TDMA (Time Division Multiple Access) is a method of digital wireless communications transmission allowing a large number of users to access a single radio-frequency channel without interference. A unique time slot is allocated to each user within each channel. Thus although the multiple users transmit and receive at a common frequency, there is no danger of collision because their respective transmissions are separated in time.
Frequency hopping is a technique where a transmitter and receiver alter frequency in synchronism with each other so that different data packets may be transmitted at different frequencies. Frequency hopping has obvious military and security applications since it is difficult for an eavesdropper to intercept a transmission for sufficient time to obtain useful information, before the transmitter frequency changes and the eavesdropper is no longer able to intercept the transmission. However, even apart from such consideration, frequency hopping is useful to minimize the risk of data loss owing to a frequency jam, since the larger the number of different frequency channels used to convey different data packets, the smaller is the number of data packets that may become lost if a specific frequency channel suffers from noise or becomes blocked.
WO 9403002 entitled “Frequency hopping time-diversity communications systems and transceivers for local area networks” discloses a system utilizing adaptive frequency-hopped spread spectrum modulation to communicate over noisy communications channels. Individual packets of data are transmitted with FSK modulation using two frequencies chosen from a larger set. An error coding system is used in which data on the quality of reception at each network transceiver is used to alter the gain of the receiver, the bit rate of the transmission, and the specific frequencies employed by the network for the purpose of optimizing communication error rate.
Communication systems must employ a communications protocol that handles transmission errors. Typically, the receiver sends an acknowledge signal to the transmitter on correct receipt of a data packet and the receiver awaits receipt of the acknowledge signal before sending the next packet. When frequency hopping is used, the retransmission of the same data packet may be at a different frequency and this may well improve the quality of transmission if the previous transmission were lost owing to a noisy or otherwise blocked channel. However, in TDMA systems, the “faulty” data packet is re-transmitted during the same time slot since it must be conveyed along the same channel and the TDMA time-slot defines which channel is associated with a given transmission. TDMA systems endeavor to pack in as many time slots as possible, since this increases the number of data channels and makes the most efficient use of bandwidth.
In summary, TDMA allows data packets belonging to different transmissions to be conveyed in different time slots at a single frequency; and frequency hopping allows data packets belonging to the same transmission to be conveyed at different frequencies. U.S. Pat. No. 5,134,615 (Freeburg et al.) entitled “Frequency agile TDMA communications system” published Jul. 28, 1992 discloses a TDMA system wherein different time slots are associated with different frequencies so as to permit the system to be compatible with different communications protocols operating at different transmission frequencies.
There appears to be no suggestion in the art to use frequency hopping in a TDMA system for reducing data loss and improving throughput.