TDMA

TDMA, as defined in IS-54 and IS-136, triples the capacity of cellular frequencies, through by dividing a 30 kHz cellular channel into 3 timeslots, which supports 3 users in strict alternation. Future systems may also utilize half-rate voice coders, which will allow 6 users in one 30 kHz channel. Hughes Network Systems is promoting the concept of E-TDMA, which uses dynamic timeslot allocation to avoid the waste of timeslots when one side of the conversation is silent. Among polite company (i.e. people that don't talk over each other), this technique can almost double the spectral efficiency of TDMA once more, to about 10:1 over analog.

TDMA As An Air Interface

In Time Division Multiple Access, the available spectrum is divided into a series of very tightly defined radio channels, and each channel is divided into time slots. The time slots are grouped together to form frames. TDMA allows multiple users to share the same radio channel by assigning the data packets from each conversation to a particular time slot. As an example: imagine several streets converging into one street. The cars on all of the streets must merge into the single street in order to pass. In a TDMA network, the base station acts like a traffic cop allowing one car from each street to pass to the single street. When the cop has allowed one car from each street to move forward, he then allows a second car from the first street to pass. In this example, the multiple streets are multiple conversations, cars are data packets from each conversation, the cop is the base station and the single street is the shared radio channel. The number of streets converging into one illustrates the number of time slots in a frame. Allowing multiple customers access to the same radio channel by the dividing the channel into time slots gives this transport mechanism it's name, Time Division Multiple Access.

TDMA Based Mobile Phone systems

In the telecommunications industry, we tend to call a specific type of mobile telephone system TDMA. This is because these phone systems use the Time Division Multiple Access air interface method. There is another name given to these systems, and that is American Digital Cellular (ADC), but this name is not commonly used. When speaking if TDMA mobile phone systems, we can either be referring to IS-54 or IS-136.

IS-54

The first American Digital Cellular system was called IS-54. The predecessor to IS-54 was the Advanced Mobile Phone System (AMPS). AMPS is commonly referred to as analog cellular. In order to increase the number of users supported in AMPS, IS-54 introduced digital voice channels. Through the use of a vo-coder it was possible to allow up to three users to share the same voice channel. It is important to note that the control channel in IS-54 was still analog.

IS-136

The ability to use a digital control channel is the pivotal point of most of the newest system enhancements. Interim Standard 136 (IS-136) details the exact functionality of the digital control channel. The first version of IS-136 was known as revision 0, it has since been updated by revision A. Revision 0 introduced a lot of new services and features to American Digital Cellular. These new features include: enhanced user services, sleep mode, private systems and enhanced security.

Enhanced user services including short messaging, and the display of the telephone number of the incoming call. Sleep mode which gives the telephone longer battery life when in standby. Instead of the phone constantly monitoring the control channel, it "goes to sleep" and only wakes up periodically to check for new messages on the control channel. New specifications which allow digital services to be used for Private Systems (PSIDS) or Residential Systems (RSIDS). Enhanced security and validation against fraud is achieved by not sending the ESN/Mobile Number combination over the air. Networks components can be added to enhance security against fraud and eavesdropping even more. An enhancement to the method of selecting adjacent channels makes the hand-off process more efficient.

The newest version of IS-136, revision A, introduces an enhanced vo-coder, over the air activation, calling name ID, enhanced hand-offs and priority access to control channels. The new A-CELP vocoder has a higher sampling rate and offers cleaner, crisper audio quality. Over The Air activation allows network operators to program information into telephones right over the airwaves, thus simplifying the activation process. The ability to deliver both the name and telephone number on an incoming call. Priority access to control channels gives priority to certain users or call types to gain access to the network when the system is fully loaded.