The Need for Speed:
WWG Calls on QNX to Fast-Forward Cell Phone Testing

Bernd Herzmann, Wavetek Wandel Goltermann

By the year 2003, there will be an estimated 500 million cell phone subscribers on the planet. But you don't have to be a slave to marketing projections to know that mobile phone usage is rampant and growing at an astronomical rate. Simple observation is enough to tell you that cell phones are one of the most pervasive inventions since, well, the telephone. And they're getting less expensive, too. So much so, that in some countries cell phones are virtually replacing traditional wired phones.

The challenge

With all this demand for more and more cell phones, manufacturers are under the gun to keep up the supply - and get their phones to market before their competitors. But how? Shortening the test phase is one way. Although testing is an absolute must in mobile phone production, it is a bottleneck that slows overall output of a production line.

Wavetek Wandel Goltermann (WWG) has been in the business of designing and manufacturing test equipment for almost 80 years. Recognizing the need for a faster mobile phone test system, we challenged ourselves to design a new platform with architecture that allows for parallel as opposed to serial test measurements.

The result of our efforts is the Wavetek 4400 series, rolled out in June of this year. Using new measurement techniques made possible by QNX, this mobile phone test series (the 4400M for manufacturing, and the 4400S for post-sales service) cuts overall testing time by up to 50 percent while maintaining outstanding measurement accuracy. The 4400 series can be used for tests supporting all forms of GSM (Global System for Mobile Communications) technology.

GSM 101

To understand why the 4400's testing techniques are so unique in the industry, we first need to know a little about the way a GSM network operates. GSM is a digital cellular radio standard with networks in place in over 200 countries worldwide. GSM networks provide almost complete coverage in western Europe and growing coverage in the Americas, Asia, and elsewhere. In this system, geographical service areas, which can be as small as an individual building (such as an airport) or as large as 20 miles across (such as a small city), are divided up into cells theoretically organized in a hexagonal honeycomb structure.

In each cell, mobile phone users (or mobile stations) communicate over the radio (Um) interface with a base station, the link between the wired network and any cell phone in the base station's cell (see Figure 1).

Base stations, in turn, communicate over the Abis interface with base station controllers (BSC), which are responsible for coordinating the radio traffic of their base stations and cell phone users - assigning traffic channels, transferring calls from one cell to the next when customers approach the boundary of a cell (handovers), and so on. BSCs also connect to mobile service switching centers (MSC) over the A interface. MSCs, which are linked directly to the wired network, administer all calls in their region, switch calls between mobile users and between mobile and fixed users, and handle mobility management operations.

All the components - base station, base station controllers, and mobile service switching centers - are involved with each cell phone call. Booking a call, checking authorization, assigning traffic channels, and changing cells all require an intensive exchange of information between components known as signaling.

TDMA time slots

The GSM system uses a combination of Time- and Frequency-Division Multiple Access (TDMA/FDMA) to divide up the bandwidth among as many users as possible (see Figure 2). The FDMA part involves the division of the 25MHz bandwidth into 124 carrier frequencies spaced 200kHz apart. One or more carrier frequencies are assigned to each base station.

Each encoded voice-data stream is divided into a package called a multi-frame, which is divided again into 26 frames using a TDMA scheme. Each of the frames, in turn, is made up of eight burst periods or time slots, the fundamental unit of time in this TDMA scheme. A burst period or time slot lasts approximately 0.577 ms (or 120 ms per 26 frames per 8 burst periods per frame).

Cell phones in call mode use an assigned time slot and carrier frequency (physical channel), which means they share the carrier with up to seven other cell phones. Phones in idle mode (i.e. not in a call) don't transmit at all.

A cell phone user or base station may only transmit data during the time slot assigned to it, and the radiated power must be within the power/time template for the entire duration of a time slot (see Figure 3). Outside of this assigned time slot, it must not emit any power. Since a time slot is only 0.577 ms long, this means a cell phone or base station must increase transmitted power very quickly and abruptly decrease power again once it has transmitted data.

Testing 1, 2, 3

Given the complexity of a GSM network, a misaligned or faulty cell phone can cause big problems in a network. In fact, a defective phone has the potential to block communications in a whole cell. Because of this, measurements have to be taken on various channels within the supported frequency bands to make sure certain functions of the cell phone work effectively and within given limits. Key functions that have to be tested include:

Power/time response - This measurement is the decisive criterion for interference-free transmission. According to GSM specs, the power of a cell phone must remain precisely within the power/time template so that it doesn't disturb adjacent time slots.

Transmitted power - In GSM, a transmitter can apply one of many power control steps. A cell phone in the vicinity of a base station must transmit at lower power than one at the boundary of a cell. Since GSM cell phones transmit in burst mode, the peak power is measured.

Frequency error - This is a measurement of how well a cell phone can align itself to a base station frequency.

Phase error - This expresses the modulator quality. Both the peak phase error (maximum phase error in a burst), and the RMS phase error (mean phase error for the duration of a burst) are compared to given limits.

Receiver sensitivity tests - The sensitivity of a cell phone receiver is measured indirectly. A data package is sent with a very low output level to the cell phone. The original package is compared to the one received and the number of incorrect bits are counted. This measures bit-error rate, which is the percentage proportion of incorrectly transmitted bits among all bits transmitted. Permissible bit-error rates are defined in the GSM specs.

Received signal strength - Every cell phone reports the received signal strength to the base station, so it's important to check that the received signal strength indicator is aligned correctly.

The short list

Despite the complexity of GSM communications, the software requirements of a cell phone test system are actually quite simple - a platform that's reliable, stable, and fast. Realtime performance is necessary for fast measurements and, especially, for fast signaling, which must work 24 hours a day, 7 days a week to avoid production delays. Based on these requirements it was relatively easy to narrow down our choice of OS.

Linux, RT-Kernel, NT, Windows 95, and of course, QNX, were all on our short list. While Linux had an appealingly low price and POSIX compliance, it also had less-than-great realtime performance and a much-too-complicated graphical programming environment. RT-Kernel offered good realtime performance and ease of use, but lacked a stable user interface. In addition, we would have had to maintain the OS ourselves.

The two Microsoft products we looked at, NT and 95, offered realtime performance that was either questionable or non-existent. While NT has lots of development tools, overall we felt the OS was too large and we weren't confident of the support we would receive from the company. We found QNX, on the other hand, to be stable, reliable, and well documented. More importantly, it's very robust and has excellent realtime performance. In fact, we believe the realtime behavior of QNX is the best on the market.

Results on the double

QNX's distributed processing also made it possible for us to build a unique feature into the 4400: parallel testing.

Our system consists of several DSPs that perform parallel tasks. These tasks send results and signaling information in parallel to the host PC to be processed in parallel and in real time by QNX. The 4400 can test receiver sensitivity and the complete set of transmitter signal quality parameters including the power/time template and phase/frequency error, simultaneously rather than sequentially. Typically, this capability reduces test time by 50 percent without reducing measurement accuracy.

The 4400 also takes advantage of the inherent frequency-hopping capability in GSM phones to reduce cell phone test time even further. Normally, signaling time is wasted with instructions to the cell phone to change the channel and then repeat the test procedure. With the 4400, all measurements can be executed while the phone is hopping through the frequency band, making reassignment of the channels unnecessary. Individual measurements are assigned to each channel and then compared to specs. This method is as accurate as measuring sequentially but it reduces test time by up to 20 percent.

The realtime performance of QNX comes into play with the transmitter power measurement. Alignment of the transmitter power stages is usually a time-consuming process. With the 4400, however, this alignment is a lot faster due to fast peak power measurements that detect, measure, and store twenty times per second - faster than any other test instrument on the market. This speeds up the board-level test by approximately 30 percent.

Another interesting feature of the 4400 is that it supports cross-band channel assignment, a feature necessary for testing dual-band mobile phones. (It's expected that in two to three years, almost all mobile phones will be able to communicate on at least two different GSM bands, which enables international roaming). The effect on production when switching to dual-band mobiles is enormous because, in addition to the existing test cycle on one GSM band, it has to be repeated on the second GSM band. Because of this, the production test time increases by almost 100 percent. However, with the 4400, both frequency bands can be tested continuously, which reduces the test time of dual-band mobile phones considerably.

Going to market

In addition to helping cell phone manufacturers save time in production, QNX also helped us get the 4400 to the finish line much sooner - and reduce development costs by an estimated man year.

QNX's memory protection helped us quickly locate and fix bugs during the development phase of the project. With the complexity of realtime projects, it can be difficult, if not impossible, to find and fix bugs. Memory protection helped significantly in allowing us to shave time off this tedious part of product development.

Another factor that sped up development was the fact that we didn't have to write absolutely all of the code ourselves. Since drivers are implemented as user-level processes and can be written without intimate knowledge of the entire application, we were able to contract a lot of them out to smaller software companies.

We believe that using standards in development is faster - at least 25 percent faster than without a standard - so QNX's POSIX compliance helped move our development along as well.

And when we needed some extra assistance, especially while we were implementing a key-related user interface (as opposed to a mouse-oriented one), the support of experienced QNX developers was there.

QNX's small size and modular architecture meant we could, and can continue to, configure the 4400 as needed. While this is certainly an advantage for us, modularity also makes our tester a secure long-term investment for our customers - new boards, processing power, bandwidths, and frequency ranges can be easily added to support new technologies and standards.

Looks matter

Currently there are two versions of this tester series: the 4400M, designed to run completely by remote control on a production line without a GUI, and the 4400S, for use in the post-sales repair of cell phones - complete with GUI. The GUI for the 4400S, which was built with the Photon microGUI and PhAB, has a user-friendly design allowing easy navigation through various menus and detailed online help. Currently there is only an English version of the 4400. But in future we want to use the supplements available for Photon to create different language interfaces.

Overall, we found we were productive with Photon quite quickly. It was easy to learn and, of course, the application builder shortened our development work considerably.

Because Photon is very small and scalable, we were able to tailor a system exactly to our needs. Beyond that, we were able to produce a GUI that really sets the 4400 apart from any other measurement device on the market. Most cell phone test systems don't have nearly as sophisticated a graphical user interface as the 4400 and certainly the vast majority have the same old steel- blue GUI of most measurement equipment and technical applications. With Photon and PhAB we were able to produce a GUI with a VGA high-resolution color screen that reduces fatigue and enables higher operator efficiency.

Thinking ahead

Since the system is modular - indeed, we designed it to be reused - the 4400 will serve as a platform for five further products. New applications and new technologies can be quickly integrated. For example, the 4400 is going to be developed further to incorporate other technologies such as cdmaOne and IMT 2000/UMTS.

For more information on Wavetek Wandel Goltermann now known as Acterna please visit www.acterna.com