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TDMA IS-136 (Time Division Multiple Access)




Key Data


TDMA (Time Division Multiple Access) is a second-generation technology used in digital cellular telephone communication, which divides each cellular channel into individual time slots in order to increase the amount of data that can be carried. Several different mutually incompatible implementations of TDMA technologies are in use worldwide, the most prolific being GSM (Global System for Mobile Communications). However, the implementation that is commonly referred to as TDMA is that defined by IS-136 by the Telecommunication Industries Association (TIA).

TDMA forms part of the evolution from first-generation analog systems to second- and then third-generation digital systems. It builds upon the original analog Advanced Mobile Phone Service (AMPS), using the same frequency band of 800MHz, but also operates in the Personal Communication Services (PCS) band of 1,900MHz in the US. Although TDMA could be considered as the least technologically advanced of the second-generation mobile systems, it has proven very popular in the US and developing world as a simple upgrade from analog to digital services. As of December 1999, there were approximately 36 million TDMA subscriptions, accounting for 9% of the digital market.

Although TDMA is currently incompatible with other second-generation systems, there is now a common upgrade path to IMT-2000, which should become the world-wide standard for third-generation mobile communication.

TDMA TECHNICAL DETAILS

TDMA enhances the AMPS service by dividing each of the original 30kHz analog channels into three digital time-division channels, thereby tripling the capacity of the system (called D-AMPS).

Like AMPS, D-AMPS uses frequency ranges within the 800 and 900 MHz spectrum. Each service provider can use half of the 824-849MHz range for receiving signals from cellular phones and half the 869-894MHz range for transmitting to cellular phones. The receiving channels are called reverse channels and the sending channels are called forward channels. The division of the spectrum into sub-band channels is achieved by using frequency division multiple access (FDMA). The TDMA processing is added to each sub-band channel created with FDMA to triple the number of channels available.

TDMA IS-136 was first specified in 1994 and is an evolution of the older IS-54 (also known as Digital AMPS or D-AMPS) standard. IS-54 used the three time-division channels for the voice information only, while IS-136 also used TDMA on the control channel.

A Digital Control Channel (DCCH) increases paging capacity, and sharing TDMA traffic and control on the same digital radio improves efficiency and reduces hardware costs. DCCH also provides the platform for a new generation of advanced wireless capabilities.

TDMA supports text messaging, caller identification and closed-user groups. Using a hierarchical cell structure, it is possible to overlay extra capacity in particular hotspots and offer different services to particular subscribers or areas within the network.

IS-136 supports a variety of digital value-added services, at the same time as being able to coexist with the AMPS network. The inherent compatibility between AMPS and TDMA, coupled with the deployment of dual-mode wireless handsets, ensures ubiquitous network access for the subscriber whether in an analog or digital serving area.

TDMA is designed to allow for seamless interworking and infrastructure sharing with IS-136 TDMA networks at 800MHz and 1,900MHz, as well as the analog AMPS networks. This allows new PCS operators to offer full wide-area coverage from day one through infrastructure sharing or roaming agreements with 800MHz operators in the same geographical area.

The newer IS-136+ and IS-136HS (based upon Enhanced Data Rates for Global Evolution [EDGE] standards) allow a higher bit rate transmission, along with the introduction of General Packet Radio Service (GPRS) data throughput can be increased to over 473Kbs per channel. This packet-switched upgrade can be overlaid on existing networks and allows the system to retain its backward compatibility.

A combined GPRS-136HS technology, known simply as EGPRS, is an ideal bearer for any packet-switched application, including internet connections using TCP/IP. From the end user's point of view, the EGPRS network is an extension of the internet via wireless access.

THE FUTURE

The enhancements available to convert an existing TDMA system to a high-throughput packet-switched system can bring some of the proposed advantages from IMT-2000 to existing networks not needing the increased radio spectrum allocation the third-generation (3G) system requires. It can also be seen as part of a gradual progression to 3G systems, eventually allowing interoperability with the other networks using the IMT-2000 standard.