Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-05-28
2003-02-11
Bost, Dwayne (Department: 2681)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S342000, C370S350000, C375S130000, C375S142000, C375S147000, C375S150000
Reexamination Certificate
active
06519237
ABSTRACT:
TECHNICAL FIELD
The present invention pertains to the field of wireless communication technology. More specifically, the present invention pertains to a method and apparatus that uses parallel operations to speed the process of acquiring Code Division Multiple Access (CDMA).
BACKGROUND ART
Wireless telephony, e.g. cellular phone use, has become a widely available mode of communication in modern society. Code Division Multiple Access (CDMA) spread spectrum systems are among the most commonly deployed wireless technology.
CDMA utilizes digital encoding for every telephone call or data transmission in order to provide privacy and security. Unique codes are assigned to every communication, which distinguish it from the multitude of calls simultaneously transmitted over the same broadcast spectrum. Users share time and frequency allocations and are channelized by unique assigned codes. The signals are separated at the receiver in a known manner so that the receiver accepts only signals from the desired channel.
Cell phones can roam throughout the country and appear to operate seamlessly to the user. This feature is possible because hundreds of base stations, located throughout the country, allow relatively local transmission and reception of data signals. The roaming feature of cell phones is accomplished by using hard and soft hand-offs between base stations. However, because cell phones have the capability to communicate with any of the different base stations, assuming contractual access and operating system compatibility, a system and method is needed for the cell phone to determine if there are any nearby base stations and which of them have the strongest power signal. Furthermore, a need for a method of how to identify each of the base stations is required.
It is desirable for the cell phone to communicate with the base stations having the strongest signal. This will typically provide the best quality of service in terms of signal reproduction at the cell phone. While the strongest power signal is usually the closest base station, natural or man-made geographical obstructions may prevent the closest base-station from providing the strongest signal. Thus, for example, the second-closest base station may provide the strongest transmission signal. The strongest signal is rated in terms of the power of the signal received at the cell phone.
CDMA systems transmit waveforms from multiple base stations occupying the same frequency band. Hence, multiple signals combine their amplitude on the airwaves. However, because these signals are encoded with a pseudonoise (PN) sequence, they have special properties. First, they can only be interpreted by a receiver that knows the given PN sequence and the phase shift used on the PN sequence. To a user without the code, the signal appears as noise. To the user given the code, the signal is only not noise because it contains usable information once the signal is decoded. Hence the term “pseudo” noise. A CDMA standard establishes a plurality of usable codes which are assigned to users for channelization. The term code division multiple access is descriptive of the process because it divides a limited number of usable codes among multiple users according to their need, capability, and contracted service.
CDMA digital cellular systems, such as that specified in the IS-95 CDMA standard, uses two specific pilot PN code sequences, e.g. one for the in-phase condition and one for the quadrature phase condition, that every base station transmits in quadrature as a pilot signal. The pilot signal is a pseudonoise binary sequence which may be generated by a Linear Feedback Shift Register (LFSR). Each base station is assigned a phase shift at which it can transmit the specific codes. Cell phones can identify and distinguish different base stations by detecting pilot energy at different phase shifts. Hence, the phase shift is the key to uniquely identifying local base stations.
Referring to Prior Art
FIG. 1
a
, a circular diagram
100
of the pilot PN code sequence is illustrated. Box
102
a
represents the first bit of the 32,768 binary values used for the PN code sequence. This PN code sequence is one of the two PN code sequences used in the quadrature modulation. Similarly, box
102
b
represents the last bit of the 32,678 binary values. Hence, the sequence has a period of 32,768 values. Because the code is periodic, it repeats itself. The pilot PN sequence can be phase shifted among a plurality of base stations so that they do not all transmit the same code at the same phase shift. Rather, they transmit the same code at different phase shifts. Because the sequence is a PN sequence, all transmissions of the code appear as pseudonoise unless they are evaluated at the precise phase shift at which they are transmitted. The IS-95 CDMA standard divides the length of the 32,768 bit pilot PN sequence into a total of 512 possible phase shifts, each separated by a phase shift of 64 bits, e.g. 32,768/64=512, as shown by item
106
. The base stations are synchronized to each other by a standard time process, such as Global Positioning System (GPS) timing.
A cell phone checks for phase shift by generating the same single specific code sequence inside the cell phone itself, then correlating it to the input signal. However, while the cell phone can easily generate the single specific code, it does not know the phase shift of the base stations with respect to the received input signal. This is because the cell phone is not synchronized to any base stations when it is first trying to acquire base stations in the CDMA system. Thus, the cell phone must phase shift its internally generated specific code, by offsetting the code sequence, and then correlate it again to the input signal. A good correlation value usually means that two identical signals, e.g. identical PN sequences, are in phase. This process of phase shifting the internally generated specific code and correlating it to the input signal is repeated until the cell phone has checked all the possible phase shifts of the code. The process and apparatus used to phase shift the specific code and to perform the checking is referred to as a searcher, among other names. The base stations constantly transmit their pilot signal in a periodic fashion, so it is always available to be received by the cell phone, barring interference, etc.
Referring to Prior Art
FIG. 1
b
, a search/correlate operation on a hypothetical PN pilot signal
150
, transmitted at different phases from different base stations, and on a pilot PN sequence, provided within a cell phone, is illustrated. This figure provides an illustration of how a pilot PN sequence is transmitted from a plurality of base stations, of when the pilot PN sequence is generated within a cell phone, and how the search proceeds to phase shift the internally generated pilot PN sequence to check all the different possible phase shifts of a PN sequence transmitted by a base station.
Prior Art
FIG. 1
b
is comprised of a transmitted PN pilot signal
152
a
, with a phase shift, from Base Station A; a transmitted PN pilot signal
152
b
, with a phase shift, from Base Station B; and a transmitted PN pilot signal
152
c
, with a phase shift, from Base Station C. The three dots appearing throughout the figures represents the periodic and repeating nature of the transmitted PN pilot signal. Each PN pilot signal from the base stations is the same PN sequence of 32,768 bits. However, the starting point, and subsequent sequential bit locations, are offset among between the base stations by the 64 bit offset specified in the IS-95 CDMA standard. Hence, Base station B signal
152
b
is offset from Base station A signal
152
a
by an offset
156
a
, which is a multiple of 64 bits. Similarly, Base station C is offset from base station B by an offset
156
b
, which is a multiple of 64 bits.
In another scenario, there is no input signal because there is no service provided in a specific region, or because the signal is blocked by a natural or man-made obstruction. The controller within
Iraninejad Mehraban
McDonough John
Bost Dwayne
Koninklijke Philips Electronics , N.V.
Old Gregory V.
Zawilski Peter
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