The present invention relates to wireless communication systems. More specifically, the present invention relates to the control of contention-based wireless access in communication systems.
FIG. 1 illustrates a simplified wireless spread spectrum code division multiple access (CDMA) or time division duplex (TDD) or frequency division duplex (FDD) communication system 18. The system 18 comprises a plurality of Node Bs 26, 32, 34, a plurality of radio network controllers (RNCs) 36, 38, 40, a plurality of UEs 20, 22, 24 and a core network 46. The plurality of Node Bs are connected to the plurality of RNCs 36, 38, 40, which are, in turn, connected to the core network 46. Each Node B 26, 32, 34 communicates with its associated user equipment (UE) 20, 22, 24. Data signals are communicated between UEs and the Node B over the same spread spectrum. Each data signal in the shared spectrum is spread with a unique chip code sequence. Upon reception, using a replica of the chip code sequence, a particular data signal is recovered.
In the context of a CDMA system, signals are distinguished by their chip code sequences (codes) and separate communication channels are created using different codes. Signals from the Node B to the UEs are sent on downlink channels and signals from the UEs to the Node B are sent on uplink channels.
In many CDMA systems, a random access channel (RACH) is used for some uplink communications. A RACH is capable of carrying packets of data from multiple UEs. Each packet is distinguishable by a combination of time slot and code. For detection by the Node B, the packets have a sequence which also distinguishes it from other packets. The RACH is a contention-based uplink transport channel which may carry control information from the UE to set up an initial connection with the Node B, for example, to register the UE after power-on to the network or to perform location updates or to initiate a call. Transmissions are sent using repeating frames, each having a plurality of time slots, such as fifteen time slots with only one or two time slots per frame typically dedicated to RACH. When a packet is transmitted over the RACH, it may last for multiple frames. Those frames however, are not necessarily consecutive because a back-off process must be performed between each transmission to control the rate at which UEs access the RACH.
A UE may attempt a RACH transmission and select a time slot using one of N code identifiers, for example in a TDD CDMA system, one of eight midambles. If no other UE transmits in the same slot with the same midamble and if there is sufficient transmission power, then the UE's RACH transmission succeeds. If another UE transmits in the same slot with the same midamble, then they both fail. This transmission error is known as a collision error. Generally, whenever two or more UEs transmit using the same channel in a wireless system, a collision occurs. Another type of transmission error results when there is insufficient transmission power. The necessary power is generally a function of the channel, the interference, and other PRACH transmissions in the same slot.
In some communication systems, such as with a 3GPP system, there is a relatively long delay, on the order of seconds, before which a UE realizes a transmission error has occurred and decides to retransmit the failed packet. The recommended operating condition for the RACH is therefore preferably biased toward having very few collisions or insufficient transmission power errors. The failed packet may be retransmitted on data link layer 2 (L2) or data link layer 3 (L3) depending on the mode of operation.
The radio access network has no prior information regarding which RACH codes, or more generally which channels were transmitted. The detection of transmitted transport block sets (TBS) or bursts is performed at the receiver, where the number of UEs that transmitted using the detected code is unknown. In the event of a RACH transmission error, the cause remains unidentified. The error might be the result of a code collision or insufficient transmission power.
A parameter of dynamic persistence (DP) is defined which is set by the RNC to avoid saturation of the RACH. The DP level (DPL) is broadcast from the Node B to the UEs and the UEs adjust their rate of access to the RACH time slots as a function of DP. A RACH constant value (CV) parameter is defined which is managed at the RNC and is used by the UEs to determine the power of RACH transmissions.
In current systems, the DP parameter, RACH CV parameter, and other parameters are set and adjusted in order to avoid collisions and insufficient transmission power errors or, in the alternative, to maintain a predetermined target collision error and target insufficient transmission power error probability. The DP parameter is generated at the Node B and the RACH CV is generated at the RNC.
A prior art method of controlling these parameters utilizes the number of successful and failed UE transmissions in a time slot for individual system frames. Another prior art method broadcasts these parameters to the UEs, which then adjust their uplink transmission accordingly. It is difficult, however, to appropriately control these parameters because they are separately generated at the Node B and RNC and because the cause of the transmission error remains unknown.
Accordingly, there exists a need for an improved method of controlling parameters in a contention-based channel wherein the cause of transmission errors is identified and the rate at which such errors occur is identified and controlled by adjusting parameters at the Node B.