EETimes India - December 1, 2008 - (Page 8)

In Focus | Mobile handsets Meet WiMAX power amp challenges continued from page where N used is the number of subcarriers assigned to the user, and Ntotal is the total number of subcarriers available. For example, if a user is assigned one subchannel made up of 24 subcarriers, the net gain that is achieved relative to the BS that is transmitting on all 841 allocated subcarriers is 10*log(841/24)=15.4 dB. The other subcarriers are made available to other users, and they can use these simultaneously. Another technique to address the link imbalance is adaptive modulation. In this case, the mobile transmits using a lower order modulation compared to the BS. For example, the mobile could transmit QPSK or 16QAM signals, while the BS transmits using 64QAM. Because the SNR required to receive QPSK or 16QAM is lower than 64QAM, using a lower order modulation allows the MS to communicate with the BS using less transmit power (although uplink throughput is reduced, since fewer bits are transmitted per subcarrier with lower order modulation). For example, the SNR required for QPSK-1/2 is 5 dB as compared to 10.5 dB for 16QAM-1/2 and 20 dB for 64QAM-3/4 modulation. If the MS transmits with QPSK, the BS can tolerate 5.5 dB more link loss than with 16QAM. When subchannelisation and adaptive modulation are combined, a network operator can effectively balance the uplink and downlink budgets, and the network will operate bi-directionally. The downside is that when these techniques are used, the uplink throughput will be lower than the downlink throughput. Subchannelisation limits the number of subcarriers available for mobile transmission, and lower order modulation means that fewer bits are transmitted on each available subcarrier. WiMAX cell power With all these in mind, let us examine what the transmit power profile looks like across a WiMAX cell. A common misconception is that mobile stations transmit at maximum power only at the edge of a cell, and at lower power when mobiles are closer to the BS. In reality, 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 towards the BS. When it is at the extreme cell edge, path loss will be very large, so the mobile device will be transmitting at maximum power with the most robust modulation. As a result, uplink data rates will be relatively low. However, with the high MS transmit power and robust modulation, the BS will be able to receive transmissions from the MS, and the link is sound. As the mobile moves closer to the BS, path loss decreases. The signal level at the BS increases, and the SNR improves, since the received signal is now farther above the noise floor. In response, the BS may instruct the mobile to start reducing power to minimise potential for interference between different mobile stations. However, as soon as the signal level supports a higher order modulation, the BS will instruct the mobile to switch modulations in order to increase overall network capacity. Going back to our example comparing QPSK and 16QAM, suppose a transmitter operates at +23dBm and it just achieves the 5 dB SNR required for QPSK when it is at the edge of the cell. As it moves closer to the BS, path loss drops and the BS may ask the MS to reduce its transmit power. However, as soon as the path loss has decreased by 5.5 dB, the BS will instruct the MS to switch to 16QAM-1/2, and will increase transmit power back to +23dBm, since the MS will now be able to achieve a 10.5dB SNR. Therefore, a mobile will typically transmit at higher powers until it is close enough to the BS to achieve 16QAM operation (or even 64QAM in many instances), at which point power is reduced. Figure 1 was derived using parameters from a WiMAX Forum whitepaper. It shows the modulation that is achievable as a function of distance from the BS. Since we use the parameters in the whitepaper, maximum available path loss is calculated assuming a 10MHz channel bandwidth at 2.5GHz, with 3 subchannels and 10 dB penetration loss. In calculating the path loss, we have assumed a COST231 suburban model at 2.5GHz with 32m BS height and 1.2m MS height. This analysis has assumed the presence of slow (lognormal) fading, but is somewhat simplified, since we assume a fixed 5.5 dB fade margin. In reality, of course, fading is a random process, and closed loop power control will be used to help mitigate its effects. However, for the sake of this analysis, the conclusions are valid, as fading will simply blur the boundaries between the different modulations. Note that the red ring, labelled QPSK-1/8 represents QPSK-1/2 modulation with a repetition factor of 4. This is the most robust modulation scheme, and it can be seen that it is indeed required at maximum range. In our analysis, we calculate that with +23dBm transmit power, a MS must use QPSK-1/8 for mobiles from 0.9 km to 1.35 km from the BS. At closer distances, the MS is able to use higher order modulations, and network capacity is therefore increased. For example, the MS is able to use 16QAM-1/2 modulation at distances from 0.45 to 0.6 km from the BS. Since 16QAM-1/2 modulation transmits 2 bits per symbol, while QPSK-1/8 transmits only 0.5 bits per symbol, one can see that the throughput in the green ring is 4 times higher than in the red ring. We can also estimate the required transmit power as a function of range. At the edge of each of the zones in figure 1, the MS Read more online InGaP HBT vs. CMOS for mobile handset power amplifiers Know which technology is better suited for power amplifiers in mobile applications. • Extending battery life and solving RF power amp problems • Understanding RF power amplifiers www.eetindia.com will be transmitting at maximum power. It will decrease its transmit power as it moves towards the BS, until it has sufficient power to achieve the next modulation order. At this time, it will increase transmit power again to maximise capacity. Figure 2 shows the expected transmit power as a function of distance, showing the impact of adaptive modulation. It can be seen that transmit power is significantly reduced only when the maximum modulation order has been achieved, which in this case is 64QAM-3/4. If the maximum modulation order was instead 16QAM-3/4, then the transmit power would be monotonically reduced once the 16QAM-3/4 rate was achieved. It should be noted that the presence of fading will result in significant changes to this curve. In a real-life fading environment, additional margin may be required to counteract fading effects, and one would expect that transmitting at maximum power would occur less frequently. However, the overall trend shown in the figure is correct, and shows that mobile stations will be required to transmit at high powers not only at the cell edges, but also at much closer distances in order to achieve higher-order modulation. Read the full article to know the benefits of higher power transmission for mobile WiMAX, plus power transmission limitations and more. 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Table of Contents Feed for the Digital Edition of EETimes India - December 1, 2008

EETimes India - December 1, 2008 Contents National Semiconductor WiMAX Goes Beyond Mobile Devices Mobile Handsets Integrate WLAN Meet WiMAX Power Amp Challenges NGSA - 08, Energy INDIA, ICDCIT 2008, REA-2008, ICON 2008, ICPCM 2008

EETimes India - December 1, 2008

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