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-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 0.625 msec (800 or 1600 times/second) – can track multi-path changes Power is updated is steps of 0.5 - 1 dB Lectures 11-12: CDMA System Elements Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 15-20 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-

Adv. Wireless Comm. Systems - CDMA- 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 Adv. Wireless Comm. Systems - CDMA-