Advanced TDMA

TDMA substantially improved upon the efficiency of analog cellular. However, like FDMA, it had the weakness that it wasted bandwidth: the time slot was allocated to a specific conversation whether or not anyone was speaking at that moment. Hughes’ enhanced version of TDMA extended time division multiple access (ETDMA) attempts to correct this problem. Instead of waiting to determine whether a subscriber is transmitting, ETDMA assigns subscribers dynamically. ETDMA sends data through those pauses which normal speech contains. When subscribers have something to transmit, they put one bit in the buffer queue. The system scans the buffer, notices that the user has something to transmit, and allocates bandwidth accordingly. If a subscriber has nothing to transmit, the queue simply goes to the next subscriber. So, instead of being arbitrarily assigned, time is allocated according to need. If partners in a phone conversation do not speak over one another, this technique can almost double the spectral efficiency of TDMA, making it almost 10 times as efficient as analog transmission.