Microwave Engineering Europe - July/August 2008 - (Page 24)

24 WiMAX DESIGN Figure 3 shows the modulation that is achievable as a function of distance from the BS with +24.5 dBm transmit power. In this ﬁgure, we again plot achievable modulation as a function of distance from the BS (and the dashed lines show the ranges for +23 dBm from Figure 1 for reference). Note that the maximum distance has increased from 1.35 to 1.5 km, as discussed above. However, it is more important to note that users can now achieve higher order modulations over a wider range. For example, for 16QAM-1/2 modulation the maximum range is now 0.7 km, versus 0.6 km for +23 dBm. As a result, each user will achieve higher throughput over a wider range, and the network aggregate capacity will be increased accordingly. With every additional user who can transmit at a higher power level, overall network capacity increases. It is important to understand that all users would need to transmit at a higher transmit power in order to allow cell sizes to expand. However, each and every higher power user added to the network increases overall network capacity. Finally, it is relatively straightforward to calculate the capacity increase seen by increasing transmit power from +23 to +24.5 dBm. We know how many bits per symbol can be transmitted for each modulation scheme, and we know the relative areas that can be covered for each modulation scheme, for both power levels. When this information is used to calculate relative capacity, we ﬁnd that it increases by 24 percent when transmit power is increased from +23 dBm to +24.5 dBm. Even if the maximum cell size remains ﬁxed at 1.35 km when the transmit power is increased to +24.5 dBm (as would be the case if networks were rolled out assuming +23 dBm devices) the capacity still increases by 18 percent when devices are able to transmit at higher power. Limitations of power So, now we understand why higher transmit power is important in a WiMAX network; it allows overall network throughput to increase, and in a ‘greenﬁeld’ deployment, it would allow for larger cell sizes, and therefore reduce deployment costs. So why not transmit even more power? There are three important factors that limit our ability to transmit at higher power: PA efﬁciency, available supply voltage, and regulatory requirements. Figure 3: Achievable modulation versus distance with +24.5 dBm transmit power. PA efﬁciency In PAs, efﬁciency is the measure of the RF power out versus the DC power in. For example, if a PA has a 10 percent efﬁciency, it would consume 3.55 W to transmit at +25.5 dBm (355 mW). If the PA efﬁciency could be doubled to 20 percent, then the peak power consumption drops to 1.7 W. Today’s state-of-the-art WiMAX PAs, like SiGe Semiconductor’s SE7262, operate with gretaer than 20 percent efﬁciency (See sidebar: Why is PA efﬁciency so low for WiMAX?). The PA efﬁciency has a direct impact on battery life for mobile devices. Of course, the PA is not working all of the time, so the average power consumption will be considerably lower than the peak power consumption quoted above. For instance, transmit duty cycles for WiMAX devices are typically about 40 percent when the MS has data to transmit. Therefore the average power consumption for a 20 percent efﬁciency PA will be about 680 mW if the PA is transmitting at maximum power. Furthermore, often there will be no data to transmit, and in this case, the device will transmit very infrequently (essentially, it transmits only ranging messages to let the BS know that it is still in the cell). In the end, however, the PA power consumption can have a signiﬁcant impact on battery life, and it is important that PA efﬁciency is as high as possible. Available supply voltage Mobile WiMAX devices will be powered directly from the mobile station’s battery, and battery supply voltages vary signiﬁcantly during use. When freshly charged, the battery will operate at about 4.8 V. The supply voltage drops as the battery discharges, and the minimum practical supply voltage before the device shuts down is typically 2.7 V. Most manufacturers want to use the battery for as much of this range as possible, and therefore specify that the power ampliﬁer must faithfully deliver fully rated power at 3.3 V (and occasionally 3.0 V). Delivering high power under these conditions imposes some signiﬁcant challenges. As most circuit designers know, a low supply voltage requires a high current, which implies a very low output impedance. Consequently, matching the low impedance PA output to a 50 Ohm antenna is difﬁcult to achieve. If higher output powers are required, the impedance becomes even lower, and it becomes increasingly difﬁcult to achieve a good broadband match between the PA and the antenna. Regulatory requirements Regulatory requirements also place a serious constraint on how much power a PA can deliver. An ideal linear PA produces only the original frequency from the input signal. In real-world implementations, PA nonlinearities introduce new frequencies through intermodulation distortion (IMD), and these out-of-band signals can interfere with users in adjacent channels (referred to as spectral regrowth or spectral leakage). Regulatory bodies have imposed strict regulations on the amount of power that can be emitted out of band. For example, for mobile devices in the 2.5 GHz band, the FCC speciﬁes4 that the emissions must be below 25 dBm/MHz, measured 5.5MHz outside the device’s assigned band. Since this limit is an absolute power measurement, as output power is increased, more and more rejection of out-of-band emissions is required, and the power ampliﬁer must be made more and more linear. Microwave Engineering Europe ● 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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