WiMAX Receiver Basics: Operation of a Dual Channel WiMAX Receiver

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The diagram shows a simplified version of a typical WiMAX™ mobile station (MS) receiver. The bold dashed lines indicate the major feedback loops. As we work through the design, we can identify how these impact testing.

For cabled testing, RF micro-sockets switch the antennas out of circuit. Antennas can be partially modelled in test waveforms. The front end of the receiver applies analog filtering to reduce high level adjacent and non-adjacent channel (blocking) interference. Testing involves raising the wanted signal typically 3dB above the nominal sensitivity value, and adding the interferer at a significantly higher level. The interference may need to be in a number of different wireless formats (e.g. WLAN).

Switchable low noise amplifiers (LNA) are needed to meet cell edge sensitivity requirements. The noise floor of the LNA is a key influence when testing the sensitivity test of the whole device. Calibrated additive white Gaussian noise (AWGN) can be used to isolate the effect of RF hardware and demodulator performance.

The gain of the receiver chain must be constantly adjusted by the DSP, symbol by symbol if downlink data boosting is used, to avoid undue demodulation errors from compression and quantisation errors. The response to level variations for both frame-frame and during a frame can be tested by changing the data burst power, or with frames using different channel pseudo random seeds.

Once the gain has been set and signal digitized, the DSP must use the preamble to identify and then recover the signal from a variety of fading channels. While conformance testing verifies the statistical performance over tens of thousands of packets, the DSP developer needs to know which precise channel conditions cause packet recovery failures. Even shortened fading channels allow comparative, or regression, performance testing. It can be faster and is much lower cost than “real time” emulation.

Another feedback loop involves locking the frequency of the local oscillator to the incoming base station (BS) signal. Receive–transmit timing is also updated as the MS moves. These loops need to be tested for speed and tracking, before moving on to the more complex task of data recovery.

After the DSP has done its best to recover the signal, the media access control (MAC) can transmit automatic repeat request (ARQ) management messages or HARQ ACK/NACK packets to the BS as part of the constant feedback. Memory and processing constraints become the factors influencing test behavior.

Each level of control will have different response rates and control loop bandwidths. Ultimately the radio will be tested with many variables being changed simultaneously, but during design and validation the operation and responsiveness need to be assessed step-by-step

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