Time Division Multiple Access (TDMA) technology is used in digital cellular telephone communication. It divides each cellular channel into three time slots to increase the amount of conversations that can be carried. TDMAimproves the bandwidth utilization and overall system capacity offered by older FM radio systems by dividing the 30-kHz channel into three narrower channels of 10 kHz each. Newer forms of TDMA allow even more users to be supported by the same channel.
TDMA systems have been providing commercial digital cellular service since mid-1992. Versions of the technology are used to provide Digital American Mobile Phone Service (D-AMPS), Global System for Mobile (GSM) communications, Personal Digital Cellular (PDC), and Digital Enhanced Cordless Telecommunications (DECT). Originally, the TDMA specification was described in EIA/TIAInterim Standard 54 (IS-54).
An evolved version of that standard is IS-136, which is used in the United States for both cellular and Personal Communications Services (PCS) in the 850- MHz and 1.9-GHz frequency bands, respectively. The difference is that IS-136 makes use of a control channel to provide advanced call features and messaging services.
Time Slots
As noted, TDMA divides the original 30-kHz channel into three time slots. Users are assigned their own time slot into which voice or data are inserted for transmission via synchronized timed bursts. The bursts are reassembled at the receiving end and appear to provide continuous, smooth communication because the process is very fast.
The digital bit streams that correspond to the three distinct voice conversations are encoded, interleaved, and transmitted using a digital modulation scheme called Differential Quadrature Phase-Shift Keying (DQPSK). Together these manipulations reduce the effects of most common radio transmission impairments. If one side of the conversation is silent, however, the time slot goes unused.
Enhancements to TDMAuse dynamic time slot allocation to avoid the wasted bandwidth when one side of the conversation is silent. This technique almost doubles the bandwidth efficiency of TDMAover the original analog systems. Frequency reuse further enhances network capacity. In nonadjacent cells, the same frequency sets are used as in other cells, but the cells with the same frequency sets are spaced many miles apart to reduce interference.
Framing
In a TDMAsystem, the digitized voice conversations are separated in time, with the bit stream organized into frames, typically on the order of several milliseconds. A6-millisecond frame, for example, is divided into six 1-millisecond time slots, with each time slot assigned to a specific user. Each time slot consists of a header and a packet of user data for the call assigned to it. The header generally contains synchronization and addressing information for the user data.
If the data in the header become corrupted as a result of a transmission problem—signal fade, for example—the entire slot can be wasted, in which case no more data will be transmitted for that call until the next frame. The loss of an entire data packet is called “frame erasure.” If the transmission problem is prolonged (i.e., deep fade), several frames in sequence can be lost, causing clipped speech or forcing the retransmission of data.
Most transmission problems, however, will not be severe enough to cause frame erasure. Instead, only a few bits in the header and user data will become corrupted, a condition referred to as “single-bit errors.”
Network Functions
The IS-54 standard defines the TDMAradio interface between the mobile station and the cell site radio. The radio downlink from the cell site to the mobile phone and the radio uplink from the mobile phone to the cell site are functionally similar. The TDMAcell site radio is responsible for speech coding, channel coding, signaling, modulation/demodulation, channel equalization, signal strength measurement, and communication with the cell site controller.
The TDMAsystem’s speech encoder uses a linear predictive coding technique and transfers Pulse Code Modulated (PCM) speech at 64 kbps to and from the network. The channel coder performs channel encoding and decoding, error correction, and bit interleaving and deinterleaving. It processes speech and signaling information, builds the time slots for the channels, and communicates with the main controller, modulator, and demodulator.
The modulator receives the coded information and signaling bits for each time slot from the channel coders. It performs DQPSK modulation to produce the necessary digital components of the transmitter waveform. These waveform samples are converted from digital to analog signals. The analog signals are then sent to the transceiver, which transmits and receives digitally modulated radiofrequency (RF) control and information signals to and from the cellular phones.
The modulator/equalizer receives signals from the transceiver. It performs filtering, automatic gain control, receive signal strength estimation, adaptive equalization, and demodulation. The demodulated data for each of the time slots is then sent to the channel coder for decoding. Newer voice coding technology is available that produces near-landline speech quality in wireless networks based on the IS-136 TDMAstandard.
One technology uses an algebraic code excited linear predictive (ACELP) algorithm, an enhanced internationally accepted code for dividing waves of sound into binary bits of data. The ACELP coders can be integrated easily into current wireless base station radios as well as new telephones. ACELP-capable phones enable users to take advantage of the improved digital clarity over both North American frequency bands—850 MHz for cellular and 1.9 GHz for PCS.
Call Handoff
Receive signal strength estimation is used in the call handoff process. The traditional handoff process involves the cell site currently serving the call, the switch, and the neighboring cells that potentially can continue the call. The neighboring cells measure the signal strength of the potential call to be handed off and report that measurement to the serving cell, which uses it to determine which neighboring cell can best handle the call.
TDMA systems, on the other hand, reduce the time needed and the overhead required to complete the handoff by assigning some of this signal strength data gathering to the cellular phone, relieving the neighboring cells of this task and reducing the handoff interval.
Digital Control Channel
Digital Control Channel (DCCH), described in IS-136, gives TDMAfeatures that can be added to the existing platform through software updates. Among these new features are:
- Over-the-air activation Allows new subscribers to activate cellular or PCS service with just a phone call to the service provider’s customer service center.
- Messaging Allows users to receive visual messages up to the maximum length allowed by industry standards (200 alphanumeric characters). Transmission of messages permits a mobile unit to function as a pager.
- Sleep mode Extends the battery life of mobile phones and allows subscribers to leave their portables powered on throughout the day, ready to receive calls.
- Fraud prevention The cellular system is capable of identifying legitimate mobile phones and blocking access to invalid ones.
A number of other advanced features also can be supported over the DCCH, such as voice encryption and secure data transmission, caller ID, and voice mail notification.
Many vendors and service providers have committed to supporting either TDMAor CDMA. Those who have committed to CDMAclaim that they did so because they consider TDMAto be too limited in meeting the requirements of the next generation of cellular systems. Although TDMA provides service providers with a significant increase in capacity over AMPS, the standard was written to fit into the existing AMPS channel structure for easy migration.
Proponents of TDMA, however, note that the inherent compatibility between AMPS and TDMA, coupled with the deployment of dual-mode/dual-band terminals, offers full mobility to subscribers with seamless handoff between PCS and cellular networks. They also note that the technology is operational in many of the world’s largest wireless networks and is providing reliable, high-quality service without additional development or redesign.
These TDMAsystems can be easily and cost-effectively integrated with existing wireless and landline systems, and the technology is evolving to meet the quality and service requirements of the global third-generation (3G) wireless infrastructure.
