EDGE Compact and EDGE Classic Packet Data Performance
Even though cellular radio services have been extremely successful in providing untethered voice communications, wireless data services have captured only a limited market share so far. One obstacle for wireless data services is their limited peak bit rates. Existing wireless data rates, up to several tens of kb/s, may be over one order of magnitude short of what is required to make popular applications user-friendly. To accomplish these necessities we go for EDGE(Enhanced Data Rates for GSM Evolution) employs adaptation between a number of modulation and coding schemes (link adaptation) as a means for providing several hundred kb/s peak rates in a macro-cellular environment while supporting adequate robustness for impaired channels. In this paper we discuss the two phases of EDGE, classic and compact. We start our discussion with the link adaptation and incremental tendency techniques. Secondly we go for discussing EDGE classic and compact systems and their deployment scenarios, followed by downlink performance comparison and some MAC layer enhancement techniques which improve the performance.
The GSM system is the most popular second-generation wireless system today. It employs TDMA technology to support mobile users in different environments. This system, initially standardized and deployed in Europe, is currently deployed worldwide. The TDMA community adopted EDGE for high-speed data services in the third- generation, radio transmission technology proposal to ITU for IMT-2000 (UWC-136). Typically, current GSM service providerâ„¢s employ 3/9 or 4/12 reuse plans and they may not impose a different frequency reuse plan for EDGE, which is thus termed EDGE Classic. However, in North America, initial deployment using 1MHz in each direction is being considered due to limited spectrum and the potential need to re-deploy spectrum currently used for ANSI 136 systems. This implies very aggressive frequency reuse having a minimum of only three 200-kHz frequency carriers. This means allocating one frequency to each of the three sectors per base station and reusing the frequency set everywhere (1/3 reuse) and providing control signaling with extra reuse protection in the time domain, which is named EDGE Compact due to its compact spectrum requirement.
MODULATION AND CODING ADAPTATION
The basic concept of EDGE is to provide higher data rates per radio time slot than is possible with GMSK modulation. This allows the support of existing services with a lower number of time slots. In addition it allows the introduction of new services with up to 59.2 kb/s per timeslot or almost 480 kb/s per carrier in multi- slot operation, hence offering an evolution path for GSM to support multimedia applications.
Radio link formats
Discussions in the ETSI workshops resulted in selection of 8PSK/GMSK to provide higher rates than the GMSK modulation with small envelope fluctuations and to provide backward compatibility to GSM and GPRS. The EDGE concept can be seen as an extension of GPRS for packet service, which is called EGPRS. ETSI has also combined EDGE with circuit switched data modes, and these modes are called ECSD. Efficient link adaptation is a key feature for EDGE and has been jointly developed with EDGE enhanced modulation. With a high degree of compatibility in the bandwidth and symbol rates with GSM, EDGE provides higher rates for users with good signal to interference plus noise ratios (SINR). This is achieved by employing lower channel-coding redundancy and/or 8PSK, which carries 3 bits per symbol (as opposed to 1 bit per symbol achieved by GMSK). Table 1 shows the bit rate provided by different MCS (Modulation and Coding Scheme) modes. An EGPRS capable terminal will have 9 modulations and coding schemes available compared to 4 for GPRS.
Radio link control
Radio link control selects among the MCS options, in response to SINR or equivalent quality measures. Link adaptation explicitly changes MCS modes based on link quality estimates, and is also called mode selection. Hybrid ARQ transmits additional redundancy bits after errors are observed. It is made possible by sending the packets with different puncturing patterns from the same mother code during retransmission. This allows data transmission to begin with low redundancy and increases redundancy only when errors occur, thus adaptively changing the effective date rates. The criterion for selecting a particular data-rate as proposed is defined by S=Rc(1-BLERc) where Rc and BLERc are the data-rate and BLER (Block Error Rate, where a block is the RLC (radio link control) block.) for the transmission mode chosen. Figure 1 shows the throughput as a function of SIR for different modes. It is found that this threshold criterion is generally effective in achieving a high aggregate system throughput, but the QoS (Quality of Service) for individual users can be degraded as system load increases. Furthermore, link adaptation requires the receiver to continuously perform quality measurements and provide timely feedback to the transmitter, so typical operation may be with somewhat higher thresholds.
EDGE Compact and EDGE Classic
GSM systems are usually planned on the basis of 4/12 (4 base stations, 3 sectors each, per cluster) or 3/9 frequency arrangements. The carriers that contain broadcast control channels (BCCH carriers) are required to transmit continuously and without hopping on control time slots to facilitate handoff measurements, control channel acquisition, and so on. These carriers usually are arranged in a 4/12 reuse pattern. Traffic channels can frequency-hop and, on non-BCCH carriers, they can use discontinuous transmission (based on voice-activity detection), and if so, typically are arranged in a 3/9 reuse pattern. These arrangements provide the strong SIR protection typically required for delay-intolerant voice services and non-acknowledged control channels. EDGE Classic is defined to be a system using continuous BCCH carriers that are typically in a 4/12 or 3/9 reuse pattern and which requires at least 2.4 MHz bandwidth in each direction. Additional traffic carriers, if available with higher total bandwidth, can be deployed under a lower reuse factor. Some system operators, particularly those in North American where 3G spectrum has been partially allocated for PCS, have to re-allocate in-service spectrum to deploy EDGE.
In that case, EDGE Compact may be used for initial deployment using as little as 1 MHz in each direction allowing only three 200-KHz frequency carriers. This means allocating one frequency to each of the three sectors per base station, and the frequency set is reused at every base station (1/3 reuse for EDGE Compact mode). While good spectrum efficiency is achieved, the provisioning of common control functionality, such as system broadcast information, paging, packet access and packet grant, cannot be deployed with 1/3 reuse. 4/12 or 3/9 reuse is required for reliable control channels. In order to achieve adequate co-channel reuse protection for the control channels, reuse in the time domain is exploited, which requires frame synchronization of base stations. Figure 2 shows an example with 4 timing groups in addition to 1/3 frequency reuse to obtain 4/12 reuse for the control channels.
EDGE Compact uses discontinuous transmission based on a 52 -multiframe2 (a multi-frame consisting of 52 frames) and designates different time slots and frames for sending control information. In blocks (blocks are non-overlapping and are each comprised of timeslots from the same timeslot number of 4 successive frames) when a sector belonging to one of the time-groups transmits or receives common control signaling (serving time-group), the sectors belonging to other (non-serving time-groups) are idle. This creates an effective reuse of 3/9 or 4/12, which is necessary for control signaling, while allowing 1/3 reuse for the traffic channels. Specifically, slots 1, 3, 5 and 7 are used for timing groups 0, 1, 2 and 3, respectively, to send common control information on frames 0-3, 21-24, 34-37 and 47-50. Frames 12, 25, 38 and 51 are also not allowed for traffic as they are reserved as idle frames or to send timing advance, frequency correction or synchronization information. More frames can be allocated for control signaling as needed. Therefore, up to 32 frames or 8 blocks per 52 multi-frame can be allocated for traffic channels on the designated control slots. This is 2/3 that of a regular slot capacity in which 48 frames in a 52 multi-frame are used for traffic in a given non-BCCH slot. When using 3 time-groups (i.e., effectively 3/9 reuse), one of the 4 time-groups is unused and it is instead used as a traffic channel. Figure 3 shows the control channel BLER distribution for both 3/9- and 4/12-reuse based systems. For about 90% of the cases, the BLER is better than 4% and 15% for 4/12 and 3/9 reuse, respectively, corresponding to the overall average BLER of about 2.4% and 5.2 % (not shown in the figure), respectively. The performance of the 3/9-reuse system may not be reliable enough at the tail end of the distribution. However, since the traffic channel performance is highly correlated with that of the control channel, i.e., a mobile station with poor control channel BLER most likely cannot support reliable traffic performance, the tail end performance may not be very crucial. The control channel performance for EDGE Classic is expected to be similar because the same reuse factor is employed for the control channel.
The minimum spectrum required for Compact deployment is 600 kHz and that for Classic is 2.4 MHz (neglecting guard bands in both cases). Therefore, at 2.4 MHz and above, there exists the option of either Compact or Classic deployment. The choice of system is partly dependent on the performance of the systems. The performance in turn is dependent on the reuse configuration employed in the deployment. For the purposes of this study and to eligible valid comparisons, the reuse configurations are such that control channels are always at 4/12 reuse while traffic channels are at 1/3 reuse whenever possible. The exceptions are the traffic channels of a Classic control (BCCH) carrier, which are at 4/12 reuse. We also consider the same control-channel capacity (one active slot of a carrier) for both cases under all scenarios. Table 2 and the text following describe the scenarios considered: