Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2001-02-15
2004-01-13
Tse, Young T. (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C370S342000
Reexamination Certificate
active
06678311
ABSTRACT:
BACKGROUND
I. Field
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for high data rate CDMA wireless communication.
II. Description of the Related Art
Wireless communication systems including cellular, satellite and point-to-point communication systems use a wireless link comprised of a modulated radio frequency (RF) signal to transmit data between two systems. The use of a wireless link is desirable for a variety of reasons including increased mobility and reduced infrastructure requirements when compared to wire line communication systems. One drawback of using a wireless link is the limited amount of communication capacity that results from the limited amount of available RF bandwidth. This limited communication capacity is in contrast to wire-based communication systems where additional capacity can be added by installing additional wire line connections.
Recognizing the limited nature of RF bandwidth, various signal processing techniques have been developed for increasing the efficiency with which wireless communication systems utilize the available RF bandwidth. One widely accepted example of such a bandwidth-efficient signal processing technique is the IS-95 over the air interface standard and its derivatives such as IS95-A and ANSI J-STD-008 (referred to hereafter collectively as the IS-95 standard) promulgated by the telecommunication industry association (TIA) and used primarily within cellular telecommunications systems. The IS-95 standard incorporates code division multiple access (CDMA) signal modulation techniques to conduct multiple communications simultaneously over the same RF bandwidth. When combined with comprehensive power control, conducting multiple communications over the same bandwidth increases the total number of calls and other communications that can be conducted in a wireless communication system by, among other things, increasing the frequency reuse in comparison to other wireless telecommunication technologies. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both of which are assigned to the assignee of the present invention and incorporated by reference herein.
FIG. 1
provides a highly simplified illustration of a cellular telephone system configured in accordance with the use of the IS-95 standard. During operation, a set of subscriber units
10
a-d
conduct wireless communication by establishing one or more RF interfaces with one or more base stations
12
a-d
using CDMA-modulated RF signals. Each RF interface between a base station
12
and a subscriber unit
10
is comprised of a forward link signal transmitted from the base station
12
, and a reverse link signal transmitted from the subscriber unit. Using these RF interfaces, a communication with another user is generally conducted by way of mobile telephone switching office (MTSO)
14
and public switch telephone network (PSTN)
16
. The links between base stations
12
, MTSO
14
and PSTN
16
are usually formed via wire line connections, although the use of additional RF or microwave links is also known.
In accordance with the IS-95 standard, each subscriber unit
10
transmits user data via a single channel, non-coherent, reverse link signal at a maximum data rate of 9.6 or 14.4 kbits/sec depending on which rate set from a set of rate sets is selected. A non-coherent link is one in which phase information is not utilized by the received system. A coherent link is one in which the receiver exploits knowledge of the carrier signals phase during processing. The phase information typically takes the form of a pilot signal, but can also be estimated from the data transmitted. The IS-95 standard calls for a set of sixty-four Walsh codes, each comprised of sixty-four chips, to be used for the forward link.
The use of a single channel, non-coherent, reverse link signal having a maximum data rate of 9.6 of 14.4 kbits/sec as specified by IS-95 is well suited for a wireless cellular telephone system in which the typical communication involves the transmission of digitized voice or lower rate digital data, such as a facsimile. A non-coherent reverse link was selected because, in a system in which up to 80 subscriber units
10
may communicate with a base station
12
for each 1.2288 MHz of bandwidth allocated, providing the necessary pilot data in the transmission from each subscriber unit
10
would substantially increase the degree to which a set of subscriber units
10
interfere with one another. Also, at data rates of 9.6 or 14.4 kbits/sec, the ratio of the transmit power of any pilot data to the user data would be significant, and therefore also increase inter-subscriber unit interference. The use of a single channel reverse link signal was chosen because engaging in only one type of communication at a time is consistent with the use of wireline telephones, the paradigm on which current wireless cellular communications is based. Also, the complexity of processing a single channel is less than that associated with processing multiple channels.
As digital communications progress, the demand for wireless transmission of data for applications such as interactive file browsing and video teleconferencing is anticipated to increase substantially. This increase will transform the way in which wireless communications systems are used, and the conditions under which the associated RF interfaces are conducted. In particular, data will be transmitted at higher maximum rates and with a greater variety of possible rates. Also, more reliable transmission may become necessary as errors in the transmission of data are less tolerable than errors in the transmission of audio information. Additionally, the increased number of data types will create a need to transmit multiple types of data simultaneously. For example, it may be necessary to exchange a data file while maintaining an audio or video interface. Also, as the rate of transmission from a subscriber unit increases, the number of subscriber units
10
communicating with a base station
12
per amount of RF bandwidth will decrease, as the higher data transmission rates will cause the data processing capacity of the base station to be reached with fewer subscriber units
10
. In some instances, the current IS-95 reverse link may not be ideally suited for all these changes. Therefore, the present invention is related to providing a higher data rate, bandwidth efficient CDMA interface over which multiple types of communication can be performed.
SUMMARY
A novel and improved method and apparatus for high rate CDMA wireless communication is described. In accordance with one embodiment of the invention, a set of individually gain-adjusted subscriber channels are formed via the use of a set of orthogonal subchannel codes having a small number of PN spreading chips per orthogonal waveform period. Data to be transmitted via one of the transmit channels is low code rate error correction-encoded and sequence-repeated before being modulated with one of the subchannel codes, gain-adjusted, and summed with data modulated using the other subchannel codes. The resulting summed data is modulated using a user long code and a pseudorandom spreading code (PN code) and upconverted for transmission. The use of the short orthogonal codes provides interference suppression while still allowing extensive error correction coding and repetition for time diversity to overcome the Raleigh fading commonly experienced in terrestrial wireless systems. In the exemplary embodiment of the invention provided, the set of sub-channel codes are comprised of four Walsh codes, each orthogonal to the remaining set and four chips in duration. The use of a small number (e.g. four) sub-channels is preferred as it allows shorter ort
Baker Kent
Qualcomm Incorporated
Tse Young T.
Wadsworth Philip
Yafuso Byron
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