seminar on wireless communication
TRANSCRIPT
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Power Control in Mobile Radio
Bhupendra Singh
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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
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Characteristics of radio channel
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FIGURE 1.The physical environment of radio system[7]
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Characteristics of radio channel
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
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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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Characteristics of radio channel
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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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Characteristics of radio channel
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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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Characteristics of radio channel
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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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
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Requirement of power control in mobile radio
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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Requirement of power control in mobile radio
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Near Far EffectFIGURE 4. Ideal CDMA System[8]
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Requirement of power control in mobile radio
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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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
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Aspects of power control algorithms
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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Aspects of power control algorithms
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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Aspects of power control algorithms
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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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
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Classification of power control algorithms
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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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Classification of power control algorithms.
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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Classification of power control algorithms.
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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Classification of power control algorithms.
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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Classification of power control algorithms.
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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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
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How to choose appropriate design parameter
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FIGURE 6. Power regulator as a control prospective[3]
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How to choose appropriate design parameter
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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How to choose appropriate design parameter
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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How to choose appropriate design parameter
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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How to choose appropriate design parameter
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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How to choose appropriate design parameter
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Local loop:
FIGURE 7.The local loop of power control algorithms[4]
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How to choose appropriate design parameter
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Local loop:
FIGURE 8.The local loop of power control algorithms[4]
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How to choose appropriate design parameter
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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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How to choose appropriate design parameter
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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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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
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Power control algorithms in CDMA IS-95
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FIGURE 11.Block diagram of power control in CDMA IS-95[5]
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Power control algorithms in CDMA IS-95
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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Power control algorithms in CDMA IS-95
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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Power control algorithms in CDMA IS-95
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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Power control algorithms in CDMA IS-95
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.
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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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ANY QUESTION??
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