Briefing
Technical Briefing
Technical Briefing
Technical
EDGE Evolution – Sweating the Assets 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 had access to fixed-line telephones. Nevertheless, GSM has been around for almost two decades and interest now seems focused on 3.5G and 4G technologies. So perhaps the time has come for GSM to step politely aside, content in the knowledge of a job well done. Not so. Work is continuing apace to enable GSM operators to offer significantly enhanced data services through their existing GSM/GPRS infrastructure without the need to migrate directly to next-generation technologies such 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 upgrade path can be very problematic and expensive. Not all operators have new spectrum in which to operate UMTS, which requires at least one clear 5 MHz block. Even for those that have the option for new spectrum, there is the consideration of the very large investment required in licence fee and new network build. In addition, operators that have not at this stage already begun investment in UMTS should certainly be considering the implications of the potential availability of LTE in only two years’ time. An upgrade once to UMTS and then again to LTE within a very short timeframe is certainly not desirable. Thus if GSM/GPRS could be given a significant performance boost then it may be able to bridge the gap to 4G. This should be a very attractive option for many operators and this is exactly what EDGE Evolution is about.
UMTS
UMTS/HSPA LTE
GSM/GPRS
GSM/EGPRS
EDGE Evolution
The Status Quo Almost all existing GSM networks are using GPRS and more than 400 of these have added EDGE capability. However, while GPRS has seen the germination of more advanced services such as the Multimedia Messaging Service (MMS) and limited web browsing, the theoretical bit rate of 171.2 kbit/s has proved stubbornly elusive. Limitations in mobile station multi-slot capability and error protection coding schemes have restricted bit rates for most operators to below 50 kbit/s in the downlink and below 20 kbit/s in the uplink. Cost per bit also remains very high with consequently low revenue generated from data services. The situation 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 the data rates achievable in a GSM-based channel. Potentially, the first phase of EDGE Evolution provides bit rates in excess of 1 Mbit/s in the downlink and in excess of 400 kbit/s in the uplink. However, to appreciate fully the advancement that comes with EDGE Evolution it is necessary to understand that it is an integral part of the wider changes that bring an all-IP, and UMTS-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 IP Multimedia Subsystem (IMS). This last point is important because, with the IMS in place, EDGE Evolution is intended as an upgrade for voice as well as data services.
February 2009
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Briefing
Technical Briefing
GPRS
Technical Briefing
EDGE
GPRS Core Network
IMS 3GPP Evolved Packet Core (EPC) BSC
BSC
Gb
GPRS
Gb
EGPRS
BTS
EGPRS2
Iu-PS
BSS/ GERAN
BSS
Technical
EDGE Evolution
GPRS Core Network BSC
Evolved GERAN
BTS
BTS
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 including higher level modulation schemes, mobile station diversity reception, adaptive and improved error protection schemes, uplink dual 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 when compared to the standard GMSK. It should be no surprise, then, that this strategy is extended for EDGE Evolution with the introduction of two more high-level modulation schemes, 16QAM and 32QAM. These two modulation schemes offer a basic improvement factor over GMSK of four times and five times respectively. Another significant addition is the option for a feature allowing the use of a higher symbol rate. The standard symbol rate of 271 ksps is increased to 325 ksps. This results in a nominal increase in physical layer throughput of approximately 20% for any given modulation scheme. However, there will also be a corresponding increase in the transmitted radio bandwidth, which will need to be allowed for in the frequency plan and in receiver design.
16QAM and 32QAM
Improved HARQ EDGE Evolution
Turbo Coding
Higher Symbol Rate
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. Core to the HARQ enhancement is the introduction of a shorter transmission time interval and variable radio block sizes. These improvements bring increased average throughput, reduced latency and more flexibility for the more rapid exchange of the small 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 the potential 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.
February 2009
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Briefing
Technical Briefing
Technical Briefing
Technical
Channel Structure Enhancements One of the headline changes for EDGE Evolution is the introduction of dual carrier operation. This is applicable only in the downlink direction. It means that information for one mobile can be split and transmitted on two different radio carriers, thus doubling the throughput rate. Therefore, dual carrier operation requires the use of two receivers in the mobile station. As for current GPRS/EDGE, multi-slot operation allowing up to all eight timeslots is included in the EDGE Evolution standards. However, it is acknowledged that practical implementations so far have always been limited to fewer than the maximum number of slots; typically four slots in the downlink and two in the uplink. This trend is expected to continue for EDGE Evolution, but the assumption is that the standard mobile station build will be improved to permit the use of up to five downlink slots and four uplink slots. The performance of the radio channel can be further enhanced through the use of dual antennas in the mobile station, providing a downlink receiver DL Po tential c. 1.2 diversity gain. In addition it is assumed that noise Mb it /s Freque cancellation techniques will be applied in the ncy 1 mobile station receiver. These advances should Freque provide an overall improvement in effective ncy 2 coverage and edge-of-cell performance. c. 3 dB RX diversity gain UL pote ntial c. 473
k b it /s
BTS
Thus, with the expected five slots in the downlink, each with the potential to carry 118.4 kbit/s, and using two radio carriers, the first phase of EDGE Evolution has a theoretical maximum bit rate in the downlink of almost 1.2 Mbit/s. Using four timeslots in the uplink direction, each with the potential to carry 118.4 kbit/s, means overall uplink potential for 473 kbit/s.
Conclusion EDGE Evolution offers significant performance gains that should be of interest to operators both with and without an existing development plan through UMTS. It has the potential to increase significantly the working life span and capability of a GSM system with a relatively small CAPEX investment. In many cases BTS enhancement can be done with a software upgrade and a corresponding increase in backhaul capacity. Lead times can be much shorter than those required to upgrade to UMTS. EDGE Evolution also provides a radio interface ready for the co-timed improvements to the radio access and core networks as they migrate to an all-IP and IMS infrastructure. This leaves an operator’s network in a condition appropriate for subsequent upgrade directly to LTE.
Need more? Delivered on your site or as regularly scheduled public events, Wray Castle offers courses covering a comprehensive range of technologies. Please call us today to learn how we can help your engineers to appreciate the full benefits of EDGE Evolution, or visit our website for full course details and public course discounts. European Office: Telephone +44 (0) 1539 742 742 North American Office: Telephone +1 800 594 5102
February 2009
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