Low Power Design
Strategies for Wireless Devices
Low power design is the biggest technical
challenge facing the next generation of consumer silicon. Managing the
soaring development costs of that silicon is the biggest commercial
As next generation mobile phones evolve from supporting only voice
transmission to the transmission of audio, images and video,
substantial baseband processing power will be required to handle the
increased digital transmission, reception, compression and
decompression of substantially larger data streams. General Packet
Radio Service (GPRS) and High Speed Circuit Switched Data (HSCSD) use
multiple time slots, processed in parallel to increase the data rate.
They increase baseband processing alone by a factor of eight and they
are becoming standard features in today’s mobile phones.
If handsets were neither handheld nor cost sensitive, adding these
features would be a matter of either turning up the processing clock
or using a more powerful processor in the phone. Sadly, these
solutions are unacceptable in the handset market because they will
increase power consumption, product size, and/or product cost. Product
cost and battery life are the two most important attributes of a
mobile handset. Wireless service providers commonly give handsets to
end-users for free in order to get customers. Since the providers give
the phones away, they want the cheapest implementation consistent with
acceptable performance. After cost, the most important features are
standby time, talk time and the phone’s feature set, in that order.
The phones with the longest battery life and the smallest overall size
command the highest prices. The question is how a designer meets
increasing processing requirements for next generation phones without
sending power consumption, cost, or end-product size through the roof.
Traditionally, performance increases have been achieved in one of
1) increasing the clock frequency;
2) adding extra
3) selecting a more powerful (and more expensive)
highly parallel processor with a memory-hungry instruction set.
Increasing the clock frequency can require a higher supply voltage
that increases the power consumption exponentially. Some processors
enable increased performance at lower clock speeds, but they are
expensive from a total system point of view since they cost more and
require substantially more program memory to store the larger
instructions. Larger memory results in higher cost and greater power
consumption, as well as a larger footprint design. Adding a second
processor (e.g. for speech processing) again increases the cost, size
and power drain of the end product. To wit, all of these traditional
solutions end up destroying the most important competitive advantages
of the phone - low cost and long standby and talk times.
Digital signal processing applications are full of relatively simple
algorithms that are repeated over an over inside the inner loops of
the code. In audio, video, or wireless baseband processing, thousands
of samples must be encoded and decoded every second. Therefore, a tiny
snippet of code that has as few as twenty operations can be
responsible for as much as 90% of the system’s total processing
requirement. Clearly, the key to improving performance is to make
these code fragments as efficient as possible.
This inevitable trade-off between power and performance suggests that
the traditional approach of using off-the-shelf DSP processors and
cores may not be suitable when performance and cost and power
consumption are equally important. Even the lowest power versions of
standard DSPs consume a lot of power when the clock frequency is high
enough. Power consumption of only 0.5 mW/MHz translates into 200 mW
with a 200 MHz clock. A handful of standard DSPs that are fabricated
in extremely small process technologies can operate with a fast clock
and low power consumption. However, the processes and the devices are
extremely expensive, violating the low cost rule for handsets.
Power Control Techniques in Wireless Systems
The primary goal of cellular radio systems is to provide communication
services to a large number of mobile users. Due to the rapid expansion
of the market in this area, the available resources have to be used
efficiently. The main issue in this thesis is methods to assign
appropriate transmission powers, given coarsely quantized
measurements, in order to meet the quality requirements from the users
despite various disturbances.
We propose a concept of a power regulator
comprising the steps of estimating relevant quantities, handling
quality specifications, and controlling the powers. With this setting,
the power controlling component relates directly to the mainstream of
the algorithms proposed to date.
For practical reasons, it is necessary to
control the powers in a distributed fashion, and these distributed
algorithms can be seen as local control loops. The effects of time
delays and power output constraints in these loops are analyzed with
respect to stability, using root locus techniques and describing
functions. We emphasize the importance of identifying these time
delays and constraints in order to choose the appropriate controller
parameters for stable operation. The relevance of the local stability
results on the overall system level is discussed, and further analyzed
in a simulation environment, which has been developed.
The literature is surveyed, and the
contributions are classified with respect to a common framework in
order to stress their similarities and differences. We show that an
integrating controller forms the basis for the most popular
algorithms. Methods for convergence analysis are investigated and
related to the theory of linear systems. These methods are applied
when proving global convergence of the integrating controller.
The power control strategies are evaluated under
more realistic circumstances in an environment simulating the
operation of a GSM system. Comparing the results when using different
power control algorithms we note that the proposed concept performs
better than the algorithms proposed to date, both in terms of
transmission quality of service and capacity.
Multiple users are accommodated through the use of time division
multiplexing (with the exception of the FDD mode of UMTS) that result
in pulsed transmissions such that the peak power is greater than the
average power. All systems employ some form of power control which
results lower power transmissions close to base stations and higher
power further away (up to the maximum defined in the standard) or
where there is a highly built-up environment or large topographic