Title: Improving Energy Conservation in mobile handsets through careful design in power transmitter functions Abstract: As the mobile handset industry transitions from 2G to 3G standards increasing demand is being placed on handsets’ power consumption. This is due to increased data rate standards such as HSDPA, additional power hungry functions such as LCD screens, and quality of service improvement specifications such as total radiated power (TRP). HSDPA and tightened TRP specifications result in decreased transmit efficiencies and combined with additional power demands result in decreased talk time. Additionally, the popularity of thin phones results in handsets with less thermal mass and this decreased thermal efficiency contributes to thermal dissipation complications. The decrease in phone sizes coupled with decreased power efficiencies has pushed energy conservation or efficiency improvements at the forefront of the issues that face mobile phone and wireless component designers. This dynamic is further exaggerated by the fact that battery technology has not kept up with the rate of increasing capacity demand. In order to achieve a minimum acceptable threshold of battery life, any increase in handset functionality and features has to be countered with an improvement in power capacity or power efficiency. All this comes at a time when customers demand increased autonomy or talk time between charges from handsets and other wireless devices; therefore, extending battery life has risen to the forefront of the challenges for handset design engineers. The paper will explore the different methods to improve handset energy conservation by increasing battery capacity and decreasing power dissipation. To keep up with the demands, no single technology or technique has surfaced with the capacity to provide a complete solution, so different combinations, ranging from system architecture partitioning, IC design techniques, and semiconductor process improvements are used to optimize energy conservation. There will be a strong concentration on the RF section, particularly the transmit section since the PA is the single highest source of power dissipation in a handset and in 3G handsets may dissipate up to 40% of the overall power. Battery Technology is not keeping up with emerging applications Battery Capacity Trends Li-Ion batteries are the mainstay of portable power for handsets; however, increased demands on the battery capacity from mobile devices have not been matched by improvement in energy density or storage capacity. Typical secondary prismatic batteries used for portable handsets have 600-880 mA-hr capacities. Some high end smart-phones require 1200-1400 mA-hr capacities to ensure acceptable talk times for consumers. Although Li-Ion energy density has been increasing by about 9 - 10% per annum, it has failed to keep pace with the increased power consumption of advanced mobile devices.

Figure 1 shows a comparison of the battery demand trend as compared to capacity improvements over the last few years.

Figure 1: Battery capacity and power consumption for maximum output power in cellular transmitters

(Source: IMS 2006 Workshop Presentation) Battery life is characterized as the ratio of the energy stored in the battery relative to the power consumption of the device attached to the battery as a factor of time, usually hours. Therefore if the power consumption or current drawn from the battery increases at a higher rate than the battery storage capability as illustrated in figure 1, battery life diminishes. This is typically the case in newer devices that incorporate camera modules, Blue-Tooth radios, and email capability.

Transmitter Design Challenges as related to the battery technology The typical discharge curve for a 600 mAH Li-Ion battery is represented in figure 2.

Figure 2: Li-Ion battery voltage discharge characteristic. In order to meet minimum output power requirements, the power amplifier has to be designed to operate at the knee voltage of the discharge curve, which for current technology is about 3.2V. However, for most of the useful life of the battery the voltage presented to the PA is higher than this knee voltage. This results in overhead or excess voltages which wastes energy, as shown in the shaded area in figure 2. Another extenuating factor is that new battery technology is being developed using different materials with higher power densities to improve overall battery life and extend the voltage usage range. Some of these new generation batteries do not exhibit as sharp a downward voltage transition as existing Li-Ion models and this more gradual voltage decline results in a wider range of useable voltage. Figure 3 represents a comparison of the technologies. The battery in figure 3 represents the standard Li-Ion battery as compared to a battery made with different anode and cathode material aimed at increasing the energy density.

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Figure 3: Comparison of useable voltage range for different technologies (Source: IMS 2006 Workshop Presentation)

How does this affect PA transmitter design? Although these technologies provide a wider useable voltage range, this wider window contributes to a degradation of system efficiency over battery life if additional methods to adapt to this new range, such as DC/DC converters, are not used. The new technologies result in batteries with the lower end of the useable range from 2.5 V to 2.8V. To guarantee that minimum power requirements are met, the system needs to be designed to be capable at these lower voltages. Lower voltage operations require larger power amplifiers to meet the power demands. This is due to the increased maximum current ratings needed to compensate for the lower voltages. The net result is that designers are forced to design less efficient PAs if they need to guarantee operation at lower DC battery voltages. Therefore, the previously mentioned issue of operating the PA in the non-optimum voltage region is exaggerated over an even wider range resulting in efficient operations.

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Also, increased current drawn from the battery results in larger voltage drops across the battery and supply line resistances resulting in decreased system efficiencies. Batteries can and do affect PA transmitter design Although improvements in battery technology has resulted in some improvement in energy density and overall battery capacity, these improvements lag the pace of demand increase from mobile devices. Additionally, some of these new technologies compound existing problems as the batteries operate over a wider voltage range resulting in increased regions of non-optimal voltage operation and therefore decreased system efficiency. In addition to challenges faced by the limitations on power supplied by the battery and the competing forces of more current hungry features, there are several radio system performance metrics that conspire against long talk time.

Radio System Performance and Physical Design Issues There are several designs challenges that the PA designer is facing when deciding the best transmit architecture and PA design technique for a certain application. A good understanding of the requirements of the final solution is crucial as architecture and designs techniques have a big impact on final performance as trade offs between different parameters have to be made. Several key topics are antenna performance, specific absorption rate (SAR), Total radiated power (TRP), available battery capacity, and target talk time.

Multi-band antennas and antenna performance impacts on the transmitter Phones today are required to operate in a number of different bands and frequencies which makes designing antennas a constantly increasing challenge. Instead of covering two frequency bands which was the standard just a couple of years ago, the basic cell phone today covers three to four bands and high end models as many as seven bands. The increased complexity has not relaxed the performance specification for the antenna; it has to be as efficient radiating and receiving and is to be of the same size, if not smaller, and cheaper as earlier versions. The net effect of all this is that antennas of today set new requirements on the transmitter and receiver that they are connected to.

Fig 4. Dual band antenna return loss vs frequency. The markers indicate edges of the GSM 900 and DCS1800 and PCS1900 transmit bands. With an increased frequency spectrum to cover, the antenna becomes less efficient for a particular band. Fig 4 above shows return loss vs frequency for a dual band antenna. If broadband operation is a requirement, as in new multi standard (WCDMA-EDGE and WCDMAGPRS phones) the difficulty of designing an optimized antenna over several octaves of frequency within space and cost constraints becomes very difficult. There will be a trade off between antenna efficiency and number of bands the antenna can cover as there are only so many degrees of freedom in antenna design. For the transmitter, this means that if in the end performance is to be kept constant into this harsh environment, the transmitter has to deliver more power to the antenna. It is therefore clear that multi band antennas put higher requirements on transmitter efficiency as degrading system efficiency at the expense of mode bands is not an option.

Total Radiated Power (TRP) and Specific Absorption Rate (SAR) : System Impacts Increasingly, phone manufactures and carriers are being held accountable for radiation emitted from the handset. Two parameters that describe the characteristics of radiated power are Total Radiated Power (TRP) and Specific Absorption Rate (SAR). TRP is a measure of the RF power emitted from a handset. Ideally phones will maintain a nearly constant power out regardless of the environment the phone is in. SAR is a measure of how much of that radiation is absorbed in a human head with a phone in standard talk positions.

TRP: It is required in the system specification that the output power from the transmitter is kept constant under various conditions such as temperature, voltage and frequency. This is normally not a problem as when this is tested as the test environment is very ideal. The antenna is replaced by an ideal load and power into the load is measured. It is clear that this is far from reality. In order to more accurately measure power that the transmitter/antenna is actually radiating and not just delivering into a fixed load, a more complex test set has to be used.

Fig 5. Illustration on how total radiated power is measured. The phone is placed next to a dummy head and radiated power from the phone is measured in all directions. In this set up, radiated power from the phone is measured with the phone close to a dummy head with the same electrical properties as a human head. The radiation is measured in a s number of angles and then averaged and max and min is determined. This is referred to as TRP (Total Radiated Power) and is getting increasingly important. The challenge for the transmitter is to deliver constant power in the ideal case as well as in the test case that the antenna and dummy head presents. SAR: The power radiated from the transmitter when the phone is close to the user’s head, i.e. the phone is in talk position as in fig 5 above, leads to heating of the users brain and too much heating is considered to be a heath hazard. As a result, there are strict requirements on SAR (Specific Absorption Rate) stated by both FCC and carriers that is a measure of how much a heating of a human brain that the phone is causing when operating in talk position. Presently the limit is 1.6W/kg. Fulfilling both TRP which is a lower limit on output power in mismatch as well as SAR which is the upper, is getting increasingly important as it is getting more and more common for operators to put requirements in this area. TRP is also going to be a part of the 3GPP specification going forward. How does this tie in with PA transmitter? In order to maintain constant power into a changing load condition, or from perfectly matched to large mismatch in RF terms, the PA will need to deliver more

power to comply with these system requirements. When the PA is designed with higher Pout to make more power margin on the link budget, transmitter efficiency will suffer. To master the balance between low and high power in mismatch is difficult to achieve but is done in RF3196, a follow up to the successful RF3166. The Power Star architecture is further enhanced in RF3196 with a power flattening circuit that is integrated. This circuitry limits high currents and thereby excessive power in mismatch conditions.

Design Techniques/challenges When transmitter PA designers are faced with the challenges of efficient PA design, they develop certain profienciences to handle radio performance issues and operate with a maximum efficiency.

Power Flattening circuit description When presented to a mismatch at the output, a PA’s load line is changed leading to increased or decreased current and output power depending on the phase of the mismatch presented. The power flattening circuit in RF3196 monitors current consumption of the amplifier thru an internal sense resistor. The power control loop in the amplifier uses the sensed current together with the collector voltage to keep the output power constant. The reason why the current is a good measure of output power of the PA can be seen in fig 6 below.

PowerStar™ Power and Current Relationship

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Figure 6. Power and current relationship showing that power and current variation closely correlates well over various phase angles in mismatch conditions.

Power Flattening Implementation As noted earlier, not only the collector voltage as in Power Star is used as a feedback parameter, but also the collector current, which is measured via a internal sense resistor in the PA module. The voltage over the sense resistor is compared to a reference voltage that corresponds to the sense resistor voltage in a 50 ohm condition.

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Figure 7. Simplified block diagram for the power flattening circuit in RF3196. If the current thru the PA, and consequently the Rsense voltage increases, the collector voltage will be decreased, keeping Pout and current variation to a minimum. The effectiveness of the circuitry can be seen in fig 75 where power variation for RF3166 and RF3196 are plotted together. Note that the power variation in RF3196 is reduced from 3.2 dB to 1.15 dB. In fig 9 the current variation in a 3:1 mismatch is shown. Max current draw is reduced from 2.6A to 2.05A. This greatly improves the capability of the handset to maintain operation at low supply voltages and increase talk time.

Fig 8 Power variation for RF3196 and RF3166. Note the reduced variation for RF3196.

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Fig 9. Current variation for RF3196 and RF3166. Note the reduced variation for RF3196

Performance into mismatch What may not be apparent to all handset designers is that it is critical that the transmitter works well under mismatch, not only in 50 ohms which is the ideal case, in order to exhibit low current consumption in real case. Even designing for higher current consumption in 50 ohms may actually mean better talk time. As noted earlier, when the phone is used and placed close to the head of the user, a mismatch is introduced that affects the performance of the power amplifier. In fact any usage model that differs from free space will affect the transmitter behavior. Whether the phone is lying on a table, mounted in a cradle for hands free operation, or on the users desktop will present different impedances and mismatch to the transmitter’s antenna. In fig 10 current consumption of a single ended PA with an assumed 2 dB front end loss is shown. The VSWR numbers represent actual mismatch at the antenna. It is seen that average current consumption is 313 mA at 23 dBm Pout and this is what would be seen in lab tests when measuring current consumption in ideal condition. With an antenna mismatch of 6:1, the current increases to over 365 mA for some phase angles which lead to a significant reduction of talk time.

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Fig 10. Current vs phase for a single ended PA with a post PA loss of 2 dB with no isolator used. VSWR numbers are related to antenna node. One way of solving the problem for a WCDMA PA is to insert an isolator between the coupler and the duplex filter as shown in fig 11. A typical isolator use for WCDMA applications has an insertion loss of approximately 0.5 dB. At a first glance, one would expect this to lead to higher current consumption as the total insertion loss from PA output to antenna increases but experiments however show that the typical PA current with isolator can be found to be equal to the case when no isolator is used.

Fig 11. Typical WCDMA front end architecture with optional isolator inserted between coupler and duplex filter. Losses of individual components are shown as well.

In fig 12 the same PA as previously used is once again used but with an isolator, max current is now below 325 mA. For the worst phase angle, peak current is 40 mA lower with isolator while current in 50 ohms is the same. Why does the isolator appears to be lossless? The answer is that in the case when an isolator is used, the PA is slightly retuned. As the PA will now always operate in a near ideal environment due to the isolation that the isolator provides, it can be tuned to have less linearity margin. The way to achieve margin is to lower the loadline of the PA, meaning increasing output power capabilities and hence increasing current consumption. With isolator the PA can be tuned with less linearity margin than if an isolator was not used. The increase in post PA loss with isolator is compensated for with the fact that the PA operates in a near ideal environment and can be tuned for with less power headroom to meet linearity.

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Fig 12. Current vs phase for a single ended PA with a post PA loss of 2 dB and an isolator used. VSWR numbers are related to antenna node.

One solution to increase efficiency of the transmitter: Multi State PAs

A challenge for the PA designer is to keep the current as low as possible over a large range of output power levels. Higher output power means a larger amplifier that will cause lower efficiency at reduced output power levels. An ideal amplifier is one that can be scaled in size depending on required output power. When high output power is required, the PA is big, but at lower power it is size reduced. One way to achieve this is to implement an amplifier topology that consists of two amplifiers of different size. These two amplifiers can either be connected is series or in parallel as shown in fig 13. RF3266 internally uses two parallel amplifiers. At low output powers only the smaller amplifier part is used but when more power is required, the larger part is used as well, boosting power capability. The end result is an amplifier that is adequately sized for its power range, thereby always operating at near optimal condition. The challenge with this design is to minimize gain steps when changing from one mode to another. A 10 dB shift in gain is typical and the transceiver has to be capable of compensating for this. Not only the gain changes when switching state, the phase changes as well and a 25 deg change is not uncommon. This is also something that the transceiver has to compensate for as the WCDMA system puts hard requirement in sudden power and phase changes of the uplink signal. A further enhancement of the two state PA described here is to add another third state used for very low power levels, below 8 dBm. One way to achieve this is to reduce bias current when in the low power state. This will give three states: High power with the whole PA active, medium power achieved by running part of the PA, and low power by running part of the PA like in medium power but with reduced bias current. The net effect of current consumption reduction can be seen in fig 14.

Fig 13. Basic architecture of a multi state PA. The different amplification stages can either be connected in parallel (top) or in series (bottom). A digital control signal, LPM control, selects if the amplifier is in high or low power mode.

Fig 14. Battery current over a range of output power levels. At 3 dBm the two state PA shows 57% reduction in current consumption compared to a standard single state PA and the three state PA greater current reduction

A More Advanced solution: DC-DC converters Maybe the most sophisticated solution to reduce current consumption in a power amplifier is to use a DC-DC converter that adjusts the collector voltage to optimal levels depending on output power requirements. This allows the amplifier to always operate at maximum efficiency unlike the two or three state PA earlier described that has discrete switch points. The DC/DC converter efficiently transforms the battery voltage that in a handset normally swings between 3.0 and 4.5 depending on charge, to a level that the PA actually needs to operate most efficient. A common kind of converter uses PWM (Pulse Width Modulation) to adjust the output level. The output voltage from the converter is a variable duty cycle square wave and that requires filtering by an inductor and capacitors in order to end up with the DC voltage the PA needs. Ripple in the voltage to the PA will degrade the PA’s ability to amplify its signal without adding spurs and degrading the RF spectrum. The necessity of the filtering components and sensitivity to layout is the downside of the DC-DC converter as the inductor required to get proper filtering is large, approximately 2x2 mm. If the layout problems can be mastered and the cost of the converter and necessary components is acceptable, the DC-DC converter is a very good solution when it comes to current saving. What are the benefits to the user? Quite often costly isolators can be eliminated form the design. This is validated by many open market WCDMA solutions today. Theoretically such benefits could be found in EDGE and GPRS systems as well, but lack of a commonly adopted probability distribution function (PDF) of output power is an inhibitor.

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Fig 15. DC-DC converter with LC network. Summary: Phone designers today are faced with a difficult choice between increasingly power hungry functions and a seemingly stagnant portable power source. Additionally, increasing system performance demands are driving excess power margins to be designed in to ensure compliance. This challenge has to be met on several fronts, with technology choices, more advanced PA architectures, and increased use of power management in the transmitter to afford a satisfactory user experience for the all important consumer. RFMD is contributing several techniques to reduce power consumption and will continue to do so to enable future portable developments to b a success.