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Briefing Technical Briefing Technical Briefing Technical

www.wraycastle.comFebruary 2009 © Wray Castle Limited

The Need for More Performance

GSM/GPRS must surely be one of the most successful communications technologies the world has ever known. After all,more than half the world’s population has a mobile phone based on GSM technology, far more people even than have hadaccess to fixed-line telephones. Nevertheless, GSM has been around for almost two decades and interest now seemsfocused on 3.5G and 4G technologies. So perhaps the time has come for GSM to step politely aside, content in theknowledge of a job well done. Not so. Work is continuing apace to enable GSM operators to offer significantly enhanced dataservices through their existing GSM/GPRS infrastructure without the need to migrate directly to next-generation technologiessuch as UMTS or WiMAX.

It may seem inevitable that a GSM operator would wish to upgrade as soon as possible to UMTS and HSPA, but this upgradepath can be very problematic and expensive. Not all operators have new spectrum in which to operate UMTS, which requiresat least one clear 5 MHz block. Even for those that have the option for new spectrum, there is the consideration of the verylarge investment required in licence fee and new network build. In addition, operators that have not at this stage alreadybegun investment in UMTS should certainly be considering the implications of the potential availability of LTE in only twoyears’ time. An upgrade once to UMTS and then again to LTE within a very short timeframe is certainly not desirable. Thusif GSM/GPRS could be given a significant performance boost then it may be able to bridge the gap to 4G. This should be avery attractive option for many operators and this is exactly what EDGE Evolution is about.

The Status Quo

Almost all existing GSM networks are using GPRS and more than 400 of these have added EDGE capability. However, whileGPRS has seen the germination of more advanced services such as the Multimedia Messaging Service (MMS) and limited webbrowsing, the theoretical bit rate of 171.2 kbit/s has proved stubbornly elusive. Limitations in mobile station multi-slot capabilityand error protection coding schemes have restricted bit rates for most operators to below 50 kbit/s in the downlink and below20 kbit/s in the uplink. Cost per bit also remains very high with consequently low revenue generated from data services. Thesituation has somewhat improved with the introduction of Enhanced Data rates for Global Evolution (EDGE), but because of similar multi-slot and error protection limitations it rarely exceeds half of its potential of more than 470 kbit/s. Additionally, for both technologies, latency remains relatively high, further degrading the data experience for subscribers.

The Aims for EDGE Evolution

The changes for EDGE Evolution arrive in releases 7 and 8 of the standards and the most obvious improvement is to thedata rates achievable in a GSM-based channel. Potentially, the first phase of EDGE Evolution provides bit rates in excessof 1 Mbit/s in the downlink and in excess of 400 kbit/s in the uplink. However, to appreciate fully the advancement that comeswith EDGE Evolution it is necessary to understand that it is an integral part of the wider changes that bring an all-IP, andUMTS-like, Evolved GSM EDGE Radio Access Network (GERAN), known as GERAN Evolution (GEV). Coupled with this

is the progression through soft switching toward an all-IP based core network with real-time services controlled by an IPMultimedia Subsystem (IMS). This last point is important because, with the IMS in place, EDGE Evolution is intended as anupgrade for voice as well as data services.

EDGE Evolution – Sweating the Assets

GSM/GPRS GSM/EGPRS

UMTS UMTS/HSPA

LTEEDGE

Evolution



To this end, the key design aims for EDGE Evolution can be summarized as:

• a 50% improvement in spectrum efficiency for both data and voice services• a 100% increase in uplink and downlink bit rates• improved radio coverage by increasing the downlink sensitivity by 3 dB• improved service availability at the edge of the cell• reduced latency to below 450 ms for initial access and below 100 ms after initial access

These performance objectives are achieved through the introduction of several new features on the air interface includinghigher level modulation schemes, mobile station diversity reception, adaptive and improved error protection schemes, uplinkdual symbol rate and multicarrier operation in the downlink.

Modulation and Coding Enhancements

One of the key changes for EDGE was the introduction of 8PSK modulation. This triples the physical layer throughput whencompared to the standard GMSK. It should be no surprise, then, that this strategy is extended for EDGE Evolution with theintroduction of two more high-level modulation schemes, 16QAM and 32QAM. These two modulation schemes offer a basicimprovement factor over GMSK of four times and five times respectively.

Another significant addition is the option for a featureallowing the use of a higher symbol rate. The standardsymbol rate of 271 ksps is increased to 325 ksps. Thisresults in a nominal increase in physical layer throughput of approximately 20% for any givenmodulation scheme. However, there will also be acorresponding increase in the transmitted radiobandwidth, which will need to be allowed for in thefrequency plan and in receiver design.

The error protection scheme is also enhanced: firstly, with the introduction of turbo coding, for more robust error protection;and secondly, with an improved HARQ process for faster retransmission and a more adaptive error protection overhead. Coreto the HARQ enhancement is the introduction of a shorter transmission time interval and variable radio block sizes. Theseimprovements bring increased average throughput, reduced latency and more flexibility for the more rapid exchange of thesmall data volumes associated with VoIP.

Based on these enhancements, a total of 13 new modulation and coding schemes are introduced. The net result is that thepotential physical layer rate for a single timeslot rises to a maximum of 76.8 kbit/s when using 16QAM and 118.4 kbit/s when

using 32QAM.

www.wraycastle.comFebruary 2009 © Wray Castle Limited

Briefing Technical Briefing Technical Briefing Technical

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