Multiplex communications – Communication over free space – Repeater
Reexamination Certificate
1999-10-15
2003-12-16
Bost, Dwayne (Department: 2681)
Multiplex communications
Communication over free space
Repeater
C370S342000, C375S145000
Reexamination Certificate
active
06665277
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to wideband code division multiple access (WCDMA) for a communication system and more particularly to cell search for WCDMA using primary, secondary and tertiary synchronization codes.
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 net generation wideband code division multiple access (WCDMA) communication systems as described in U.S. Pat. No. 6,345,069, entitled
Simplified Cell Search Scheme for First and Second Stage
, issued Feb. 5, 2002, 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) sown 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 use 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 (IPC) 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.
A WCDMA base station must broadcast primary or first (FSC) and secondary (SSC) synchronization codes to properly establish communications with a mobile receiver. The FSC identifies the slot timing from the transmitting base station. The SSC further identifies a group of sixteen scrambling codes, one of which is assigned to the transmitting base station. Referring now to
FIG. 1
, there is a simplified block diagram of a circuit of the prior art for generating primary and secondary synchronization codes. These synchronization 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,−1,1}. 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 FSC 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 synchronization code symbols. These synchronization code symbols include the FSC and SSC symbols transmitted in parallel. These synchronization code symbols are not modulated by the cell-specific long code, so a mobile receiver can detect the FSC and SSC transmitted by an unknown base station. Proper identification of the FSC and SSC by the mobile receiver, therefore, limits the final search to one of sixteen scrambling codes that specifically identify a base station within the cell to a mobile unit.
Referring to
FIG. 2
, there is a match filter circuit of the prior art for detecting the FSC and SSC generated by the circuit of FIG.
1
. The circuit receives the FSC 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 16 respective PN signals to produce 16 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 16 output signals and adds them to produce a sequence of output signals at terminal
250
corresponding to the oversampling rate n.
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 16 taps
250
-
284
that produce 16 respective parallel tap signals. Inverters
285
invert tap signals corresponding to negative elements of the Lindner sequence. Adder circuit
286
receives the 16 output signals and adds them to produce a match signal MAT at output terminal
288
in response to an appropriate FSC or SSC.
Referring now to
FIG. 3A
, several problems arise with this method of FSC and SSC transmission and detection. During first step or first stage acquisition, the receiver must match the 256-chip FSC on a first perch channel to identify a base station transmission. In the second stage of acquisition, the mobile receiver must match the SSC to determine which of 32 possible groups of 16 synchronization codes (ScC) are being transmitted and complete frame synchronization by determining which of 16 time slots is the first in the frame. Finally, during third stage acquisition, the receiver must determine which of 16 codes in the code group is being transmitted. This detection scheme, therefore, limits the mobile receiver to identification of a maximum of
512
base stations corresponding to the 32 groups of 16 scrambling codes each. With the current proliferation of base stations and mobile receivers, however, this has become an unacceptable limitation. Furthermore, simply increasing the number
Bost Dwayne
Brady III Wade James
Neerings Ronald O.
Ramos-Feliciano Eliseo
Telecky , Jr. Frederick J.
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