Powering VoIP over Wimax

18 Aug 2008


So Wimax systems that use a 2.5-GHz radio system instead of a 3.5-GHz system are not just reducing their chance of signal loss in direct proportion to the difference in frequencies, but by a much larger value.

However, there is one challenge - albeit not a fatal one - that is proving to be difficult to resolve.

Power-hungry handsets

One of the key selling features used to move a customer to Wimax is offering VoIP service in addition to broadband data services. This certainly was the case with Buzz Broadband.

Of course, customers are naturally assuming that since VoIP service is part of the Wimax service, then Wimax handsets will soon be available. However, handset designers are beginning to worry whether they will be able to design a handset with long-enough battery life. The hang-up is the power amplifier used in the handset's transmitter. Put simply, it's not very efficient.

While it's not a big deal when using a PC or a USB Wimax-phone tethered to a PC's massive battery, it does becomes a big deal in handsets. The power consumed by the power amp is currently about 1W to 1.5W, which cuts significantly into the life of a handset's battery.

One measure of a power amp's efficiency is its power added efficiency (PAE) rating. PAE is the ratio of the power going into the power amp to the output power of the transmitted signal. A high PAE means that most of the input power from the battery is converted to signal output power. A low PAE means that a large portion of the input power is lost to heat or otherwise wasted.

The PAE for a GSM phone is 50% to 55%. For a WCDMA phone, it's 40% to 45%. For a mobile Wimax phone, it's 10% to 20%.

The culprit for the low PAE is the transmission method used in Wimax: orthogonal frequency division multiplexing. OFDM uses multiple carriers. That imposes a major load on the power amp, because OFDM requires that the power amp deliver very high linearity over a wide dynamic range. That can only happen by boosting power consumption.

The dilemma that designers find themselves in is that Wimax requires a signal power output level of about 250 mW. Yet, for the power amp to operate in its linear operating range so it can handle the multiple signals being multiplexed at a given time, its actual power output needs to be about 2.5W. That is, the power amp consumes 2.5W of power from the battery to produce a 250 mW signal.

Power amp manufacturers are scrambling to boost the PAE of their devices. Their immediate goal is to boost PAEs to 20-25%, and then to 40-45% levels. However, handset manufacturers can't wait - the launch date for mobile Wimax is fast approaching.

Power/data tradeoff

So designers have two choices in using today's power amps. One: lower the data rate for the uplink. Two: lower the output power of the handset.

By using simpler modulation technologies - QPSK instead of 64-level QAM for example - and fewer carriers, then the designers can reduce the linearity requirements of the power amp.


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