Mobile Wireless Communications Today (cont.)
by Puneet Gupta
Time Division Multiple Access (TDMA) IS-136 Technology
TDMA is so named because frequency bands available to the network are divided into time slots, with each user having access to one time slot at regular intervals. Three users share a 30 kHz bandwidth (IS 136) by splitting a 30 kHz carrier into 3 time slots. TDMA was first specified as a standard in EIA/TIA Interim Standard 54 (IS-54). IS-136, an evolved version of IS-54, is the United States standard for TDMA for both the cellular (850 MHz) and PCS (1.9 GHz) spectrums. TDMA IS-136 is an evolved form of TDMA IS-54. Unlike IS-54, IS-136 utilizes time division multiplexing for both voice and control channel transmissions. Digital control channel allows residential and in-building coverage, dramatically increased battery standby time, several messaging applications, over the air activation and expanded data applications (GSM also has the same characteristics). The digital control channel allows for the creation of micro cell applications making it suitable for wireless PBX and paging applications. TDMA networks transmit at a higher data rate on a relatively low bandwidth channel resulting in chances of co-channel interference. As described above for GSM, the time slot structure allows the mobiles to conserve battery power and to collect information about other channels. IS-136 specifies a "sleep mode" that instructs the compatible cellular phones to conserve power. IS-136 handsets are not compatible with IS-54.
TDMA makes more efficient use of available bandwidth than the previous generation analog technology. TDMA IS-136 exists in North America at both the 800MHz and 1900MHz bands. IS-136 TDMA normally co-exists with analog channels on the same network. One advantage of this dual-mode technology is that users can benefit from the broad coverage of established analog networks while IS-136 TDMA coverage grows within, and at the same time take advantage of the more advanced technology of IS-136 TDMA where it exists. TDMA networks have increased the capacity of the analog networks (using the same bandwidth) by 3 times.
The universal wireless communication consortium (UWCC) is a group of more than 100 telecom carriers and vendors of mobile products and services, which focuses on efforts to develop services based on IS-136 TDMA and IS-41 WIN. The Global TDMA Forum (GTF) of UWCC focuses on both technical and market led developments. IS-136 revision A has introduced several new features like Adaptive channel allocation (ACA) depending on the instantaneous channel quality determined by the level of interference, the Private System Identification (PSID) which allows development of large scale corporate private systems either as multi location or in-building closed user groups, two way short text messaging (SMS, 256 chars) etc. IS-136 revision B, as standard includes all IS-136+ proposals from the UWC-136 RTT proposal for voice and circuit switched features. Notable features are packet data service, mobile assisted handoff, improved SMS and intelligent roaming.
Major US carriers using TDMA are AT&T Wireless Services, Bell South and Southwestern Bell.
Next: Code Division Multiple Access (CDMA) IS-95 (cdmaOne)