How much transmit power do WiMAX nets need?

To achieve the long ranges it requires, a WiMAX network must have an optimized power profile—from the basestation to the components in the mobile device. High transmit power is important for long range, but how high can WiMAX go? Designers must find the optimal balance between high transmit power and low power consumption to ensure robust links, high data rates and good range for WiMAX services.

Mobile WiMAX networks will achieve coverage of ~1km per basestation, and providers will deploy numerous techniques to achieve this long range, including high transmit power, subchannelization and adaptive modulation.

A typical WiMAX basestation transmits at power levels of approximately +43 dBm (20 W), and the mobile station typically transmits at +23 dBm (200 mW). There is a large difference between downlink power and uplink power, so while a mobile can easily receive transmissions from a basestation, the mobile's relatively low transmit power makes it difficult for the basestation to hear it.

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One way to combat this mismatch is with a technique called subchannelization. In effect, each mobile concentrates its power over a subset of all available subchannels, and the other subcarriers are simultaneously made available to other users.

Another technique to address the link imbalance is adaptive modulation. In this case, the mobile transmits using a lower order modulation compared with the basestation. For example, the mobile could transmit QPSK or 16QAM signals, while the basestation transmits using 64QAM. Because the SNR required to receive QPSK or 16QAM is lower than 64QAM, using a lower order modulation allows the mobile station to communicate with the basestation using less transmit power. The SNR required for QPSK-1/2 is 5 dB, for instance, compared with 10.5 dB for 16QAM-1/2 and 20 dB for 64QAM-3/4 modulation. If the mobile station transmits with QPSK, the basestation can tolerate 5.5 dB more link loss than with 16QAM.

When subchannelization and adaptive modulation are combined, a network operator can effectively balance the uplink and downlink budgets, and the network will operate bidirectionally.