that produce sixteen respective parallel tap signals. Inverters
285
invert tap signals corresponding to negative elements of the Lindner sequence. Adder circuit
286
receives the sixteen output signals and adds them to produce a match signal MAT at output terminal
288
in response to an appropriate PSC symbol. This match result is subsequently stored in a buffer memory. In this manner, the match filter circuit samples the entire PN sequence at lead
200
for one frame of 10 milliseconds. Thus, the match filter detects a PSC symbol common to each time slot and an SSC symbol corresponding to each respective time slot. After the 10-millisecond sample period of first-stage cell acquisition, the match filter is reset and repeats the match process for a next frame. The previously acquired time of the PSC symbol match as well as the sixteen SSC symbols are passed to a second-stage of cell acquisition.
Referring to
FIG. 3
, there is a diagram showing cell acquisition of the prior art. The first-stage cell acquisition begins at time
301
when the first-stage
300

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