Articles

Putting EDGE phones to a test

(This article was submitted by Matthias Weber, Director of Product Marketing for Willtek Communications in Ismaning, Germany, and Petri Toivonen, Test Manager for Nokia Mobile Phones in Finland.)

EDGE or "Enhanced Data rates for Global Evolution" is a technology that was specified in 1997, but until recently had difficulties in making it to the market. In Europe, where the EDGE specifications were laid down, there has always been the discussion what should come first, Wideband CDMA (UMTS) or EDGE. How should EDGE be positioned - as an alternative to Wideband CDMA or as a complement? In the USA, EDGE was meant to pave the way towards 3G for the TDMA operators, where it was adopted in a slightly different way as "EDGE Compact" in order to cope with the low bandwidth available for mixed TDMA/EDGE networks. But this became obsolete when many of the TDMA operators decided to replace TDMA with GSM. Now even in Europe operators see the benefit of EDGE and are implementing it. One mobile phone manufacturer has started to deliver EDGE-capable phones to the European, Asian and US markets, which makes it obvious that we need to start taking a more proactive look at where EDGE differs and what needs to be measured in addition to GSM, since EDGE phones will still be using 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 three bits per symbol, in contrast to GMSK modulation where only one bit per symbol is transferred.

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 wireless terminals are mainly on the physical layer, the impact on terminals 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 3π/8 per symbol, which ensures that the signal never reaches the origin.

This effect has an impact on the type of amplifier that can be used and may result in separate TX paths for GMSK and 8-PSK, as we will see in the next section. The type of amplifier also affects the battery life. This means that the standby time and the talk time will be constrained.

GPRS uses several coding schemes; in EGPRS, the different coding schemes are combined with a choice of modulation formats: GMSK or 8-PSK. This helps to maintain the connection at reduced data rates when radio conditions get worse. Table 1 shows the different Modulation and Coding Schemes (MCS) for EDGE.

Table 1: Modulation and coding schemes (MCS) for EDGE

How does EDGE affect mobile phone design?

The new modulation requires a completely new transmitter design. The envelope is no longer constant and therefore the amplitude is changing, so for EDGE a different power amplifier is chosen.

A common approach for RF designers is to split the transmit path into two: a path for GSM (GMSK) transmission and one for EDGE (8-PSK) transmission. It is interesting to note though that the physical receive path remains the same. The receiver architecture is able to cope with the different types of modulation without the need to have two separate paths.

Figure 1 below shows the principle block diagram of an EDGE enabled phone, where the different TX paths for GSM and EDGE can be clearly identified.

Other parts of the mobile phone that need modification include the demodulator, which needs to demodulate the signal without advance knowledge of the modulation type that the base station applies. This process is called blind demodulation. Also, the increased data rate implies that the channel codec must have greater capacity.

Figure 1: Typical block diagram of an EDGE enabled phone

Tests required for EDGE-capable phones

Since EDGE-capable phones still support GSM for voice, EDGE testing increases the overall test time. Therefore it is important to focus on those test criteria that are most significant for this new technology.

In the block diagram above we see that there is one RX path, which is usually tested as part of standard GSM testing. If the receiver quality is good for GSM it is also good for EDGE. If this were not the case, there would be a flaw in the design of the receiver! Furthermore there are two TX paths, one for GMSK and one for 8-PSK, which means that in addition to the standard GSM tests, the 8-PSK transmitter needs to be tested as well.

The tests foreseen for an EDGE transmitter are:

• Power measurements, including the power-time-template

• Modulation quality measurements

Power measurements

One important quality factor with all RF systems is transmit power. The maximum achievable power level is key to a good data transfer rate in rural areas with a large cell radius. As the transmitter is in a different operating mode when applying 8-PSK modulation, measurements are essential to ensure that the power level lies within the allowable limits. Note that the cell radius can be lower with 8-PSK modulation than with GMSK due to the increased number of symbols.

To minimize unnecessary interference with adjacent cells and to ensure proper handover between cells, the power control mechanism in GSM and GPRS uses further power steps with exact levels.

In manufacturing and after repair, these levels are usually calibrated with the help of a test set, with correction values being stored in the mobile phone.

Measurements of power versus time use the same principles for 8-PSK as in plain GSM but with a modified template; the template takes into account that the 8-PSK signal amplitude varies between symbols, depending on the exact symbol sequence. This 8-PSK attribute makes it more difficult to use cheap, nonlinear amplifiers and may require hardware and/or software designs to correct these nonlinearities. The power-time template for 8-PSK signals is shown in Figure 2; the flat part at the beginning and the end of the burst applies to the tail bits, which have been chosen such that only minimal variations of the power level occur.

The limits of the power level steps are identical to those known from GMSK modulation.

Figure 2: Power-time template for 8-PSK bursts

Modulation quality

8-PSK requires modulation quality measurements that are different from GMSK. The beauty of these measurements is that they provide parameters which are more likely to allow you to track down the source of any potential error.

The key measurement is that of the error vector magnitude or EVM for short, which is the distance in the I/Q diagram between the measured and the ideal signal (see Figure 3); the EVM is measured for each symbol separately. Standard measurement results to observe are the RMS-averaged EVM (for all the symbols of a burst), the maximum EVM within the burst, and the 95% EVM. The latter is the error vector magnitude not exceeded by 95% of the symbols within a burst; the parameter effectively disregards the highest 5% of all error vectors.

The error vector can be broken down into two components; one component is tangential i.e. in the direction of the circle (phase error) and the other is vertical to the circle (magnitude error).

These parameters are not part of the mandatory measurements but can be helpful in tracing a problem with the modulator.

Figure 3: Error vector definition

Another parameter which is determined inside the test set but not necessarily shown or assessed is the amplitude change or droop, a measure of the stability of the power level during the burst. An amplitude change may be caused by on-chip temperature changes during the RF transmission. Finally, if the I and/or the Q component of the modulator has an unwanted DC component, this can be determined in the origin offset suppression measurement.

All these parameters, whether mandatory or not, are well known from the US-TDMA system which applies a slightly different modulation (π/4 DQPSK) but with very similar calculations of error parameters. Table 2 summarises the measurement parameters with their test limits according to the ETSI standards.

Table 2: Test limits

Test scenario

A typical test scenario for a final test can be seen in Table 3. This table describes the test steps and their reasons.

In this table there are no RX tests on EDGE receivers for the above-mentioned reason. This, of course, only applies to a final test in a service or manufacturing environment. When performing type approval testing, EDGE receiver measurements are certainly an issue!

Conclusion: EDGE-capable mobile phones must be tested in manufacturing as well as in service centres. It is evident that testing needs to go beyond the standard GSM measurements. The additional test effort can, however, be limited because it is possible to rely on transmitter measurements. Test time can thus be reduced to a minimum instead of increasing it unnecessarily. It is important to understand that GSM tests are still required because GMSK modulation is complemented, not substituted by 8-PSK.

Following this test philosophy, test engineers can minimize the impact of EDGE on the test time and concurrently ensure that network quality is not compromised by a defective mobile phone.