1. Field of the Invention
The present invention generally relates to the field of wireless communication systems. More specifically, the invention relates to supplemental channel assignments in code division multiple access communication systems.
2. Related Art
In wireless communication systems, such as code division multiple access (xe2x80x9cCDMAxe2x80x9d) system, communication generally takes place between a mobile unit and a base station. Communication occurs over communication paths called channels. For example, a channel may be provided by radio frequency (xe2x80x9cRFxe2x80x9d) signal propagation between a base station antenna and a mobile unit antenna. In CDMA systems each user uniquely encodes its message signal into a transmission signal in order to separate the message signal from those of other users sharing a channel. Several different techniques and methods of coding are used for encoding and decoding the information in transmission signals. For example, some commonly used coding methods, also referred to as xe2x80x9cspreading codesxe2x80x9d, are Walsh functions (also referred to as xe2x80x9corthogonal spreading codesxe2x80x9d) and pseudo-random noise codes, also called a PN (pseudo-noise) codes. The intended receiver, knowing the code sequences of the user, can decode the transmission signal to receive the message. The encoding of the message signal spreads its spectrum so that the bandwidth of the encoded transmission signal is much greater than the original bandwidth of the message signal. For this reason CDMA is also referred to as xe2x80x9cspread spectrumxe2x80x9d modulation or coding.
Communication channels are further characterized, for example, according to function. A forward channel, for example, refers to a channel used for communications from the base station to a mobile unit, and a reverse channel refers to a channel used for communications from a mobile unit to the base station. A fundamental channel is established whenever communication is initiated between a base station and a mobile unit, for example, if a user makes a phone call. A fundamental channel is a channel used for carrying a combination of user data, signaling information, and power control information. A fundamental channel can include both a forward fundamental channel and a reverse fundamental channel.
A supplemental channel is a channel that may optionally be provided in addition to a fundamental channel and which operates in conjunction with the fundamental channel to provide higher data rate services. Provision of a supplemental channel is also referred to as xe2x80x9cassignmentxe2x80x9d of the supplemental channel. Supplemental channel assignments are made by the base station, and not by the mobile unit, in a cdma2000 system. A supplemental channel may include a forward supplemental channel or a reverse supplemental channel or both. A supplemental channel is used for communicating user data, but not for signaling information, i.e. information used by the communication system, for example, to control the flow of traffic. Thus, communication between a base station and a mobile unit entails establishment of one forward fundamental channel, one reverse fundamental channels and, optionally, the assignment of one or more supplemental channels. The specifications for various types of communication channels in a cdma2000 system may be found in the cdma2000 standards, in particular, the standard published as TIA/EIA/IS-2000-5, Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems. 
FIG. 1 illustrates an example of channel assignments by a base station to a mobile unit in an exemplary CDMA wireless communication system. Exemplary communication system 100 shown in FIG. 1 comprises part of a cdma2000 spread spectrum communication system. As shown in FIG. 1, base station 102 communicates with mobile unit 112 over a communication channel provided by RF signal propagation between base station antenna 104 connected to base station 102 and mobile unit antenna 111 connected to mobile unit 112. Mobile unit 112 may optionally be connected to a computer, such as a personal computer, for example, PC 114. PC 114 can be connected to mobile unit 112 by serial cable 113, for example.
The communication channel includes fundamental channel 106, which includes both a forward and reverse fundamental channel, indicated in FIG. 1 by an arrow which points in both the forward and reverse directions. As described above, fundamental channel 106 can be used for carrying a combination of user data, signaling information, and power control information. The communication channel further includes forward supplemental channel 108, indicated in FIG. 1 by an arrow which points in the forward direction from base station 102 to mobile unit 112. The communication channel also includes reverse supplemental channel 110, indicated in FIG. 1 by an arrow which points in the reverse direction from mobile unit 112 to base station 102. As described above, forward supplemental channel 108 and reverse supplemental channel 110 can be used for carrying user data, indicated in FIG. 1 by the notation xe2x80x9cDataxe2x80x9d in forward supplemental channel 108 and the same notation in reverse supplemental channel 110.
As seen in FIG. 1, the notation xe2x80x9cESCAM, UHDM, SCRM, Dataxe2x80x9d in fundamental channel 106 indicates types of information which may be carried by fundamental channel 106. Data refers to user data information and other non-specified information which may be carried by the channel. ESCAM (xe2x80x9cExtended Supplemental Channel Assignment Messagexe2x80x9d) and UHDM (xe2x80x9cUniversal Handoff Direction Messagexe2x80x9d) are prescribed forms of communication which the base station can transmit to the mobile unit to make assignments of supplemental channels to a mobile unit. The ESCAM and UHDM contain necessary information for the establishment and assignment of a supplemental channel to a mobile unit, for example, the data rate that will be used on a particular supplemental channel. SCRM (xe2x80x9cSupplemental Channel Request Messagexe2x80x9d) is a prescribed form of communication which a mobile unit can transmit to the base station to request from the base station various actions regarding supplemental channels. For example, a mobile unit may request a supplemental channel to be assigned or an assigned supplemental channel to be canceled, , or may request a supplemental channel with a particular data rate to be assigned. In general, the base station tries to accommodate as many mobile units as possible, and therefore the base station has control over supplemental channel assignments. Thus, FIG. 1 shows one example of channel assignments by a base station to a mobile unit in an exemplary cdma2000 spread spectrum communication system.
One scenario which commonly occurs is that a mobile unit may receive a supplemental channel assignment with a reverse supplemental channel data rate that is too high for the mobile unit processor. In such a situation the mobile unit may not be capable of processing and sending data on the reverse supplemental channel at the required assigned data rate, and therefore the mobile unit may xe2x80x9ccrashxe2x80x9d, or reset and lose the call, or exhibit other undesirable behavior. There are a number of possible responses for the mobile unit to try to adapt to the situation. For example, the mobile unit can operate in discontinuous transmission mode (xe2x80x9cDTXxe2x80x9d). In DTX mode the mobile unit switches its transmitter off on the reverse supplemental channel autonomously. The base station recognizes that the mobile unit is operating in DTX mode, so the call is not lost, however there is a waste of system resources because, for example, the assigned reverse supplemental channel is not being used and data transmission from the mobile unit is being interrupted.
As another example, the mobile can send a SCRM requesting a lower reverse supplemental channel data rate. The base station requires a certain amount of time to respond to such a request, may take as much time as it needs for response, and may, in conformance with the cdma2000 standard, even ignore the request. In conformance with the cdma2000 standard, the mobile unit continues transmitting on the assigned reverse supplemental channel at the high data rate until a reverse supplemental channel reassignment is received from the base station. Thus, the mobile unit can still crash while attempting to adapt to the situation by requesting a lower reverse supplemental channel data rate.
Thus, there is a need in the art for a mobile unit to adapt to a situation in which the mobile unit has received from a base station in a spread spectrum system a reverse supplemental channel assignment with a reverse supplemental channel data rate that is too high for the mobile unit processor""s capabilities. There is also need in the art for a mobile unit in a spread spectrum system, in a situation in which the mobile unit has received from a base station a supplemental channel assignment with a reverse supplemental channel data rate that is too high for the mobile unit processor""s capabilities to adapt to the situation without being wasteful of mobile unit or system resources.
The present invention is directed to method and apparatus for determining optimum data rate on the reverse supplemental channel in wireless communications. The invention overcomes the need in the art for a mobile unit to adapt to a situation in which the mobile unit has received from a base station in a spread spectrum system a reverse supplemental channel assignment with a reverse supplemental channel data rate that is too high for the mobile unit processor""s capabilities. Moreover, the invention provides an adaptation for a mobile unit in a spread spectrum system, in a situation in which the mobile unit has received from a base station a supplemental channel assignment with a reverse supplemental channel data rate that is too high for the mobile unit processor""s capabilities, that avoids wasting either mobile unit or system resources.
In one aspect of the invention, a mobile unit receives an assigned reverse supplemental channel data rate. For example, the assigned reverse supplemental channel data rate can be part of a supplemental channel assignment from a base station to the mobile unit in a cdma2000 spread spectrum communication system. The mobile unit utilizes a profiling table to determine a maximum feasible reverse supplemental channel data rate corresponding to a given forward supplemental channel data rate. For example, the profiling table can be constructed by testing the mobile unit to determine what combinations of forward and reverse supplemental channel data rates and operating conditions cause the mobile unit""s processor to xe2x80x9ccrashxe2x80x9d. The mobile unit communicates data over the reverse supplemental channel at a data rate which does not exceed the maximum feasible reverse supplemental channel data rate. For example, the mobile unit can construct service data units using filler to lower the communication data rate below the maximum feasible reverse supplemental channel data rate.