1. Technical Field of the Invention
The present invention relates to radio telephone systems. More particularly, the present invention relates to an improved method for encoding and decoding a supplementary signal so that greater amounts of data may transmitted by a radio call without any reduction in performance of nearby radio calls.
2. Description of the Related Art
Communication systems take many forms. In general, the purpose of a communication system is to transmit information-bearing signals from a source, located at one point, to a user destination, located at another point some distance away. A communication system generally consists of three basic components: transmitter, channel, and receiver. The transmitter has the function of modulating the information signal into a form suitable for transmission over the channel. The function of the channel is to provide a physical connection between the transmitter output and the receiver input. The function of the receiver is to demodulate the received signal so as to produce an estimate of the original information signal.
Analog and digital transmission methods are used to transmit a information signal over a communication channel. The use of digital methods offers several operational advantages over analog methods, including but not limited to: increased immunity to channel noise and interference, flexible operation of the system, common format for the transmission of different kinds of information signals, improved security of communication through the use of encryption, and increased capacity.
To transmit a information signal (either analog or digital) over a bandpass communication channel, the information signal must be manipulated into a form suitable for efficient transmission over the channel by modulating the information signal. Modulation involves varying some parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the modulated wave matches the assigned channel bandwidth. At the receiver point of the communication channel, a receiver re-creates the original information signal from a degraded version of the transmitted signal by a process known as demodulation.
For multiplexed communication systems such as a code division multiple access (CDMA) system, the system typically consists of many remote units (i.e., subscriber units). Each subscriber unit requires a communication channel for short or discrete intervals of time rather than continuous service on a communication channel at all times. The discrete intervals of time are known as packets or power control groups. Therefore, communication systems have been designed to incorporate the characteristic of communicating with many remote units for brief intervals of time on the same communication channel.
A Direct Sequence (DS) Code Division Multiple Access (CDMA) system is a cellular system where all subscriber units transmit information signals on the same frequency band simultaneously. Similarly, base stations transmit information signals intended for a particular subscriber unit by transmitting the information signals on the same frequency band as base station originated transmissions to other subscriber units. By necessity, the transmitted bandwidth from a base station is much larger than the information rate of the information signal, i.e. carrier bandwidth is large compared to the message bandwidth.
In digital communications, and CDMA communication systems in particular, communication clarity and throughput is controlled by the amount of data that can be transmitted over a channel. For every increase in the data rate transmitted over a channel, there is a corresponding increase in either the quality of a call (e.g. voice) or the number of simultaneous calls that are carried by a channel. Sometimes an increase in data rate per channel can realize both improved quality and call carrying capacity. Any method that improves the data rate per channel is useful where the number of channels are scarce.
In cellular systems, including CDMA, there is a limited number of channels available to carry a transmission. When many transmissions occur on a cellular system, some of the transmissions will occur on the same channel. A way to limit interference between cells that share the same frequency band is to reduce the power of each transmitter according to the power received at the receiver. Consequently, power control must be implemented to adjust the transmitter to maintain a constant received signal strength at the receiver. Any method that improves the ability to maintain constant levels of received signal strength is useful.