Microwave Engineering Europe - July/August 2008 - (Page 23)

WiMAX DESIGN 23 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, labeled 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 +23 dBm transmit power, an 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 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 maximize 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 once 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 Figure 2 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. Beneﬁts of high power The beneﬁts of higher power transmission from the mobile WiMAX terminal are signiﬁcant. Consider the effect of increasing the transmit power by 40 percent, from +23 dBm (200 mW) to +24.5 dBm (281 mW). First, it would require a larger power ampliﬁer (PA). Assuming that losses after the PA are 1 dB, the output power from the PA must increase from 250 mW (+24 dBm) to 355 mW (+25.5 dBm). There are two beneﬁts to transmitting at higher power. First, transmitting at this higher output power increases the maximum range. Using parameters from the WiMAX Forum [3], maximum mobile to BS distance is increased from 1.35 to 1.5 km when the output power is increased from 23 to 24.5 dBm, so that the overall coverage area increases by 23.5 percent. In principle, one might expect that a network operator could deploy 23 percent fewer base stations, and realize a cost savings. However, this effect may be of only limited beneﬁt, since many networks will have been designed with cell sizes assuming +23 dBm uplink transmit power, so cell sizes may already be ﬁxed. The second beneﬁt is more signiﬁcant, however. If an MS is able to transmit at higher power, then it can achieve the SNR required for higher order modulation when it is further from the BS. This improves overall network capacity, so increases overall spectral efﬁciency. RF testing out of control? Only Keithley gives you the RF test tools you need to rein in today’s devices and tame tomorrow’s challenges. MODEL 2920 RF SIGNAL GENERATOR MODEL 2820 RF SIGNAL ANALYZER ■ Test the most complex signal structures, including 802.11n WLAN MIMO and 802.16e Wave 2 WiMAX. Conﬁgure a 4x4 MIMO test system costeffectively. Generate and analyze signals up to 6 GHz repeatybly and accurately with our instruments’ software-deﬁned radio architecture. Reduce your time to market and cost of test with MIMO systems optimized for R&D and production test. ■ ■ ■ Go to www.keithley.com/tame and try a demo. www.keithley.com/contact info@keithley.de Microwave Engineering ● July/August 2008 ● www.mwee.com http://www.keithley.com/tame http://www.keithley.com/tame http://www.keithley.com/contact 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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