I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a channel structure for communication systems.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known, CDMA has significant advantages over these other techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," and assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention and incorporated by reference herein. The CDMA system can be designed to conform to the "TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", hereinafter referred to as the IS-95 standard. Another code division multiple access communication system includes the GLOBALSTAR communication system for world wide communication utilizing low earth orbiting satellites.
CDMA communication systems are capable of transmitting traffic data and voice data over the forward and reverse links. A method for transmitting traffic data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled "METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION", assigned to the assignee of the present invention and incorporated by reference herein. In accordance with the IS-95 standard, the traffic data and voice data are partitioned into traffic channel frames which are 20 msec in duration. The data rate of each traffic channel frame is variable and can be as high as 14.4 Kbps.
In the CDMA system, communications between users are conducted through one or more base stations. A first user on one remote station communicates to a second user on a second remote station by transmitting data on the reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on the forward link of the same base station, or a second base station, to the second remote station. The forward link refers to transmission from the base station to a remote station and the reverse link refers to transmission from the remote station to a base station. In IS-95 systems, the forward link and the reverse link are allocated separate frequencies.
The remote station communicates with at least one base station during a communication. CDMA remote stations are capable of communicating with multiple base stations simultaneously during soft handoff. Soft handoff is the process of establishing a link with a new base station before breaking the link with the previous base station. Soft handoff minimizes the probability of dropped calls. The method and system for providing a communication with a remote station through more than one base station during the soft handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled "MOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM," assigned to the assignee of the present invention and incorporated by reference herein. Softer handoff is the process whereby the communication occurs over multiple sectors which are serviced by the same base station. The process of softer handoff is described in detail in copending U.S. Pat. No. 5,933,787 entitled "METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION", filed Dec. 11, 1996, assigned to the assignee of the present invention and incorporated by reference herein.
Given the growing demand for wireless data applications, the need for very efficient wireless data communication systems has become increasingly significant. An exemplary communication system which is optimized for data transmission is described in detail in copending U.S. Pat. No. 5,930,230 entitled "HIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEM", filed May 28,1996, assigned to the assignee of the present invention, and incorporated by reference herein. The system disclosed in U.S. Pat. No. 5,930,230 is a variable rate communication system capable of transmitting at one of a plurality of data rates.
A significant difference between voice services and data services is that the former requires a fixed and common grade of service (GOS) for all users. Typically, for digital systems providing voice services, this translates into a fixed and equal data rate for all users and a maximum tolerable value for the error rates of the speech frames, independent of the link resource. For the same data rate, a higher allocation of resource is required for users having weaker links. This results in an inefficient use of the available resource. In contrast, for data services, the GOS can be different from user to user and can be a parameter optimized to increase the overall efficiency of the data communication system. The GOS of a data communication system is typically defined as the total delay incurred in the transfer of a data message.
Another significant difference between voice services and data services is the fact that the former imposes stringent and fixed delay requirements. Typically, the overall one-way delay of speech frames must be less than 100 msec. In contrast, the data delay can become a variable parameter used to optimize the efficiency of the data communication system.
The parameters which measure the quality and effectiveness of a data communication system are the total delay required to transfer a data packet and the average throughput rate of the system. Total delay does not have the same impact in data communication as it does for voice communication, but it is an important metric for measuring the quality of the data communication system. The average throughput rate is a measure of the efficiency of the data transmission capability of the communication system.
A communication system designed to optimize transmission of data services and voice services needs to address the particular requirements of both services. The present invention provides a channel structure which facilitate transmissions of data and voice services.