Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-06-30
2002-03-26
Olms, Douglas (Department: 2661)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S335000, C370S329000
Reexamination Certificate
active
06363060
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to wireless communications. More particularly, the present invention relates to an improved method of achieving synchronization with, and identifying a received signal in an asynchronous code division multiple access (CDMA) system.
II. Description of the Related Art
The International Telecommunications Union recently requested the submission of proposed methods for providing high rate data and high-quality speech services over wireless communication channels. One of the proposals was issued by the European Telecommunications Standards Institute (ETSI), entitled “The ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT Candidate Submission”, hereafter referred to as WCDMA. The contents of these submissions is public record and is well known in the art, and describes the use of PERCH channels in a WCDMA system as discussed herein.
FIG. 1
illustrates the parts of a frame transmitted on the WCDMA PERCH channel by each base station in a WCDMA communication system used to permit the mobile station to acquire synchronization with the base station.
A frame is 10 milliseconds in duration and consists of 40,960 chips. A frame is divided into 16 slots, each slot having 2560 chips. Each slot can then be thought of as being divided into 10 consecutive parts, each part consisting of 256 chips. For the purposes of this disclosure, the 10 parts of each slot are numbered from 1 to 10, with 1 being the earliest transmitted 256 chips of each slot.
The first 256 chips (part 1) of each slot in the frame consist of two orthogonal sequences, which are transmitted on top of one another. The first of the two orthogonal sequences is the primary synchronization code (PSC) sequence. The PSC sequence is the same sequence for every slot and for every base station in a WCDMA system. The second of the two orthogonal sequences transmitted in part 1 is the secondary synchronization code (SSC). One of seventeen possible SSC sequences is transmitted in each slot.
Parts 2 through 5 of each slot include broadcast data such as the system identity of the transmitting base station and other information that is of general use to all mobile stations in communication with that base station. Parts 6 through 10 of each slot are used to carry a pilot signal that is generated in accordance with an Orthogonal Gold code as defined by the aforementioned UTRA standard.
Since the PSC and SSC signals are transmitted during the same 256-chip part of each frame, each is transmitted at half the power of the signals in the other parts. In other words, the PSC signal is transmitted at a power of 3 dB less than the signals in parts 2 through 10 of each slot. The SSC signal is also transmitted at −3 dB compared to signals in parts 2 through 10. Though this makes PSC and SSC detection more difficult, it keeps the transmission signal power constant throughout each frame.
FIG. 2
illustrates the apparatus used to generate the PERCH channel used for initial system acquisition in the proposed WCDMA Third Generation communication system. Primary Synchronization code (PSC) generator
1
generates a predetermined 256 chip sequence that is used for the first stage of system acquisition described later herein. The PSC is the same for all base stations in the communication system and is punctured into the first 256 chips of each slot of each frame.
In WCDMA systems, each base station spreads its transmissions using an orthogonal Gold code. The generation of orthogonal Gold codes is well known in the art. In WCDMA, all of the Gold codes are generated using the same generator polynomial. There are a total of 512 possible timing offsets of the Gold code for a given base station. These offsets are measured with respect to the start of a frame and not with respect to any centralized timing signal. The time-offset Gold code is truncated at the end of each ten millisecond frame, and then repeats from the offset point at the start of each frame.
WCDMA base stations transmit a secondary synchronization code (SSC) that serves two functions. First, the secondary synchronization code is used to identify the frame timing of a base station. Second the secondary synchronization code provides a group identification (GI), which narrows down the orthogonal Gold code offset to a subset of sixteen of the possible 512 offsets. In the proposed WCDMA systems, there are 32 different Group Identities, each associated with a set of sixteen Gold code offsets.
The group identification is provided to SSC outer coder
2
. The group identification is mapped to one of 32 possible 16 element code words wherein each of the elements takes on one of seventeen possible values. The code words are selected as comma free codes such that any cyclic shift of any of the code words results in a vector that is not a legitimate code word. The elements of the code word are then provided to SSC inner coder
3
which maps each of the elements of the code words into a 256 chip sequence. Each of the possible 256 chip SSC sequences into which an element of the code word can be mapped is orthogonal to any other sequences used to encode an element of a code word. Each of the possible 256 chip SSC sequences is also orthogonal to the 256 chip sequence used by the PSC. Each of the sixteen 256 chip SSC sequence is added to the PSC sequence punctured into the first 256 chips of part 1 of the slots in each frame.
The PSC sequence and the SSC sequence are summed in adder
6
. Because the sequences are orthogonal to one another they can be distinguished from one another at the receiver and will not, in a single path analysis, interfere with one another. In addition, broadcast common data is punctured into parts 2 through 5 of each slot of the frame. The remaining 1280 chips (occupying parts 6 through 10) of the slots in each frame consist of the remaining unpunctured chips of the orthogonal Gold code sequence used to spread the transmissions from the base station. The first 1280 chips of the orthogonal Gold code sequence within each slot is punctured out by the PSC/SSC and common broadcast information.
FIG. 3
illustrates the current state of the art in acquiring synchronization in a WCDMA communication system. The signal is received at antenna
10
and provided to receiver (RCVR)
11
. Receiver
11
down converts, amplifies and samples the received signal and provides the samples to Primary Synchronization code (PSC) detector
12
. The PSC is redundantly transmitted in part 1 of each of the sixteen slots of each frame. The PSC is transmitted at a very low power using very weak coding that is prone to false detection. In order to reduce the probability of false detection to an acceptable level, currently contemplated systems accumulate three full frames of samples into a buffer.
The following description will assume that the sampling is 1× and real samples only are taken. In reality the WCDMA system uses QPSK modulation so the sampling will be complex and oversampling is desirable to increase the likelihood of accurate detection.
Slot buffer
14
is a circular buffer that is capable of holding 2560 samples. The elements of slot buffer
14
are initialized to zero at the start of slot timing acquisition. The first 2560 samples are provided directly to slot buffer
14
. Thereafter, the samples received over the remainder of three frame periods are summed in summer
13
with corresponding accumulated sample values stored in slot buffer
14
in accordance with equation (1) below:
ACCUM_SAMP(i)=ACCUM_SAMP(i)+NEW_SAMP(i+2560n), (1)
where i is a slot chip number between 0 and 2559, ACCUM_SAMP(i) is the i
th
value stored in slot buffer
14
, NEW_SAMP(i) is the i
th
sample received and n is a slot number from 0 to 47 (corresponding to the number of slots in 3 full frames).
For the first 30 milliseconds of signal accumulation, switch
30
is set so that the values output by summer
13
are stored back into slot buffer
14
. At the completion of the signal accumulation period, switch
30
moves
Baker Kent D.
Olms Douglas
Pizarro Ricardo M.
QUALCOMM Incorporated
Wadsworth Philip
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