Putting EDGE Phones To The Test

March 16, 2006

By Matthias Weber, Willtek Communications, and Petri Toivonen, Nokia Mobile Phones

Introduction

EDGE, or Enhanced Data rates for Global Evolution, is a technology that was specified in 1997, but only really emerged in the market over the past two years. In the United States, EDGE was originally intended to pave the way for TDMA operators to transition to 3G. As such, it was adopted in the slightly modified "EDGE Compact" format to cope with the low bandwidth available for mixed TDMA/EDGE networks. This version of EDGE became obsolete, however, when many U.S. TDMA operators started to switch to GSM. Now, even European operators understand the benefits of EDGE and have implemented it in addition to their W-CDMA networks. With a wide variety of EDGE phones available, we need to consider where EDGE differs and what needs to be measured in addition to GSM – since EDGE phones still use the GSM mode for voice calls.

What's new with EDGE

EDGE adds a new modulation scheme to the data protocols of both GPRS (General Packet Radio Service) and HSCSD (High Speed Circuit Switched Data), allowing EDGE to triple the rates for these data services. When EDGE is applied to GPRS, it will be referred to as EGPRS (Enhanced GPRS), while HSCSD becomes ECSD (Enhanced Circuit Switched Data). However, since HSCSD is not very widespread this article concentrates on EDGE and GPRS.

EDGE offers a choice between two modulation methods: GMSK and 8-PSK. The latter transmits 3 bits per symbol, compared to only one bit per symbol for GMSK.

This means that the introduction of EDGE has the biggest impact on Layer 1, or the physical layer; it has a limited impact on the upper layers. In the network, EDGE parameters only affect the base station and not the core network. Even though the additional requirements for mobile phones are also mainly on the physical layer, the impact is much higher. The amplitude of the signals transmitted by the mobile phone is no longer constant because it does not rotate on a circle as it does for GMSK. It can actually change from one phase state to any of the eight phase states, with the effect that the amplitude may cross near the origin of the I/Q plane. In order to avoid this, the whole I/Q plane rotates by 22.5° or 3p/8 per symbol, which ensures that the signal never reaches the origin.

Click Here To Download:
•White Paper: Putting EDGE Phones To The Test