Microwave Engineering Europe - July/August 2008 - (Page 21)

WiMAX DESIGN 21 (CDMA) transmit powers for both the BS and MS are similar to those used in WiMAX. However, because WiMAX uses much higher modulation orders to achieve higher throughput, WiMAX requires a much better SNR than cellular. For the mobile transmitter, high modulation orders require a PA with much better linearity and greatly complicates PA design compared to GSM or CDMA. You might notice that there is a large difference (approximately 20 dB) between downlink power (from the BS to the MS) and uplink power (from the MS to the BS), so mobile WiMAX networks are severely uplink limited (this is also the case for cellular networks, of course). This means that, while a mobile can easily receive transmissions from a BS, the mobile’s relatively low transmit power makes it difﬁcult for the BS to hear it. One way to combat this mismatch is by using a technique called subchannelization, where only a subset of all of the available subchannels is used for any particular user. In effect, each mobile concentrates its power over a smaller range of frequencies, and the net signal gain is: 10*log(Ntotal/Nused), where Nused 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 [1]. If the MS transmits with Figure 2: Transmit power versus distance from basestation. QPSK, the BS 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 bi-directionally. The downside is that when these techniques are used, the uplink throughput will be lower than the downlink throughput; subchannelization limits the number of subcarriers available for mobile transmission, and lower order modulation means that fewer bits are transmitted on each available subcarrier. Power proﬁle of a mobile WiMAX cell With all of the above explanations in mind, let’s 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. To understand why this is the case, 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 minimize 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 +23 dBm and it just achieves the 5 dB SNR required for QPSK when it is at the edge of the cell. As is 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 +23 dBm, since the MS will now be able to achieve a 10.5 dB 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. This is shown in Figure 1. Figure 1 was derived using parameters from a WiMAX Forum whitepaper [2]. It shows the modulation that is achievable as a function of distance from the BS. We use the parameters in the whitepaper, so, for example, maximum available path loss is calculated assuming a 10 MHz channel bandwidth at 2.5 GHz, with 3 subchannels, and 10 dB penetration loss. In calculating the path loss, we have assumed a COST231 suburban model at 2.5 GHz with 32 m BS height and 1.2 m 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, Microwave Engineering ● July/August 2008 ● www.mwee.com http://www.mwee.com

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Microwave Engineering Europe - July/August 2008 Contents News Comment Cover Feature: Effective EM Simulations with Micro−λ Resolution in Macro-λ Objects — General Huygens Box Implementation RF CMOS: Programmable Transceiver IC Minimises OEM Inventory for Femtocells CAD/EDA: Software-Defined Radio Platforms CAD/EDA: Cadence Enhances RF Verification While AWR Delivers an Improved Microwave Office How to Meet the Design Challenges of WiMAX Power Amplifiers Products Calendar

Microwave Engineering Europe - July/August 2008

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