Multiplex communications – Generalized orthogonal or special mathematical techniques – Particular set of orthogonal functions
Reexamination Certificate
1998-02-27
2001-03-06
Olms, Douglas W. (Department: 2732)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Particular set of orthogonal functions
C370S206000, C370S320000, C370S335000, C370S342000, C370S441000, C375S260000, C375S308000, C375S329000
Reexamination Certificate
active
06198719
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to radio communications. More particularly, the present invention relates to mobile communication in a code division multiple access system.
II. Description of the Related Art
Multiple access techniques are designed to make efficient use of the limited radio frequency spectrum. They allow users to access the same band of frequency without interfering with each other. Examples of such techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA wireless technology, governed by Electronic Industry Association/Telecommunication Industry Association Interim Specification-95 (IS-95), employs a spread spectrum technique for the transmission of information. A spread spectrum system uses a modulation technique that spreads the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal.
A form of frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only part of a signal is typically affected by a frequency selective fade, the remaining spectrum of the transmitted signal is unaffected. A receiver that receives the spread spectrum signal, therefore, is affected less by the fade condition than a receiver using other types of signals.
The spread spectrum technique is accomplished by modulating each base band data signal to be transmitted with a unique wide band spreading code. Using this technique, a signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz. Typical examples of spread spectrum techniques are found in M.K. Simon;
Spread Spectrum Communications
, Volume I, pp. 262-358 McGraw-Hill (1994).
In a CDMA-type radiotelephone system, multiple signals are transmitted simultaneously on the same frequency. A particular receiver determines which signal is intended for that receiver by the unique, orthogonal spreading code, referred to as a Walsh code, in each signal. The signals at that frequency, without the particular spreading code intended for that particular receiver, appear to be noise to that receiver and are ignored.
The demand for reduced error transmission has been on the rise in recent years. New wireless services have emerged recently, such as broad band multimedia communications, that require lower bit error rates. Multimedia applications may require bit error rates (BER) as low as 10
−10
.
Convolutional encoding and RS code with appropriate interleaver depth are presently used to reduce the BER. However, more robust multiple access radio communications for both broad band terrestrial applications as well as satellite communications are needed.
SUMMARY OF THE INVENTION
The present invention encompasses a method for transmitting a code division multiple access signal in a radiotelephone system. The system is comprised of a radiotelephone that communicates with a base station. The radiotelephone is assigned user identification codes by the base station that are unique from other radiotelephone user identification codes. In the preferred embodiment, the radiotelephone is assigned two unique codes.
The transmitter in the base station generates a plurality of Walsh codes using the unique user identifications. In the preferred embodiment, two Walsh codes are generated. Each Walsh code is orthogonal to the other Walsh code generated in that particular radiotelephone. Additionally, both Walsh codes are orthogonal to the Walsh codes used by any other radiotelephones in the system.
The two Walsh codes are used to cover an information signal to be transmitted from the base station to the radiotelephone. This information signal can be digitized voice data, video data, or any other type of digitized information.
The covered information signal is then modulated for transmission. In the preferred embodiment, quadrature phase shift keying modulation is used. The resulting modulated signal is orthogonal to other signals transmitted by the radiotelephone and to all other communication signals in the radiotelephone system.
Since the cross correlation of Walsh codes is zero, the present invention lowers the symbol error rate of the system and increases system capacity. Data transmission using the present invention results in a bit error rate less than 10
−10
at E
b
/N
o
=4.5 dB. In other words, complex error correction normally required in a radio system is not needed using the present invention.
In the preferred embodiment, the base station of the present invention is a satellite that communicates with terrestrial radiotelephones able to receive satellite signals. In alternate embodiments, the present invention is used in terrestrial base stations.
REFERENCES:
patent: 5353352 (1994-10-01), Dent et al.
patent: 5550809 (1996-08-01), Bottomley et al.
patent: 5675344 (1997-10-01), Tong et al.
Faruque Saleh
Lo Wing
Maveddat Payam
Crane John D.
Hom Shick
Nortel Networks Limited
Olms Douglas W.
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