Death to “half-power handsets”

Technology News |
By eeNews Europe

It is one of the mobile industry’s dirty secrets, but nearly all of today’s 4G handsets do not transmit at full power. Instead, 4G handset manufacturers have been producing "half-power handsets," due partly to limitations of RF front-end components, but more often because of the mistaken belief that reducing transmit power is the best way to maximize battery life.

For end users, battery life is still one of the key metrics when selecting a handset. This is why, as more is asked of the battery in mobile handsets, manufacturers have been so keen to pursue any strategy that reduces current consumption. As a result, handset manufacturers have looked to minimize the peak handset transmit power wherever possible.

Ironically, the fact is that in the data-centric 4G world, far from extending battery life, backing off handset transmit power as far as possible is actually increasing the drain on batteries. Not only that, but "half power handsets" also significantly reduce network coverage and data rates for all users.

A new approach is needed to overcome these significant performance limitations and consign half-power handsets to the trash can. It may seem counter-intuitive, but the best way to extend handset battery life is deceptively simple – turn the transmit power up to eleven.

Low power, mo’ problems
In the voice-centric 2G and 3G era, the strategy of backing off transmit power made good sense and fitted neatly with network bandwidth allocation strategies. Network bandwidth allocation in those networks tended to focus on two key metrics: number of simultaneous users, and latency for voice calls.

A handset power consumption comparison between 2G, 3G and 4G transmissions.

Schedulers in the base station tended to maximize network ‘size’ by allocating the minimum amount of bandwidth to each user, increasing the number of simultaneous users, and limiting peak allocations. This low-bandwidth, low-power approach has one major benefit – minimizing latency for each user, which was historically important for voice calls.

The transition to 4G has resulted in a significant change to the network, with dynamic bandwidth allocation on a per-timeslot basis, which is much better suited to ‘bursty’ data requirements. This allows the base station to allocate almost the entire channel to a single handset for a single timeslot, maximizing the instantaneous data rate.

However, in LTE the transmit power from the handset is directly proportional to the number of Resource Blocks allocated in the frequency domain, with up to 20 dB (100x) difference between the low-data-rate voice and high-rate bursts of data. This pushes up both the average and peak transmit power of the RF Power Amplifier (PA) in the handset. For the highest data rates, the handset is also able to use highly efficient 16QAM transmissions, which push the peak power up by a further 1 dB (25%).

As a result, many handset manufacturers and chipset vendors have released 4G products that fall way short of the 3GPP specifications for transmit power, claiming that it’s not possible for them to transmit at full power.

FCC reports show how today’s LTE handsets and terminals are often incapable of transmitting at full power, falling short by 2.5 dB or more even in the relatively easy 700-MHz bands. Manufacturers have taken advantage of spec loopholes that allow Maximum Power Reduction (MPR) of up to 3 dB for high-bandwidth LTE transmissions – just half the specified output power.

An example FCC report showing how handset transmit power is being backed off by manufacturers.

This may make life easier for the RF front-end designer, but halving the peak transmit power requires handsets to transmit for twice as long for a given data payload – significantly degrading battery life. During those extra timeslots, the power-hungry LTE modem and apps processor have to stay awake, burning even more power.

More than that, it also hogs precious network resources, halving network throughput. It also means that more often than not LTE network coverage is limited by the handset’s transmit performance rather than the basestation’s. Nujira’s own network modeling has suggested that coverage can be reduced by as much as 32% as a result of half-power handsets.

The benefits of ‘high-power, short-time’ transmissions
Turning the transmit power up may not be the most obvious way to increase battery life. However, the drawbacks associated with half-power handsets are now impacting the user experience, and handset manufacturers need to look at the slightly more counter-intuitive option of eliminating MPR, and transmitting at full power in very short bursts.

Enabling handsets to transmit at full power increases the peak current requirement. But the resulting transmission time of each handset is dramatically minimized, which can bring significant battery power savings.

Researchers in the Department of Electronic Systems at Aalborg University in Denmark have studied the efficiency of ‘low-power, long-time’ transmissions compared to ‘high-power, short-time’ transmissions. The results found that allocating each user the maximum bandwidth and allowing for full-power transmissions for very short bursts resulted in a 24% increase in handset energy efficiency. Interestingly the study also showed that ‘high-power, short-time’ transmissions had the added benefit of a 300% increase in the total network throughput.

It may seem counter-intuitive, but it is clear that increasing the transmit power, and minimising the amount of time a handset is transmitting, will improve handset energy efficiency.

TD-LTE needs full transmit power too
For TD-LTE networks the benefits are even more significant, and full-power transmissions are a necessity, rather than a luxury. TD-LTE networks use the same spectrum for both downlink (receive) and uplink (transmit), but the timeslots are allocated asymmetrically to reflect the overall network traffic statistics, which are typically a 3:1 or 4:1 ratio of download versus upload. This allows the TD-LTE operator to maximize network capacity, by keeping all timeslots allocated.

Although the TD-LTE standard supports several different frame configurations, the majority of TD-LTE operators, including China Mobile, are deploying configuration 2 – a 5-ms frame with 4:1 downlink:uplink. This means that across the whole network, only 20% of the time is available for handsets to transmit.

Because TD-LTE handsets can only transmit for 20% of the available time, when they are transmitting they will on average use five times the instantaneous bandwidth, and therefore five times the instantaneous RF transmit power (a 7 dB increase), compared to transmitting the same data payload over a continuous FDD LTE connection. Despite this, tests by Nujira show that you can achieve a 40% saving in energy-per-bit, even though the instantaneous power consumption during a TD-LTE burst can be double that of FD-LTE. This is because the RF front end is significantly more energy efficient when transmitting at high power and high bandwidth.

A chart of handset energy consumption in FD-LTE vs TD-LTE.

As a result of the growing importance of TD-LTE networks, particularly in China and the US, it will be increasingly important for handsets to be able to meet the specifications and support full-power transmissions.

No more MPR
New RF front-end technologies such as Envelope Tracking and Antenna Tuning can now enable handsets to transmit at the full 200 mW/23 dBm even with high-bandwidth LTE signals. This boosts data rates, increases network capacity, extends battery life, and eliminates the need for MPR altogether.

The industry should be demanding full-bandwidth, full-power transmissions from handset suppliers to deliver a true 4G experience to users. Moreover, it is important that support for full-power transmissions on the handset side is complemented with a similar shift to maximum bandwidth allocation strategies on the network side.

However, to effectively enforce the specifications, a unified approach needs to be adopted throughout the entire mobile supply chain. Organizations like the International Wireless Industry Consortium (IWPC) are working to improve collaboration in the mobile supply chain – and of course there is also scope for major operators to take the lead on the issue.

By setting the specifications for handsets from the top down, operators are in a unique position to demand support for full-power transmissions to improve user experiences and make the "half-power handsets" of today a thing of the past.

Turning it up to eleven may still seem a strange thing to want to do – but everyone in the industry needs to realize that the best way to extend battery life and optimize LTE network performance is to crank the transmit power up, not down.

About the Author

Jeremy Hendy VP Sales & Marketing at Nujira brings considerable experience of semiconductor sales and marketing across multiple technologies for wireless communication and digital video. Previous positions include Marketing Director of wireless USB start-up Artimi, VP Marketing for Aspex Semiconductor, and Strategic Technology Director of Cadence’s Wireless and Multimedia business unit. He started his career with Texas Instruments, and holds a first class honours degree in Electronic Engineering from the University of Liverpool.


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