Seminar on Wireless Communication

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Power Control in Mobile Radio

Bhupendra Singh

28/02/2012 1power control in mobile radio


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Contents

Characteristics of mobile radio channel

Requirement of power control in mobile radio

Aspects of power control algorithms

Classification of power control algorithms

How to choose appropriate design parameters

Power control algorithms in CDMA IS-95

References

28/02/2012 power control in mobile radio 2


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Characteristics of radio channel


FIGURE 1.The physical environment of radio system[7]


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The transmitted signal in radio channel is affected by radio wavepropagation effects, thermal noise and interfering signals.

In propagation of radio waves the effects of rain and moisture are less

nominal compared to scattering, diffraction and reflection.

The power gain of received signal is (linear scale).

=p sm

where p

=path loss, s

=shadowing fading, m =multipath fading.



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Characteristic of radio channel


Path loss:

In long time average, the power at receiver depends mainly on the

distance of the transmitter.

p

Cp is constant that depends upon wavelength and antenna parameter.

Shadow fading:

Shadow fading is referred as a slow variation in average gain over a

distance corresponding to several tens of wavelengths.gsN(0,s ).

shadow fading is modeled as Gaussian random variable.

4r

Cp


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Multipath fading:

when the waves of multipath signal reach at receiver. Depending on theirphase they interfere either constructively or destructively resulting in

multipath fading.

m a2.

Where a is Rayleigh distributed amplitude gain

FIGURE 2.Multipath propagation[8]


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Multiuser communication:Frequency division multiple access (FDMA).

Time division multiple access (TDMA).

Code division multiple access (CDMA).

FIGURE 3. Multiple access scheme[2]


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Radio resource management:

The available resources are primarily the bandwidth, and algorithms to

allocating this resource .

The goal is to maximize the capacity, while offering a sufficient quality to

users.

When a mobile station is admitted to the network. The following has tobe assigned to the MS.

A base station from set B.

A channel pair from set C.

Transmitter powers for the BS and MS.



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References



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Why we need power control?

Enhanced adjacent channel protection:Example- Near-Far problem (CDMA).

The aim of power control scheme is to maintain the received powers from

all mobiles at a constant level.



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MS2

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Near Far EffectFIGURE 4. Ideal CDMA System[8]


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Co channel interference management:

Example-Reuse of spectrum.

The aim of power control scheme is to choose the transmitter powers in

each base- mobile link to maintain sufficient transmission quality in all

communication links.

Reduced power consumption:

Example-Portable hand-held application (Battery power).

The aim of power control algorithms is to minimum use of transmitter

power to achieve the required transmission quality (to save Battery life).

Can we use common algorithms to all three problems?



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Requirement of power control in mobile radio Aspects of power control algorithms


How to choose appropriate design parameters Power control algorithms in CDMA IS-95

References



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Centralized/Decentralized Controller:

A centralized controller has all information about established connectionsand power gains.

Controls all the power levels in the network.

Decentralized controller has local information and controls the power of

one single transmitter.

Quality measure:

Speech quality is a subjective quantity.

Signal -to-Noise Ratio based.

Linear Predictive Coding based.

Spectral Distance based measure.

Psycoacoustically based measure.



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Available Measurements:

It is very difficult to measure optimal quality measurements.

Quality Indicators(OI), reflecting quality .

Received Signal Strength Indicator(RSSI),reflecting the received signal

strength at receiver.

Constraints:

The output power levels are limited to given set of values due to hardware

constraints.

The various standards include different constraints.GSM- Channels requiring the use of maximum power.

D-AMPS-Three time slot on same carrier use same power levels.



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Time Delay:Measuring and control signaling takes time .

To measure and report the measurements to the algorithms.

Example-GSM- 0.48s.

The computed power actually used in the transmitter.

Examples-GSM-0.52s.



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How to choose appropriate design parameters Power control algorithms in CDMA IS-95

References



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FIGURE 5.Classification of power control algorithms[5].


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Classification of power control algorithms.

Reverse link control:

power control from mobile to base station.

DS-CDMA reverse link power control requirement, due to Near-far

problem.

Forward link control:

power control from base station to mobile station.

No power control is required in single cell systems.

Forward power control mainly used to reduced intercell interference.



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Strength- based:

Signal strength measured at base station from mobile station.

compare with desired quality of signal.

The command to lower or raise the transmitter power is made accordingly.

SIR- based:

Measured quantity is SIR, where interference consists of channel noiseand multiuser interference.

Reflects better performance in terms of QoS and capacity.

Suffer from non- cooperative person game problem.

BER based: If the signal and interference power is constant, BER is same like SIR.

It gives better performance, when SIR does not correspond to BER.



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Closed loop power control:

Feasible in terrestrial cellular environment.

Fads occurs to rapidly, due to large propagation round trip delay.

Difficult to implement with packet switched mode of operation.

Open loop power control:

Mobile user estimates the channel states in forward link.

Compensate with path loss and large scale variations.

Limited by correlation.

Multipath fading is not compensated.



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Fixed Step Size: Power control step size is fixed to any variation in channel.

Power control is simple 1- bit command.

Performance is less, but easier implementation.

Adaptive Step Size:

power control step size is adaptive to any variation in channel.

Update power according to reverse algorithm.

Better performance, but requires additional bandwidth on channel.

Alternative is adaptive delta algorithm.



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Continuous power control: Transmit power level is controlled in continuous domain.

Discrete power control: Transmit power level is controlled in discrete domain.

Transmission power in digital systems can be updated at discrete levels.

Used ceiling and floor power control algorithms.

Higher Outage probability due to oscillations.



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Requirement of power control in mobile radio Aspects of power control algorithms




References



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How to choose appropriate design parameter


FIGURE 6. Power regulator as a control prospective[3]


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Non linear control strategy:In real systems non linear effects challenge the design. It may be possible

to improve design by using non linear elements.

Rate limiter:

Computed power is much lower then the power in use.

It may complete safe or may be a temporary fluctuation.

It limits the rate of change of the control signals.

Selector:

Selectors are generally used to address the low interference problem.Selector is a static device, it is either a maximum or a minimum selector.

We use selectors to state priorities in case of conflicts in systems.



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Global Analysis of algorithms.

We analyze different algorithms based on linearity.

Find out the convergence rate for different algorithms.

Providing global convergence to different algorithms.

How the centralized controller can be distributed.



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Local Loops:

In order to reduce signaling it is desirable to use distributed algorithms.

Local Loops, to describe a power control algorithm and its environment.

components that cause problems in local loop:

Time delay:

Time delay results in delayed signaling.

Nonlinearities:

Output power is bounded by upper and lower limit.

Power levels are quantized.



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components that cause problems in local loop:Smoothing filters:

The measurements or estimates may be corrupted by noise.

Smoothing filters are used to control rapid fluctuation due to noise.

The values are weighted together using equal weight. This is called Moving

average .

Another approach using recursion to weight values differently. It is called

Exponential forgetting.



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Local loop:

FIGURE 7.The local loop of power control algorithms[4]


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Local loop:

FIGURE 8.The local loop of power control algorithms[4]


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FIGURE 9. Basic feed back loop[4]

Stability of local loops:

Stability means no input signal or disturbance can drive any signal in loop toInfinity.

Local stability does not guarantee global stability.

Root Locus:It is used to find out stability in linear systems.


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Stability of local loops:Describing Functions:Describing functions are used to find out stability of local loop in presence of nonlinearity.

FIGURE 10.Describimg function for one non linearity[4]


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References



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FIGURE 11.Block diagram of power control in CDMA IS-95[5]


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Power control for DS-CDMA according to IS-95 standards consists of

reverse link open loop power control, reverse link closed -loop power

control and forward link power control.

Reverse link open-loop power control is basically a function of the mobile

station .

The base station takes active role in reverse link closed-loop power control

and the forward link power control.

Reverse link power control is a combined of Open-loop and Closed loop.



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Reverse link power control is a fixed step size algorithm and strength

based distributed algorithms.

The goal of open-loop power control is the estimation of a path loss and a

loss due to shadowing between the Base and the mobile station.

After that mobile transmit the initial power control signal.

Every cell site demodulator measures the received Signal-to-noise

ratio(SNR) from each mobile station.



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Mobile station receives a power control command from base station.The

command is combined with mobile station open loop estimation value todetermine the final value of transmit power.

The command has fixed step size of 0.5 db and it is transmitted at a rate

of once every 1.25 ms.

The rate of power control adjustment command must be high enough to

permit tracking of Rayleigh fading.

The delay in power control signal process should be small so that channelcondition will not change significantly.



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Increasing the update rate, result in capacity improvement under flat

fading multi cell systems.

But in case of frequency selective fading a very less improvement is there.

We can used different algorithms in both closed loop and open loop

power control ,but we have to pay for that.

In forward link power control the base station periodically change the

power transmitted to mobile station.


R f


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References[1] J. Zender, Transmitter Power Control for Co-Channel Interference

Management in Cellular Radio Systems, Proc. 4th Winlab Wksp., NewBrunswick, NF, USA, Oct. 1993.

[2] S. A. Grandhi, J. Zander, and R. Yates, Constrained Power Control,Wireless Pers. Commun., vol. 1, No. 4, 1995.

[3] An Overview of the Application of CDMA to Digital Cellular Systems andPersonal Cellular Networks, Technical Report, Qualcomm Inc., May 1992.

[4] J. C. Doyle, B. A. Francis, and A. R. Tannebaum. Feedback Control Theory.Macmillan Publishing Company, New York, NY, USA, 1992.

[5] M. Novakovic, L. Dukic, Evolution of the power control techniques . IEEE

Communication surveys, Fourth quarter 2005.

[6] M. Andersin, Z. Rosberg, and J. Zander, Distributed Discrete Power

Control in Cellular PCS, Proc. Wksp. Multiaccess, Mobility and Teletrafficfor PCS, MMT96, Paris, France, May 1996.

[7] D.Dahlhaus, T.Hunzikar, Mobile Radio Lecture.

[8] S.DAS, Analysis of downlink power control algorithms.



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