Playback apps push cell phones to Edge

Playback apps push cell phones to Edge
Cell phones are moving to faster data rates and adding such features as cameras and MP3 playback as they evolve from wireless telephones to mobile communication, information and entertainment appliances. Adding such features and capabilities requires modest advances in the underlying semiconductor technology. Additional work is needed at the system level to figure out how to implement all these new features with minimal impact on battery life. And clever tricks to save power in all areas of the cell-phone platform are needed to extend both talk and standby times.

Faster air interface speeds provide faster file transfers and add voice capacity by freeing user time slots. The latest advance in the Global System for Mobile Communications (GSM) family of standards, used by the majority of cell-phone users worldwide-including such U.S. carriers as AT&T Wireless, Cingular and T-Mobile-is Enhanced Data Rates for GSM Evolution, known as Edge. In the CDMA family of standards, the latest version is cdma2001XRTT.

Neither of these is a 3G system in the strictest sense of the term (or the ITU definition), but consumers don't care if the underlying system is 2G, 2.5G or 3G. What matters most is the user experience, including the time taken to transfer a picture file, the ability to access the phone book, the quality of the audio playback and the battery life. Nonetheless, it is useful to understand the technical challenges inside the cell phone.

Edge uses the same 200-kHz channels as the original GSM/GPRS system, as well as the same network signaling and backbone. But the data rate is accelerated from the 30 to 50 kbits/second available in GPRS to 100 to 200 kbits/second. The actual speed depends on such factors as the number of time slots available, the signal strength and the presence of interference, because the Edge protocol uses different coding schemes depending on those factors. In the ideal case, with strong signals and no interference, a Class 12 Edge device can transmit or receive data at a rate equal to four time slots at 59.2 kbits/s per slot, for 236.8 kbits/s total.

Edge achieves these higher data rates by replacing the Gaussian minimum-shift keying (GMSK) used in GPRS with eight-level phase-shift keying (PSK). That lets 3 bits of data be sent at a time instead of just 1 bit, effectively tripling the data rate without increasing the bandwidth.

This improvement isn't free. The Edge transmitted waveform is a nonconstant envelope and can't be generated by the techniques most commonly used for GSM/GPRS transmitters. To maintain the same error rate, a receiver for 8-PSK requires several decibels' more signal-to-noise ratio. Consequently, the entire radio section of the phone needs upgrading from the performance needed for GSM/GPRS. The Edge receiver needs a lower noise figure, the baseband A/D converters need a few decibels' more dynamic range and the power amplifier needs higher linearity.

To complicate matters further, the data receiver, which includes the equalizer and detector functions performed in DSP, also becomes more complex, because of the larger number of bits per symbol. Therefore, the DSP speed requirement rises from the sub-100-Mips speed needed for GSM/GPRS to well over 200 Mips for a full Class 12 Edge terminal.

Fortunately, the requirements have been well understood, and chip sets are coming to market that meet both the analog and DSP requirements for Edge terminals, so handset consumers will soon be able to reap the benefits of the higher data rates.

The industry has struggled to find the killer app for wireless data at any speed. Two interesting applications are emerging: digital imaging and music playback.

In the past few years, picture messaging has become popular in Japan over the J-Phone network. Recently, the standardization of Multimedia Messaging Service (MMS) has enabled the same capability on networks using the GSM family of standards, and camera phones are emerging in those markets.

The camera phones have created a need for low-cost imagers and the accompanying analog signal processing as well as digital processing capabilities to reduce the size of the picture file by compressing the image before storage or transmission. Of course, if a big picture file is to be stored in the phone, one can count on a lot of flash memory as well. Also, displaying received pictures creates a need for better color displays with brightness control and wide-angle viewing.

Time will tell whether the majority of cell phones will integrate a camera. Clearly, the move to higher data rates such as Edge will pave the way to sending and receiving pictures.

The other application emerging for handsets is the addition of music playback. The audio signal chain in most cell phones is intended for reproduction of voice, with the accompanying limited 3 kHz or so (monaural) bandwidth and limited fidelity. Music playback, whether from files downloaded over the air or inserted on a memory card, requires higher fidelity and stereo playback.

While it is tempting to compromise on the performance of audio as a frill, a higher-quality sound experience will generate repeat business from a satisfied user. The wireless carrier and content provider benefit from increased fees for music downloads.

Newer baseband processors include provisions for power savings in their digital-processing sections.

Also, baseband processors for newer standards, such as Edge, may include provisions for reducing the processor's core operating voltage as well as the clock speed when full performance isn't needed.

Careful attention to the RF power amplifier in a cell phone can result in additional power savings. In systems like GSM, the output power can vary over a considerable range, plus/minus 2 dB at full output, and still meet certification requirements.

While 2 dB might not seem like much, it can have a huge impact on the battery life. Consider a cell phone with a nominal +33 dBm output power (2 W). If inaccurate power control is implemented, allowing the output power to drift to the upper limit of +35 dBm, nearly 1.2 W of additional power is wasted.

New techniques, offering more precise control of a handset's output power, are emerging that hold the output power to the desired level and conserve battery power.

Doug Grant manages business development for RF and wireless at Analog Devices Inc. (Norwood, Mass.).

http://www.eet.com