Wireless communication systems have been extremely popular for more than a decade. They allow users to communicate with each other while remaining geographically mobile. These systems also allow communications to be in different modes, such as full-duplex voice, half-duplex voice, and data, as examples. An example of a wireless communication system protocol is 1xEV-DV which specifies the requirements for a system that is optimized for both data and voice communications. Although an 1xEV-DV wireless communication system serves to exemplify the invention, it shall be understood that the invention is applicable to other types of wireless communication systems.
The forward link of a 1xEV-DV wireless communication system is of particular interest herein. This is the wireless communication link for sending voice and data from a base transceiver station (BTS) to mobile communication unit (MU). According to the 1xEV-DV protocol, a compliant wireless communication system shall be backwards compatible with the IS-95 and CDMA2000 specification for the forward link. This specification provides that the forward link uses a time division (TD)/code division multiple access (CDMA) modulation scheme to send voice and data information from a base transceiver station (BTS) to mobile communication units (MUs). According to the TD/CDMA modulation scheme, time slots are used to simultaneously transmit frames of data and voice to a plurality of mobile communication units (MUs), and Walsh codes to define separate channels for communicating with the mobile communication units (MUs).
According to the IS-95 and CDMA2000 specification, a base transceiver station (BTS) uses 64 Walsh codes in the forward link to implement separate channels to communicate with mobile communication units (MUs). The maximum length of each of the Walsh codes is 64 bits. One of the Walsh code is reserved for a Pilot channel (W064). Another Walsh code is reserved for a Sync channel (w3264). Up to seven of the Walsh codes may be reserved for Paging channels (W164-W764). And, the remaining Walsh codes are reserved for dedicated traffic channels through which voice is sent to the respective mobile communication units (MUs).
The IS-95 and CDMA2000 protocols were originally developed for wireless communication systems that provide primarily time-sensitive voice communications, such as interconnect voice communications and dispatch voice communications. However, since the inception of these protocols, there has been a substantial growth in the need for the communication of packet of data (i.e., non-time-sensitive voice communications), such as webpages, objects, emails, video and other information. In response to such need, the 1xEV-DV protocol was developed for the transmission of data at substantially higher data rates, such as 3.1 mega bits per second (Mbps). Since many of today's wireless communication systems were based on the IS-95 and CDMA2000 protocols, the 1xEV-DV protocol can be used with IS-95- and CDMA2000-based systems. Thus, the forward link specification for existing IS-95- and CDMA2000-based wireless communication systems can implement the 1xEV-DV protocol.
According to the 1xEV-DV protocol, the forward link has up to 28 Walsh codes reserved for packet data channels (PDCH) for communicating data from a base transceiver station (BTS) to a plurality of mobile communication units (MUs). Each Walsh code has a length of 32 bits. Since, as discussed above, the 1xEV-DV forward link must be compatible with the forward link specified in the IS-95 and CDMA2000 protocols, the 28 Walsh codes use the same Walsh code tree structure as that of the forward link of an IS-95 or CDMA 2000 compliant system. Accordingly, the 28 Walsh codes reserved for 1xEV-DV consume 56 out of the available 64 Walsh codes for the forward link. Since there may be at least four common channels (e.g., the Pilot, Sync, and a couple of Paging channels), that only leaves four channels for transmission of voice.