Pulse or digital communications – Synchronizers – Phase displacement – slip or jitter correction
Reexamination Certificate
1999-06-08
2002-05-07
Pham, Chi (Department: 2731)
Pulse or digital communications
Synchronizers
Phase displacement, slip or jitter correction
C375S130000, C370S342000
Reexamination Certificate
active
06385264
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to wireless telecommunications systems. More particularly, the present invention relates to a novel and improved method and apparatus for mitigating the effects of destructive interference between the respective synchronization channels broadcast by two or more base stations in a code-division multiple access system.
II. Description of the Related Art
In a wireless radiotelephone communication system, many users communicate over a wireless channel. Communication over the wireless channel can be one of a variety of multiple access techniques that allow a large number of users in a limited frequency spectrum. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
The CDMA technique has many advantages. An exemplary CDMA system is described in U.S. Pat. No. 4,901,307, entitled “Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters”, issued Feb. 13, 1990, assigned to the assignee of the present invention, and incorporated herein by reference. An exemplary CDMA system is further described in U.S. Pat. No. 5,103,459, entitled “System And Method For Generating Signal Waveforms In A CDMA Cellular Telephone System”, issued Apr. 7, 1992, assigned to the assignee of the present invention, and incorporated herein by reference.
Recently, third-generation (3G) CDMA communication systems have been proposed including proposals such as cdma2000 and W-CDMA. These 3G CDMA communication systems are conceptually similar to each other with some significant differences. One significant difference is that in the cdma2000 system, each of the base stations operates synchronously. In other words, each base station in the cdma2000 system operates according to the same universal time reference. Each base station transmits a pilot channel having the same PN spreading code, but having a different PN phase offset. As a result, a mobile station can acquire the pilot channel of one or more base stations by searching through the possible PN phase offsets of the known PN spreading code. Additionally, the mobile station can distinguish among different base stations by their respective PN phase offsets, even though they are using the same PN spreading code.
However, under the currently proposed W-CDMA system standard, each of the base stations operates asynchronously. In other words, there is no universal time reference among separate base stations. In the W-CDMA system, each base station transmits a “synchronization” channel that comprises two sub-channels. The first of the two sub-channels, the primary synchronization channel, uses a primary synchronization code, c
p
, that is common to all base stations. The second of the two sub-channels, the secondary synchronization channel, uses a cyclic set of secondary synchronization codes, c
s
, that are not shared by other base stations that are not in the same code group. The mobile station in a W-CDMA system can acquire the synchronization channel of one or more base stations by searching for the primary synchronization code, c
p
of the primary synchronization channel, and then using the timing information derived from the primary synchronization channel to process the secondary synchronization channel.
FIG. 1
is a timing diagram illustrating the structure of the synchronization channel (SCH) of a W-CDMA system. In
FIG. 1
, one frame is illustrated. The one frame comprises sixteen individual slots, separated in
FIG. 1
by dashed lines. The primary synchronization channel is shown as a burst
100
of the primary synchronization code, transmitted at the beginning of each slot. The secondary synchronization channel is shown as a burst
102
of one of 17 possible secondary synchronization codes, transmitted in parallel with the primary synchronization code at the beginning of each slot.
The primary synchronization channel comprises an unmodulated code that is the same for every base station in the system, and is transmitted time-aligned with the slot boundary of the transmitting base station. The secondary synchronization channel comprises a sequence of 16 unmodulated code words that are orthogonal to each other and to the primary synchronization code. Each secondary synchronization code word is chosen from a set of 17 different orthogonal codes. The sequence on the secondary SCH indicates which of the 32 different code groups the base station PN scrambling code belongs to. 32 sequences are used to encode the 32 different code groups each containing 16 scrambling codes. The 32 sequences are constructed such that their cyclic shifts are unique. In other words, a non-zero cyclic shift less than 16 of any of the 32 sequences is not equivalent to some cyclic shift of any other of the 32 sequences. This property is used to uniquely determine both the long code group of the base station and the frame timing. It should be noted that the term “scrambling” code as used with reference to a W-CDMA system is synonymous with the term “spreading” code as used above with reference to a cdma2000 system. However, for consistency and clarity of disclosure with respect to W-CDMA based systems, the terminology “scrambling” code will be used herein to denote the code used to spread the information signal over the desired bandwidth.
During cell search, the mobile station searches for the base station to which it has the lowest path loss. It then determines the downlink scrambling code and frame synchronization of that base station. The cell search begins by using the synchronization channel. During the first step of the cell search procedure, the mobile station uses the primary SCH to acquire slot synchronization to the strongest base station. This may be done with a single matched filter matched to the primary synchronization code, c
p
, which is common to all base stations. During the second step of the cell search, the mobile station uses the secondary SCH to find frame synchronization and identify the code group of the base station found in the first step. This is done by correlating the received signal with all possible (16) secondary synchronization codes. Specifically, the mobile station correlates the sequence of 16 code words that are received against the 32 possible sequence patterns and 16 possible cyclic shifts, for a total of 32×16 possibilities. During the third and last step of the initial cell search, the mobile station determines the exact PN scrambling code used by the found base station. The scrambling code is identified through symbol-by-symbol correlation of the pilot symbols received over one or more common channels with the PN scrambling codes that belong to the code group identified by the second step.
A functional block diagram of the multiplexing of the synchronization channel (SCH) with the other downlink physical channels (dedicated channels) is shown in FIG.
2
. In
FIG. 2
, ones generator
202
generates a sequence of logical one values for 256 bits at the beginning of each slot. To be more precise, ones generator
202
generates the complex signal 1+j1. These ones are complex spread in complex spreader
208
with the primary synchronization code, c
p
, from primary code generator
206
. The primary synchronization code is common to all base stations. Together the ones generator
202
, primary code generator
206
and complex spreader
208
may be referred to as a “primary synchronization channel generator”.
Ones generator
204
(which may be the same as ones generator
202
) also generates a sequence of logical one values for 256 chips at the beginning of each slot. The ones are complex spread in complex spreader
210
with the secondary synchronization code, c
s
, from secondary code generator
212
. Together, the ones generator
204
, complex spreader
210
, and secondary code generator
212
may be referred to as a “secondary synchronization channel generator”. The in-phase (I) and quadrature-phase (Q) components
Agrawal Avneesh
Terasawa Daisuke
Baker Kent D.
Beladi S. Hossain
Pham Chi
Qualcomm Incorporated
Tran Khai
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