Time division multiple access (TDMA) is a channel access method for shared medium (usually radio) networks. TDMA allows several users to share the same frequency channel by allocating unique timeslots to each user within the frequency channel. The users transmit in rapid succession, one after the other, each using his allocated timeslot. This allows multiple terminals to share the same transmission medium (e.g. radio frequency channel) while using only part of its bandwidth. Access is controlled using a frame-based approach, and precise system timing is necessary to allow multiple users access to the bandwidth (i.e. time slot access) necessary to transmit information in a multiplexed fashion on the return channel.
TDMA is used in the digital 2G cellular systems, such as, Global System for Mobile Communications (GSM), IS-136, Personal Digital Cellular (PDC), and in the Digital Enhanced Cordless Telecommunications (DECT) standard for portable telephones. TDMA is also used extensively in satellite communications systems, such as Demand Assigned Multiple Access (DAMA) systems, and in combat-net radio systems.
In general, the majority of timeslots in a TDMA communications system are allocated to different communication units by a resource controller (also referred to as a base station, satellite, repeater, controlling node, and the like). This is done so that two or more communication units do not transmit at the same time. The remaining timeslots are designated as random access timeslots by the resource controller. Any of the communication units can transmit in the random access timeslots without previous consultation with the other communication units.
Because the random access timeslots are not assigned to a particular communication unit, multiple communication units may transmit at the same time. When messages sent by any two communication units overlap, a collision occurs, and the message packets that were transmitted are lost. The communication units listen for acknowledgements of their transmissions. If an acknowledgement is not received in response to a message packet transmission, a collision is presumed to have occurred. In response to the presumed collision, the communication units may retransmit in a future random access timeslot. Commonly, when doing such retransmissions, each communication unit waits a random length of time before retransmission so that their retransmissions do not collide a second time.
A well known method of random access to a shared resource is the ALOHA protocol. The standard ALOHA protocol has a maximum throughput of approximately 18.4%. That is, approximately 81.6% of the total available bandwidth is essentially wasted usually due to losses from packet collisions. An improvement to the ALOHA protocol is slotted ALOHA. The slotted ALOHA protocol coordinates and arbitrates random-access to a shared communication channel using TDMA timeslots to reduce collisions. The resource controller emits a signal at the start of each timeslot to let all other communication units know when the timeslot is available. Each timeslot is available to all communication units on a random access basis, and a communication unit can transmit only at the beginning of a timeslot.
The slotted ALOHA protocol has a performance advantage over standard ALOHA, with maximum throughput increasing to approximately 36.8%. A characteristic of a standard slotted ALOHA system is that all of the random-access timeslots are equally available to all communication units. Additionally, all of the random-access timeslots use the same transmission rate and timeslot size. Therefore, a standard slotted ALOHA random-access technique may consider each timeslot to have equal availability and desirability. Because of this, the TDMA timeslots are not utilized efficiently for communication units having different transmission rate capabilities and message packet sizes, thus limiting data throughput.
Therefore, what is needed is a technique for improving bandwidth utilization of random access timeslots of a TDMA system to better accommodate transmissions from communication units having different transmission rate capabilities and message packet sizes.