1. Adv. Wireless Comm. Systems - CDMA-
CDMA System Elements
Objectives
Understand when CDMA has higher capacity
Understand the purpose of power control in CDMA
Understand the advantage of soft handover
Outline
CDMA/TDMA/FDMA Capacity comparison
Open and closed loop transmitter power control
Soft handover advantage
Lectures 11-12: CDMA System Elements
Adv. Wireless Comm. Systems CDMA-

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TDMA/FDMA Vs. CDMA – Capacity Comparison
Can Shannon capacity provide an answer?
A dominance argument can be given for the following fading channel
Discrete time slotted channel
Stationary ergodic time-varying fading channel with power gain {g(i)}
Flat fading (gain varies slower than symbol period)
AWGN noise
Channel Side Information in Receiver (CSIR) and Transmitter (CSIT)
The capacity of such fading channel is given by:
Bits/transmission
Expectation is with respect to the distribution of g(i) – represented by g
P – symbol transmission power
Lectures 11-12: CDMA System Elements
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TDMA/FDMA Vs. CDMA – Capacity Comparison (cont.)
Suppose we have K users and the channel occupies a bandwidth W
TDMA
Channel is time slotted and each user transmits once per K slots with power
(namely, an average of P per time slot)
Each time slot has the
Same fading distribution
The capacity/time-slot is given by:
Bits/time-slot
The capacity/user/time-slot is then:
Bits/time-slot per user
Lectures 11-12: CDMA System Elements
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TDMA/FDMA Vs. CDMA – Capacity Comparison (cont.)
FDMA: All slots are used by all users but each occupies W/K and transmits
with power P per time slot
The capacity/user/time-slot
is given by:
CDMA: All slots and bandwidth are used by all users that are multiplexed
by code and transmits with power P per time slot
Bits/time-slot per user
Jensen’s
Inequality
Lectures 11-12: CDMA System Elements
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TDMA/FDMA Vs. CDMA – Capacity Conclusions
We saw that for time variant fading channels, CDMA has an advantage over TDMA and FDMA.
In the presented case, the CDMA spreading sequence and receiver detector are the optimal ones
For the non-optimal DS-CDMA, it is not necessarily true
Lectures 11-12: CDMA System Elements
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Transmitter Power Allocation Problem in CDMA
Transmission power is an efficient device to address unbalanced
signal received power due to:
The near-far problem in the uplink channel
Due to different path loss signals from users near BTS and users
that are further away from the BTS
Due to variable channel fading conditions that depend on the
mobile location
The near-far problem in the downlink channel
Mobiles at cell border experience higher interference compared
with located nearer the BTS
Lectures 11-12: CDMA System Elements
Adv. Wireless Comm. Systems CDMA-

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Transmitter Power Allocation Problem in CDMA (cont.)
Transmitter power is also useful to balance between
the co-channel/adjacent interference
the received power at the target receiver
From Shannon’s channel coding theorem transmission power
directly effect the channel spectral efficiency interference
Transmitter power can compensate sudden fading deep
Lectures 11-12: CDMA System Elements
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Power Control General Objectives
Eliminates the near-far effect
Mitigate multi-path fading (20-30 dB fluctuation)
Compensate changes in path loss (up to 80 dB dynamic range)
Decrease co-channel interference
Increase system capacity
Assist with handover and admission control
Increase mobile battery life
Power Control Objectives in (W)CDMA
In the uplink channel
equalize signal received power at the BTS over all users all the time
In the downlink channel
maintain the signal received power at the minimum required level
Lectures 11-12: CDMA System Elements
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Power Control in CDMA
The uplink power control comprises of two control loops
Open loop
Done at the mobile to adjust its transmission power
Rough grain power control
Slow power update rate
Closed loop
Done at the BTS
Fine grain power control
Fast power update rate
Comprises two loops:
Fast closed loop – received SIR estimation and power update
Outer closed loop – error rate estimation
Lectures 11-12: CDMA System Elements
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Open Loop Power Control
Each mobile receives a pilot signal from its BTS transmitted with power
measures the time-dependent received power
inverses the channel gain and transmits with power
where C is a constant known by all users (-73 dB in IS-95)
A problem with none reciprocal up and down links (e.g., with FDD)
Observe that the received power at the BTS is
Thus, the BTS can control its received power by just updating the
transmission power of the pilot signal
The required accuracy is about
Serves as a nominal transmission power about which fine tuning is made
Lectures 11-12: CDMA System Elements
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Closed Loop Power Control
The Fast Closed Loop
Performs fine tuning of the open loop power control
The BTS measures the received SIR, , from each mobile
Compares it with the target SIR,
If , then a down command is sent to the mobile
If , then an up command is sent to the mobile
Power control commands are sent every 1.25 or msec
(800 or 1600 times/second) – can track multi-path changes
Power is updated is steps of dB
Lectures 11-12: CDMA System Elements
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Closed Loop Power Control (cont.)
The Outer Loop
The BTS adjust the required channel quality
Measures the bit or frame error rate
Adjust the SIR target to the required FER (or BER)
Ensures that the fast closed loop achieves the channel quality
Adjusts to changing environment and mobile speed
In IS-95 only in the uplink – In WCDMA, in both links
Lectures 11-12: CDMA System Elements
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Downlink Power Control in CDMA
Downlink may also be subject to poor reception conditions
Mobiles near cell border get strong interference from other cells
Mobiles at locations that receive several strong multi-path signals
As the fast closed loop in the uplink power control – but slower
A mobile can request an up/down in transmitted power from the BTS
Requests are made based on SIR measurements at the mobile
Power update is slow – once every ms
Dynamic range is around the nominal power
In WCDMA, the outer closed loop is also applied
Power update steps of 1,2,3 dB
With soft-handoff (macro-diversity) the mobile can request power update
from all its BTS
Lectures 11-12: CDMA System Elements
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Another PC Algorithm for the Fast Closed Loop
Recall that the DS-CDMA received signal power is given by
desired signal power
MAI power
received power
transmit power
added noise power
Measured SIR at
receiver k
To suppress MAI: choose spreading sequence and detector to max
Power Control: choose such that , for every k
Lectures 11-12: CDMA System Elements
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Another PC Algorithm for the Fast Closed Loop (cont.)
We have a SIR target constraint for every transmitter k
That is, at every instance , the transmission power must satisfy or
for every transmitter k
Lectures 11-12: CDMA System Elements
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Another PC Algorithm for the Fast Closed Loop (cont.)
In a matrix form, the SIR target constraints are given by
- Transmitter power vector at time t
- Normalized form of the channel gain matrix
- The additive noise component
The PC algorithm:
Each transmitter uses the minimum required power at any instant
Each transmitter uses local information only: and receiver
feedback
All transmitter powers converge to a global minimum power that
satisfies for every k
A feedback of requires more bits than up/down commands – Therefore
further delay that result in a slower pace of power update
(11.1)
Lectures 11-12: CDMA System Elements
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Another PC Algorithm for the Fast Closed Loop (cont.)
Distributed version and convergence was first introduced in
J. Zander, “Performance of optimum transmitter power control in cellular radio systems”, IEEE Trans. On Vehic. Tech., Vol. 41, Feb. 1992
A general framework and extension is given in
R. Yates, “A framework for uplink power control in cellular radio systems”, IEEE Journal on Selected Areas in Comm. (JSAC), Vol. 13, No. 7, Sept. 1995
The following constrained PC alg. has the same properties
The PC alg. in (11.1) always transmit with some positive power
Is it optimal to use a positive power at any channel state?
For voice connection w/o macro diversity, you must! - To keep connection
For data or with macro diversity – it is not necessary
Lectures 11-12: CDMA System Elements
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Optimal PC of Fading Channels with CSI
Shannon’s channel coding Theorem has been extended for fading
channels with power control in
A. J. Goldsmith and P. Varaiya, “Capacity of Fading Channels with Channel Side Information”, IEEE Trans. On Information Theory, Vol. 43, No. 6, 1997
SINGLE USER, CHANNEL SIDE INFORMATION
Lectures 11-12: CDMA System Elements
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The Discrete-time Channel Model
Stationary and ergodic time-varying power gain sequence
Average transmitted power constraint - P
AWGN Noise PSD -
Received signal bandwidth - W
Perfect CSI
With no PC and avg power constraint P - the capacity is given by:
Bits/transmission
Assume instantaneous SIR feedback -
PC that is a function of the SIR only -
Average power constraint
(11.2)
Lectures 11-12: CDMA System Elements
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Definition: Given the average power constraint (11.1), the time varying
channel capacity is defined by
(11.3)
C(P) is the the time-varying fading channel capacity and is achieved by the
following PC algorithm:
(11.4)
where satisfies
Density function of
(projected from )
Lectures 11-12: CDMA System Elements
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Substituting (11.4) into (11.3) implies that the channel capacity is
Optimal PC properties
A “cutoff” policy – If the instantaneous SIR is below a cutoff value
, then no data is transmitted over the symbol time interval
The policy depends on the channel power gain distribution only through the cutoff value
The optimal policy is a “water-pouring” formula in the time domain
Lectures 11-12: CDMA System Elements
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Soft Handover in CDMA
Soft handover is a technique whereby mobiles in transition between cells
transmit to and receive the same signal from both base stations at the
same time
On the downlink, the mobile RAKE receiver combines the two incoming
signals
On the uplink, the MSC resolves the BTS that receives the strongest
Lectures 11-12: CDMA System Elements
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Shadow Margin with Soft Handover
Soft handover improves cell coverage – Let’s see why.
Suppose that path loss to BTS i due to log-normal shadow fading is
proportional to , where are correlated r.v.’s
An outage occurs if the path loss is above a given level , namely
system params
I.e., the shadow path loss (in dB) at distance d relative to a distance is
is above the threshold
(in dB)
Lectures 11-12: CDMA System Elements
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Shadow Margin with Soft Handover (cont.)
The outage probability with hard handover is then
Normal tail distribution
Worst case ( d = )
Consider soft handover with two base stations separated by
An outage occurs if both path losses satisfy
Lectures 11-12: CDMA System Elements
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Shadow Margin with Soft Handover (cont.)
The outage probability with soft handover is then
Worst case ( )
For every the outage probability of soft handover is lower
Example: if the required outage prob. is 0.1 and
then with hard handover
with soft handover
Lectures 11-12: CDMA System Elements
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Shadow Margin with Soft Handover (cont.)
Thus, we get a 4 dB gain with soft handover
Cells could be larger and the number of BTS is reduced
Lectures 11-12: CDMA System Elements
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