Cutting-EDGE

Wireless-service providers that aren't sold on UMTS could wind up on EDGE.

Thanks to its similar service options, EDGE will be a viable alternative to the UMTS revolution for some service providers. Here's why:

• EDGE offers an evolutionary step to wireless-data services with rates up to 384kb/s in existing GSM spectrum.

• It's plug-and-play and allows service providers to re-use existing GSM equipment.

• In addition to lower investment outlay compared to UMTS, EDGE also offers short-term, mass-market provisioning of higher data rates.

• EDGE provides a way to offer higher-bandwidth services in outlying areas where UMTS essentially constitutes overkill.

An essential first step toward 3G is GPRS, which improves GSM by providing more efficiency per subscriber. The packet-based GPRS upgrade increases data rates from today's 9.6kb/s and 14.4kb/s to more than 100kb/s, and it eliminates dial-up by providing a continuous Internet connection. GPRS complements but isn't intended to replace GSM data services such as circuit-switched. It also provides the type of data capabilities that 3G promises.

GPRS doesn't modify the GSM physical layer but instead uses a new layer-2 and -3 protocol architecture, implemented on new network elements as the packet-control unit and the GPRS service nodes. The nodes are the direct gateway to the packet-data world. EDGE helps direct high-speed data onto the network and increases GPRS' rates up to 384kb/s.

GPRS is a necessary next step in the evolution toward those technologies. It has brought GSM kicking and screaming into the packet world, and EDGE will boost capacity even more. Service providers hungry for value-added services thus will be ensured a wise investment with GPRS because GSM/GPRS core-network elements can be reused when GSM and UMTS are integrated into the same network.

Forward & Backward
Evolution is the operative word in EDGE: It uses the same frame structure, logic channel structure and the 200kHz-carrier bandwidth of existing GSM networks. The added capacity is achieved by increasing the data capacity of a single GSM time slot from the current 9.6kb/s to 48kb/s per time slot. EDGE also allows the aggregation of up to eight 48kb/s time slots to achieve the advertised 384kb/s maximum rate.

EDGE inherits GSM's racks, antennas and base stations, which add an EDGE plug-in carrier unit. Whether the EDGE unit is a replacement or an add-on depends on the traffic needs that the service provider wants to manage in the cell. The EDGE concept allows the mix of Gaussian minimum shift keying (GMSK) and EDGE modulation within one carrier unit, providing maximum flexibility.

The new EDGE carrier unit is fully backward compatible with existing GSM carrier units. Once this upgrade is completed, the EDGE network provides the bandwidth required for mobile-office applications with peak throughput in hot spots such as airports and offices.

More With Less
In principle, packet-data services with improved efficiency and asymmetric capabilities are available with GPRS and then EDGE, increasing the data throughput remarkably. Performance, however, depends on environmental conditions.

EDGE/E-GPRS Performance

	Indoor/Low-Range Outdoor	Urban/Suburban Outdoor	Rural Outdoor
E-GPRS	384kb/s (48kb/s/TS)	384kb/s (48kb/s/TS)	144kb/s/ (18kb/s/TS)
Speed	Up to 10km/h	Up to 100km/h	Up to 250km/h
Propagation Conditions	Indoor, TU3	TU50 HT100	900MHz: RA250 1.8/1.9GHz: RA130 HT 100

Figure 1.

The performance of EDGE and E-GPRS can vary significantly under different conditions.

GPRS constantly optimizes the channel coding according to radio-channel conditions in an effort to achieve the most throughput possible. Unlike GSM's 1-bit-per-symbol GMSK modulation scheme, EDGE transmits three bits per symbol.

Another of EDGE's key innovations is a link-quality-control system: The channel quality constantly is checked in order to avoid sending at a rate higher than the channel conditions can accommodate. If interference decreases or the mobile is close to the site, the data capacity is higher. If interference increases or the signal becomes weak, the data rate is reduced gradually to prevent connection loss.

Depending on the channel quality, a trade-off is made between the user-data rate and the data-protection rate. The E-GPRS link quality control has the capability to accommodate both eight-phase-shift keying modulation for higher data rates in good radio-channel conditions, as well as GMSK for poor radio-channel conditions.

UMTS' Peer?
If we define 3G as the provision of multimedia services (i.e., parallel services with different data rates and quality-of-service profiles), UMTS clearly can live up to this definition. For GSM, the major step is made with GPRS and EDGE Phase I and with the GSM release 2000 targeting on voice-over-IP capabilities and packet-bearer handover. From a service point, GSM will be on par with UMTS, although GSM always will be limited to the peak data threshold that the radio-access bearer can provide.

Thus, service providers' decisions about EDGE and UMTS must take their individual requirements into account. EDGE and UMTS shouldn't be considered competitors. Instead, together they provide a broader solution choice to meet the diverse range of needs of today's wireless-service providers.

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EDGE in Japan

In the United States, EDGE can be integrated in GSM 1900. The UWCC has established a platform that allows the introduction of GPRS/EDGE in ANSI 136 networks and offers U.S. service providers a clear path for 3G evolution.

Granted, some U.S. providers have spectrum limits that preclude introducing a classic EDGE approach. As a result, the UWCC and ETSI SMG jointly defined the "Compact EDGE" mode, which allows those with limited spectral resources to offer high-speed data services in a spectrum block of only 600kHz.

EDGE is pointless in Japan, where more efficient spectrum use, not higher data rates, is the most pressing issue. Japan's Pacific Digital Cellular system's capacity is completely exhausted, a problem that can be addressed only with the completely new standards and frequencies offered with UMTS for Japan. Furthermore, EDGE's inherent principle of smooth evolution is absolutely contrary to the Japanese revolutionary philosophy of network development.

Beutmueller (andrew.beutmueller@icn.siemens.de) and Faerber (michael.faerber@icn.siemens.de) work for Siemens. Faerber also serves as vice chairman of ETSI SMG 2.