Wireless Developer Network - Mobile Wireless Tomorrow

Mobile Wireless Communications Tomorrow (cont.)

EDGE! Will TDMA and GSM ever meet?

EDGE is a new time division multiplexing based radio access technology that gives GSM and TDMA an evolutionary path towards 3G in 400, 800, 900, 1800 and 1900 MHz bands. It was proposed to ETSI in 1997 as an evolution to GSM. Although EDGE reuses GSM carrier bandwidth and time slot structures, it is not restricted to use in GSM cellular systems only. In fact, it can provide a generic air interface for higher data rates. It provides an evolutionary path to 3G. Some call it 2.5G. It can be introduced smoothly into the existing systems without altering the cell planning. But as with GPRS, EDGE doesn't provide any additional voice capacity. The initial EDGE standard promised mobile data rates of 384 kbps. It allows data transmission speeds of 384 kbps to be achieved when all eight timeslots are used. In fact, EDGE was formerly called GSM384. This means a maximum bit rate of 48 kbps per timeslot. Even higher speeds may be available in good radio conditions. Actual rates will be lower with rates falling as one goes away from the cell site. EDGE can also provide an evolutionary migration path from GPRS to UMTS by implementing now, the changes in modulation that will be necessary for implementing UMTS later. Both High Speed Circuit Switched Data (HSCSD) and GPRS are based on something called Gaussian minimum-shift keying (GMSK) which only yields a moderate increase in data bit rates per time slot. EDGE, on the other hand, is based on a new modulation scheme that allows a much higher bit rate across the air interface. This modulation technique is called eight-phase-shift keying (8 PSK). It automatically adapts to radio circumstances and thereby offers its highest rates in good propagation conditions close to the site of base stations. This shift in modulation from GMSK to 8 PSK is the central change with EDGE which prepares the GSM world (and TDMA in general) for UMTS.

Only one EDGE transceiver unit will need to be added to each cell. With most vendors, it is envisioned that software upgrades to the BSCs and Base Stations can be carried out remotely. The new EDGE-capable transceiver can also handle standard GSM traffic and will automatically switch to EDGE mode when needed. EDGE capable terminals will also be needed - existing GSM terminals do not support the new modulation techniques and will need to be upgraded to use EDGE network functionality.

EDGE is currently being developed in two modes: compact and classic. Compact employs a new 200 kHz control channel structure. Synchronized base stations are used to maintain a minimum spectrum deployment of 1 MHz in a 1/3-frequency reuse pattern. EDGE Classic on the other hand employs the traditional GSM 200 kHz control structure with a 4/12 frequency reuse pattern on the first frequency.

How Can GSM and TDMA Converge With EDGE?

While developing the 3G wireless technology for TDMA, the Universal Wireless Communication Consortium (UWCC) proposed the 136 High-Speed (136 H-S) radio interface as a means of satisfying requirements for IMT-200 radio transmission technology (RTT). After evaluating various proposals, UWCC adopted EDGE (Actually EGPS, EDGE+GPRS) as the outdoor component of 136HS to provide 384 kbps data services. Since GSM networks can also have an evolutionary path via EDGE, this presents an interesting opportunity where the air interfaces of TDMA and GSM can converge and then evolve together. EDGE is being developed concurrently in ETSI and UWCC. The phase one of EDGE emphasizes enhanced circuit-switched data (ECSD) and enhanced GPRS (EGPS).

The TDMA terminals that support 30 kHz circuit switched services scan for a 30 kHz control channel (DCCH) according to TIA/EIA 136 procedures. If an acceptable 200 kHz EGPRS carrier exists, a pointer to this system will be available on the DCCH. On finding this, the terminal will leave the 30KHz system and start scanning of the 200 kHz system. When it finds it, it starts behaving as if it is were a GSM/GPRS terminal. To answer a circuit switched page, the mobile suspends packet data traffic and starts looking for a 30 kHz control channel. Mobile terminals that only support 200 kHz carriers immediately start looking for 200 kHz packet data system.

Will This Happen?

While EDGE provides a common air interface for TDMA and GSM to converge, there is one possible problem. GSM operators may decide to skip EDGE altogether in their migration path to 3G. By the time EDGE will be commercially available for GSM systems, 3G will already be in sight with W-CDMA and since W-CDMA will need an entirely new air interface, the additional investments in EDGE, only to be replaced by another system seems a bit unjustified. EDGE has lost favor in Europe with some wireless operators and vendors that are not convinced it will actually be adopted in force once carriers move to GPRS. As described above, the belief is that wireless service providers may be more inclined to move straight to WCDMA from GPRS. On the other hand, some North American operators have taken the position that they may not need to upgrade to WCDMA after EDGE because it doesn't offer increased speeds in the mobile environment (the ITU/UMTS definition of G3G is 384 Kbps mobile, 2 Mbps low mobility/fixed wireless). This is an especially strong point when one considers that the market demand for high-speed wireless data has yet to be fully proven.

The convergence of TDMA and GSM can't be ruled out also. Particularly in the US, operators may have more interest in moving on to EDGE to get compatibility with the TDMA networks. According to a study [1], EDGE should be available in the North American markets by 2002. The study also indicates that initially only the big operators may go in for EDGE first:

"In the end, the decision to upgrade will be made on two points. First, operators will want the technology that will both be available in a suitable time frame and will endure for the longest period of time--GSM and TDMA operators would like to avoid the continuous string of upgrades for the next five years. Second, operators will want the upgrade path that will provide the necessary data services while displacing as little voice traffic spectrum as possible."