Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-02-22
2004-03-16
Chin, Stephen (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S150000, C375S367000, C370S331000, C455S039000
Reexamination Certificate
active
06707842
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally concerns wireless mobile communications systems. In particular, the present invention relates to searching for pilot signals of alternate base stations by buffering a received signal.
2. Description of Related Art
In wireless communications technology, user data (e.g., speech) modulates a radio frequency signal for transmission and reception between a base station and a mobile unit. The radio spectrum allocated by regulatory authorities for a wireless system is “trunked” to allow simultaneous use of a spectrum block by multiple units.
The most common form of trunked access is the frequency-division multiple access (FDMA) system. In FDMA, the spectrum is divided into frequency channels comprised of distinct portions of the spectrum. The limited frequency channels are allocated to users as needed. However, once a frequency channel is assigned to a user, that frequency channel is used exclusively by the user until the user no longer needs the channel. This limits the number of concurrent users of each frequency channel to one, and the total number of users of the entire system, at any instant, to the number of available frequency channels. In addition, a single user generally will not at all times use the full capacity of the channel assigned to him. Accordingly, obtaining maximum efficiency of the available resources is difficult to achieve using FDMA solely.
Another common trunking system is the time-division multiple access (TDMA) system. TDMA is commonly used in telephone networks, especially in cellular telephone systems, in combination with an FDMA structure. In TDMA, data (speech) is digitized and compressed to eliminate redundancy, thus decreasing the average number of bits required to be transmitted and received for the same amount of information. The time line of each of the frequency channels used by the TDMA system is divided into “frames” and each of the users sharing the common channel is assigned a time slot within the frames. Each user then transmits or receives a burst of data during its assigned time slot and does not transmit or receive during other times. With the exception of delays required by the bursty data transmission, which typically are small enough to be largely unnoticeable, the TDMA system will appear to the users sharing the frequency channel to have provided an entire channel to each user.
The FDMA and TDMA combination technique is used by the GSM (global system for mobile communications) digital cellular system. In GSM, each channel is divided up in time into frames during which eight different users share the channel. A GSM time slot is only 577 &mgr;s (microseconds), and each users gets to use the channel for 577 &mgr;s out of every 4.615 ms (millisecond) time interval. 577 &mgr;s*8=4.615 ms.
Yet another method for sharing a common channel between multiple users is the code-division multiple access (CDMA) technique which uses direct sequence spread spectrum modulation. CDMA is relatively new to cellular technology and is one of the accepted techniques to be included into the next generation of digital cellular systems in the United States.
As with TDMA, the CDMA systems are typically used in conjunction with a FDMA structure, although this is not required. However, unlike the TDMA system, the CDMA system generally does not separate the multiple users of a common frequency channel using time slices. Rather, in CDMA, multiple users are separated from each other by superimposing a user-specific high-speed code on the data of each user. Because the applied code has the effect of spreading the bandwidth of each user's transmission, the CDMA system is often called a “spread spectrum” system.
Initially, the user information is digitized so that the information is represented as a sequence of “0” and “1” bits. For modulation purposes, it is common to convert this sequence of information bits into a corresponding synchronous time signal having +1 and −1 values, where +1 corresponds to a bit value of “0” and −1 corresponds to a bit value of “1”.
“Direct sequence” spreading typically is accomplished by multiplying a narrowband information signal by a much wider band spreading signal. The error and redundancy encoded digital data (speech) for each of the shared users of the CDMA channel may typically be provided at a rate of 19.2 kbps (kilobits per second). These data are then spread using a much higher frequency spreading signal, which may be provided at a rate of 1.2288 Mbps (megabits per second). Using the wider frequency spreading signal, a CDMA frequency channel can accommodate many users on code sub-channels.
The spreading signal is usually a sequence of bits selected from one of 64 different orthogonal waveforms generated using Walsh functions. Specifically, each such Walsh function typically consists of a repeating 64-bit sequence, and thus has a period of 52.083 &mgr;s (64 bits/1.2288 Mbps). A different one of the 64 different Walsh functions is utilized for each sub-channel to be included in the frequency channel. At the receiving end, a particular sub-channel can be decoded using the same Walsh function which was used to encode the sub-channel. When decoded in this manner, the desired sub-channel signal is reproduced and the signals from the other 63 sub-channels are output as low level noise. As a result, a user can distinguish its code sub-channel from other users' sub-channels on the same frequency channel.
In addition to the above channel coding, the various sub-channels also are processed using other types of coding. For instance, data on the speech traffic sub-channels typically are encrypted using a repeating pseudo random bit sequence (long code) which is unique to each different mobile unit and which has a period of 2
42
−1 bits. In order to coordinate encryption codes with the base unit, upon initial registration with the base unit the mobile unit provides its serial number to the base unit. The base unit then uses that serial number to retrieve the mobile unit's unique encryption code from the base station's database. Thereafter, the two can communicate using encrypted data, so as to provide a certain amount of privacy.
In addition to encryption coding, each sub-channel typically is encoded using an additional repeating pseudo random bit sequence (PN code). The PN code sequence, also referred to as the chipping function, utilized by a particular base station may be expressed as c(t), because it is applied as a function of time t. The PN code sequence is generated using a linear feedback shift register (LFSR) which outputs a pseudo random sequence of digital ones and zeros. These digital ones and zeros are converted to −1 and +1 symbols respectively and then filtered to give the chipping function c(t). Thus the chipping function has the property that c(t)
2
=+1. The period of the PN code sequence generated by a N-register LFSR is 2
N
−1 bits (or chips) long, though it is common to insert a zero to extend the full sequence length to 2
N
chips. Typically, the PN code is generated using a 15 bit code word and a 15-register LFSR, providing a repeating sequence of 2
15
=32,768 chips. Assuming the system uses the typical chip rate of 1.2288 MHz, the sequence repeats every 26.666 ms.
Generally, the PN code is identical for all base stations in the cellular network. However, each base station typically applies the PN code using a different time delay from the other base stations. For example, each base station generally selects from one of 512 different offsets (spaced 64 chips apart) for use in its PN code. By utilizing different offsets in this manner, a mobile unit can selectively tune to any given base station merely by using the same offset as that base station. Accordingly, it can be seen that merely time shifting a PN code sequence in this manner produces the same result as if each base station were using an entirely different PN code.
In a typical system, each
Banister Brian C.
Davis Mark
Rick Roland R.
Chin Stephen
Ha Dac V.
Mitchell Siberberg & Knupp LLP
VIA Telecom Co. Ltd.
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