Pulse or digital communications – Spread spectrum
Reexamination Certificate
1999-02-23
2001-04-03
Vo, Don N. (Department: 2734)
Pulse or digital communications
Spread spectrum
C370S342000
Reexamination Certificate
active
06212220
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to wireless communication systems, and more particularly to a mechanism for use within a wireless multiple access system, such as a Code Division Multiple Access (CDMA) system, which achieves signal orthogonalization without the use of orthogonal codes.
BACKGROUND OF THE INVENTION
The wireless communications industry has grown immensely in the past twenty years. In particular, the use of cellular or wireless phones and other such devices has been widely accepted by the public. The first generation of such cellular telephone devices operated on an analog transmission principle using a pair of frequencies per transmission session (sending and receiving) per user. Shortly after cellular devices became popular, digital cellular devices were perfected and appeared in the marketplace. Digital cellular devices operate by modulating a digital signal onto a carrier wave. Since the signal is digital, certain techniques can then be used to allow multiple users access to the same frequency spectrum, thereby effectively increasing system capacity.
One such technique, called Code Division Multiple Access (CDMA), provides a multiple access technique that offers certain benefits over analog cellular transmission techniques. In CDMA, a single radio frequency is used as a carrier for the communications of multiple users. By way of example, two CDMA wireless telephone users who are each making a call may have communications simultaneously transmitted using one or more of the same radio frequencies. Each user's signal is encoded with a pseudorandom noise (PN) code at the transmitter in such a way that it can be properly decoded at a received with minimal interference to another user's signal.
Certain types of interference problems however still exist in CDMA systems. For instance, a fading characteristic called Rayleigh fading occurs when a transmission signal is reflected from many different features of the physical transmission environment. As a result, the signal arrives at the destination receiver from many directions and each signal may have a different transmission delay. These effects can result in destructive summation of the signals, otherwise known as multi-path fading.
Certain CDMA modulation techniques can be used to mitigate the adverse effects of multipath fading and interference, while exploiting frequency reuse advantages. One such prior art modulation technique widely implemented in CDMA systems uses orthogonal codes. Using this technique, a digital data signal to be transmitted is first modulated with a pseudorandom noise (PN) code. The resulting signal is then modulated with an orthogonal code created from an orthogonal code generator to create a user signal which is orthogonal to other user signals. The orthogonal signal can be transmitted on the same frequency as an orthogonal signal created with an another code that is orthogonally related to the first orthogonal code. That is, both orthogonal codes are mathematically interrelated such that the signals produced from the modulations using each code are non-interfering when transmitted on the same frequency together.
Benefits of digital CDMA systems include a decreased requirement for transmission power which results in longer battery life, increased clarity, higher radio frequency reuse, and favorable signal-to-noise ratio results. Since the frequency spectrum can be reused multiple times in such systems, an overall increase in system user capacity also results. For specific details of the use of CDMA techniques in a multiple access system, the reader is referred to U.S. Pat. No. 4,901,307.
SUMMARY OF THE INVENTION
The present invention provides a mechanism to achieve similar results as described above without the use of an orthogonal code generator or orthogonal codes. Instead, a combination of procedures referred to herein as channel sequencing and bit strobing are used to produce two or more resultant signals that do not interfere with one another when transmitted on the same frequency. That is, the resultant signals appear to be, and actually are, orthogonal to one another. However, the process of creating these resultant signals does not require an orthogonal code generator nor orthogonal codes. As such, device power and processing resources can be conserved on a wireless device such as CDMA phone or laptop computer which results in prolonged battery life.
Instead of using orthogonal codes, the invention provides a rolling selection of channels with which a modulated information signal can again be modulated. The rolling channels are modulated with the modulated information signal to produce a resultant signal that is double modulated. The resultant signal can be transmitted without interference most of the time. However, at times when the value of the rolling channel would produce a double modulated resultant signal that is in an interfering or non-orthogonal state with another users resultant signal, a strobe signal is used to alter the selection of the resultant signal. The strobe signal essentially reverts to selecting the original singly modulated information signal, instead of the double modulated signal.
In contrast to prior art systems which always impose a second stage orthogonal code modulation technique on a first modulated PN signal, the invention uses the channel selection and bit strobe mechanism which only requires a secondary modulation process part of the time. At times when the secondary modulation process is not needed, the original modulated signal can be used as a resultant signal without interference. Neither the first nor the second modulation process within this invention requires the use of orthogonal codes. Hence, the invention results in a decrease in the amount of total modulation that is required to be performed and lowers processing and power consumption costs.
In its simplest embodiment, the invention provides a system and method for modulating an information signal in a spread spectrum communication system, such as a CDMA communication system. An input information signal to be transmitted can be a digitized voice or data signal generated by a device such as a CDMA phone or modem coupled to a computer. The system uses a first pseudorandom combiner that receives the input information signal and a pseudorandom code sequence. The pseudorandom code sequence is not an orthogonal code sequence. The pseudorandom combiner combines the information signal with the pseudorandom code sequence to produce a first combined signal. The first combined signal alone may be used at certain times as the resultant signal, without further modulation.
To make this determination, a channel sequence combiner is provided that receives the first combined signal which includes the information signal in a modulated form. The channel sequence combiner also receives a repetitive maximum length channel sequence. The repetitive maximum length channel sequence is also not an orthogonal code. The channel sequence combiner combines the first combined signal with the repetitive maximum length channel sequence to produce a second combined signal.
Finally, a selector is provided which is coupled to receive the first combined signal, the second combined signal and a repetitive strobe signal. The selector selects one of either the first combined signal or the second combined signal to produce a modulated signal based upon a value of the repetitive strobe signal. The repetitive strobe signal is of a length greater than the repetitive maximum length channel sequence.
As governed by the repetitive strobe signal, there are times when the system of the invention transmits just the first combined signal, instead of the second combined signal which requires the second modulation process.
According to more particular aspects of the invention, the repetitive maximum length channel sequence is a non-orthogonal repeating bit sequence of length N, and the repetitive strobe signal is non-orthogonal repeating bit sequence of at least length N+1. The bits 0 through N of the r
Hoffmann John
Proctor, Jr. James A.
Rouphael Antoine
Hamilton Brook Smith & Reynolds P.C.
Phu Phuong
TANTIVY Communications, Inc.
Vo Don N.
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