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How high can WiMAX power go?

Posted: 16 Dec 2008 ?? ?Print Version ?Bookmark and Share

Keywords:WiMAX. power transmit? mobile application? base station?

To achieve the long ranges it requires, a WiMAX network must have an optimized power profilefrom the base station to the components in the mobile device. High transmit power is important for long range, but 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 base station, and providers will deploy numerous techniques to achieve this long range, including high transmit power, sub-channelization and adaptive modulation.

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

One way to combat this mismatch is with a technique called sub-channelization. In effect, each mobile concentrates its power over a subset of all available sub-channels, and the other sub-carriers 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 base station. For example, the mobile could transmit QPSK or 16QAM signals, while the base station 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 base station using less transmit power. The SNR required for QPSK-1/2 is 5dB, for instance, compared with 10.5dB for 16QAM-1/2 and 20dB for 64QAM-3/4 modulation. If the mobile station transmits with QPSK, the base station can tolerate 5.5dB more link loss than with 16QAM.

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

Mobile WiMAX
A common misconception is that mobile stations transmit at maximum power only at the edge of a cell and at lower power when they are closer to the base station. This is not the case; mobile stations will transmit at high powers over a range of distances.

Consider a mobile device moving from the edge of the cell directly toward the base station. When it is at the extreme cell edge, path loss is very large, so the device transmits at maximum power with the most robust modulation. As a result, uplink data rates are relatively low. However, with high mobile station transmit power and robust modulation, the base station can receive transmissions from the mobile station, and the link is sound.

As the mobile moves closer to the base station, path loss decreases. The signal level at the base station increases, and SNR improves. In response, the base station may instruct the mobile to start reducing power. However, as soon as the signal level supports a higher-order modulation, the base station will instruct the mobile to switch modulations to increase overall network capacity.

Efficiency is essential
Undoubtedly, higher transmit power is important for mobile WiMAX networks. Networks are currently being deployed specifying that the minimum transmit power is +23dBm. Each user who enters a network transmitting at powers greater than +23dBm increases overall network efficiency. However, delivering higher transmit powers comes at a cost to power consumption.

As a result, power amplifier efficiency becomes more important.

- Darcy Poulin
EE Times

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