Testing on the EDGE Part I

We all have witnessed the explosive growth of digital cellular communications, particularly mobile phones that implement the GSM standard. When you combine the accelerating market acceptance of digital wireless with the attraction of high-speed data services, today’s digital cellular engineers are faced with significant challenges. These challenges will require more extensive testing of both the parametric and protocol components of mobile and base-station systems as they become more data-centric.

In the very near future, the mobile engineer, already hard-pressed to test the functionality of today’s mobile terminal, will be in a position of trying to test mobiles with even more advanced features:

• Triple mode with the capability to provide GSM and either TDMA or CDMA voice and roaming between the two networks; most likely, backwards compatible with AMPS analog cellular networks.
• GSM CSD, HSCSD, or ECSD as well as GSM SMS functions.
• Quad Band capable of supporting 800-, 900-, 1,800-, and 1,900-MHz bands for true international roaming across two entirely different networks, maybe even five-band with the new 400-MHz GSM capabilities being discussed in Europe.
• GPRS and EDGE capabilities allowing data rates from 9.6 kb/s to 384 kb/s or more in a mobile environment.
• Precise power control levels and the capability to change channel-coding schemes and modulation formats to handle changing channel conditions based on QOS requirements on the air interface.

Test systems generally have copied the standard that they are supposed to test with little thought given to multistandard or multifunction capabilities. Today, most test systems do an excellent job of testing the basic RF functions of a mobile or base station. Frequency, RF power levels, BER, and ACPM are all common tests.

Current test systems not only must offer the capability to test parameters, like power vs. time, but also to test software interaction with hardware. Figure 1 (see the February 2002 issue of Evaluation Engineering) shows the test limits for GSM ramp-up power, digital modulation, and ramp-down power.

With the rapid deployment of digital services, there is a growing need to test outside the basic parametric functions of the radio. With the higher software content of digital mobile radios and expanded feature sets, a critical eye is being turned toward the performance of the software internal to the mobile and the interoperability of the phone on a particular network in various channel conditions. Testing hardware requires a different approach to test-system development as we now want to look at hardware interaction with software-defined coding schemes and other parameters.

GPRS and EDGE Technologies

GPRS simply is an extension of the GSM standard to provide packet data services. GSM uses TDMA technology that allows eight users on one 200-kHz RF channel. These users are assigned to slots, with each slot’s duration in time being 577 µs. Together, all eight slots become a frame with a corresponding duration of 4.615 ms. We also know from some previous calculations on the corresponding channel capacity that the number of bits per time slot or burst is equal to 147 with GMSK modulation.

In a voice mode, the user always would be assigned one slot for transmission of voice data. The same is true for a form of wireless data service called circuit-switched data. With circuit-switched voice or data, we take advantage of the frame system used in a number of telephone networks to transmit the data. You are assigned a time slot, and that time slot always is yours to use whether there is data present or not.

With a GSM system, voice or data is sent during each slot every frame. This means that the phone is ramping up and sending data whether data is present or not and then ramping down. Circuit-switched operation is inherently inefficient because a slot always is assigned whether or not the mobile has any information to send.

Recently, designers of these mobiles have recognized this, and to save battery life, they have developed a means to shorten the burst using a method of detecting whether information is present and then discontinuing the burst. This is known as DTX. This is only a battery-saving function since the mobile still is always assigned to that particular slot.

Things change dramatically with packet data services such as GPRS or EDGE. These systems allow the mobile to transmit any number of slots either consecutively or within a frame as directed by the network. This permits the system operator to take advantage of dead air time associated with circuit-switched networks to increase capacity and data rates. GPRS and EDGE data sessions are not circuit switched and, for that reason, require a new package routing network to interface to the IP.

GPRS and EDGE Modulation Formats

GPRS is an evolution of the existing GSM modulation and channel formats. The modulation scheme is the same as for GSM and called GMSK. The GMSK digital modulation format relies on gently shifting the carrier 180° in phase to produce a binary modulation scheme capable of delivering 1 bit/symbol. Binary modulation is best displayed by looking at a rotating vector on an I/Q map and plotting the two points of the signal’s phase where the symbol designates a data symbol and, in the case of binary modulation, the data bit.

As Figure 2 (see the February 2002 issue of Evaluation Engineering) shows, there are two points in the vector rotation where you gather data, each point shifted 180° from the other. This relatively simple modulation scheme within a 200-kHz bandwidth provides good spectral performance and adequate data rates for GSM voice applications; however, it cannot supply fast data services since it only transmits 1 bit/symbol.

EDGE takes the same bandwidth allocated to GSM voice and GPRS data and effectively triples the data rates by using a new modulation scheme called 8PSK as shown in Figure 3 (see the February 2002 issue of Evaluation Engineering). Keep in mind that with any PSK modulation you want to avoid crossing through zero amplitude and, as such, the EDGE version uses a 3p/8 rotation to alleviate the associated problems that a zero crossing has on a high-efficiency amplifier used in today’s cellular phones.

With 8PSK, there are eight distinct phase changes that the decoder will look for in the binary data. With every phase transition, the symbols rotate an additional 3p/8 or 67.5°, causing a shift of the I/Q constellation relative to its previous starting position.

8PSK is a high-level modulation and provides high data rates for applications due to its 3 bits/symbol feature. However, it is inherently prone to errors in the air interface due to its fast changing phase profile and requires channel-coding schemes to correct for various QOS requirements.

Channel Coding

Channel coding in a GPRS or EDGE system protects the data that is being transported across the air interface and is implemented to correct errors in the bit stream caused by the RF environment. GPRS and EDGE use different convolutional encoding and decoding processes, bit-puncturing schemes, and interleaving processes to account for different RF channel conditions and QOS requirements.

This is required because the air interface can be highly destructive to the RF channel. If we are to realize the benefits of higher data rates, then we want to provide as little encoding as possible to preserve the data being transferred. But we also need to ensure that we do not drop the call when conditions suddenly change due to fading.

Figure 4 (see the February 2002 issue of Evaluation Engineering) shows an example of a channel-coding scheme for EDGE and indicates the relationship among the various processes. Bits of the data stream are removed or punctured to reduce the size of the bit stream using a predetermined mask for that particular channel-coding scheme. The data sent between the MS and the network is not a 1:1 burst-to-block conversion.

The data in each burst is broken into subsections and dispersed into the RLC blocks using a predetermined mapping scheme across multiple bursts in a process called interleaving. Finally, convolutional coding on the bit stream uses redundancy bits so that a decoder can detect errors in the bit stream and correct them.

The scheme shown in Figure 4 is MCS-7, which uses the 3p/8 8PSK modulation scheme. Channel-coding schemes used in GPRS and EDGE must be bidirectional, meaning that they are encoded and decoded the same way. If we look at this example from a decoding point of view, you can see that we are taking four consecutive bursts and deinterleaving the information or symbols.

After deinterleaving, the bits are depunctured and sent to a 1/3 rate convolutional decoder. Further parsing these bits provides us with the RLC blocks that are the basis for all of our signaling or protocol needed to set up an EDGE call.

While GPRS uses the same modulation scheme as GSM, there are four different channel-coding schemes to provide varying levels of protection to the packets either going or coming from the mobile. Table 1 (see below) shows the impact that different channel-coding schemes and the number of slots can have on effective data rates.

Table 1. GPRS Data Rates for Channel-Coding Schemes 1 through 4

First-generation GPRS systems will use four slots in the downlink (base station to mobile) and one or two slots in the uplink (mobile to base station) direction to transfer data. This is due to the fact that during a data session, typically more information is received than transmitted.

EDGE uses nine channel-coding schemes separate from GPRS schemes CS1 through CS4. In Table 2 (see below), we see a sample of a few of the corresponding data rates for channel-coding schemes MCS1-4 that use GMSK and MCS5-9 using 3p/8 8PSK modulation. There are dramatic increases in the data rates from 1 bit/symbol GMSK modulation to 3 bits/symbol data rates of 3p/8 8PSK and the difference that channel coding (protection) can make in effective overall data rates.

Table 2. EDGE Data Rates Using 3p/8 8PSK Modulation vs. Different Channel-Coding Schemes and Slot Combinations

GPRS and EDGE have much in common with conventional GSM systems. They share the same slot structure and timing and the same 200-kHz channel bandwidth. However, significant differences exist between a GSM voice system that is circuit switched and a GPRS or EDGE system that is packet routed.

About the Author

Rob Barden is senior product marketing manager at IFR Systems. He has been in the test equipment industry for more than 16 years and with IFR for the last three years. IFR Systems, 10200 W. York St., Wichita, KS 67215, 316-522-4981, www.ifrsys.com.

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February 2002