Downlink Power Control in Co-Channel Macrocell Femtocell Overlay Xiangfang Li (WINLAB), Lijun Qian (Prairie View A&M Univ.), Deepak Kataria (HCL America) CISS 2009, 43 rd Annual Conference on Information Sciences and Systems Ji Hoon Lee, , MMLAB

Introduction Studies show that more than 50% of all voice calls and more than 70% of all data traffic originate indoors. –V. Chandrasekhar, et al.,”Femtocell Networks: A Survey,” IEEE Comm. Mag., Sep Raising expectation on installing a home base station (femtocell) Deployment of femtocells can accommodate a large portion of existing traffic –Requirement of new applications and services  better coverage and capacity –Cost effectiveness of current cellular services

Macrocells and Femtocells Coexistence The radio interference betw. Femtocells and macrocells is a major concern. –Interference mitigation techniques are required. Downlink power control is a more critical issue. –Intensive ongoing research activities on interference mitigation and power control of femtocells in the standard bodies, such as 3GPP TSG-RAN. Most of current work focus on determining the dynamic range of the downlink transmission power of femtocells. In conclusion, detailed power control schemes and an analytic framework are required.

Network Model Notations –Macrocell NodeB: MNB p MNB –Home NodeB: HNB p HNB –Macrocell User Equipment (MUE) Distance to MNB: D Distance to HNB: d –Home User Equipment (HUE) Distance to HNB: h Distance to MNB: m –Radius of femtocell coverage: l

Different Deployment Configurations By the definitions of 3GPP TSG-RAN, –Open access or Closed subscriber group (CSG) Open access: HNBs can serve any UE in the same way as normal NodeB. CSG: HNBs only serve Ues which are a member of a particular Closed Subscriber Group. –Dedicated channel or co-channel This concerns whether HNBs operate in their own separate channel or they share a channel with an existing UTRAN network. –Fixed or adaptive downlink maximum transmit power. In this study, consider the downlink co-channel interference when macrocell and femtocell share the same spectrum.

Problem Formulation The authors address the downlink interference problem by considering the QoS at both the MUEs and the HUEs in terms of their received SINR. γ (m) MUE : SINR at the m-th MUE γ (i) j, HUE : SINR at the i-th HUE in j-th femtocell The downlink power control problem for energy efficiency maximization and interference suppression

Single Femtocell (1/3) Assume that the received power is only a fraction of transmitted power and path loss. –Fading effects are omitted. α 1 : Path loss factor from MNB α 2 : Path loss factor from HNB f 1 : Orthogonality factor for MNB f 2 : Orthogonality factor for HNB SINR of the MUE SINR of the HUE

Single Femtocell (2/3) In order to satisfy the two constraints on the SINR values (current >= target for MUE and HUE), we need If the downlink power control problem is feasible, eq. (4) (min power <= curr power <= max power) must be satisfied simultaneously.

Single Femtocell (3/3) Feasible transmission power of a HNB (upper bound of p_HNB is not a function of h)

Fundamental Capacity Limit (1/2) If the interference is dominant, i.e., The sum of the SINR of a MUE and the SIME of a HUE can be expressed as The sum of the achievable SINRs depends mainly on the relative distances m/D and d/h. If m  D+d, d/D and d/h have the dominant effects on the sum of the SINR values.

Fundamental Capacity Limit (2/2) Sum SINR values vs. d/D and h

Multiple Femtocells (1/2) Assume that each HNB will serve only one HUE for simplicity. α 3 : Path loss factor for Femto-to-Femto f 3 : Orthogonality factor for Femto-to-Femto SINR of the MUE SINR of the HUE

Multiple Femtocells (2/2) The power control problem is feasible for all N simultaneous operating HNBs within the same channel as long as 1.Matrix Y is non-singular (invertible), where Y is an N x N matrix 2.The transmission power vector p* HNB satisfies inequality (4) element- wise, where 3.The transmission power vector p* HNB also satisfies the following inequality

Centralized Solution 1.Solve the transmission power vector p* HNB using equaiton (14). 2.Check: If Yes, go to the next step; Otherwise, re-allocate the k-th HNB that has the largest value of to a different channel, and return to Step 1. 3.Check inequality (16) If it is satisfied, set tx power vector as p* HNB. Otherwise, re-allocate the HNB that requires the largest tx power to a different channel, and return to Step 1.

Distributed Solution Distributed power control schemes may be derived by applying iterative algorithms to solve equation (21): For example, using the first-order Jacobian iterations, the following distributed power control scheme is obtained The convergence properties of this type of algorithms were studied by Yates. –R. Yates, “A Framework for Uplink Power Control in Cellular Radio System,” IEEE JSAC, vol. 13, no. 7, Sept This algorithm given in eq. (22) does not enforce the QoS req. of the MUE. Thus the HNBs apply eq. (22) alone may violate the QoS of MUE. –MNB runs a similar distributed algorithm as the HNBs –When MNB cannot maintain the QoS of MUE, it need to request the strongestly transmitted HNB to switch channel…

Simulation Results Convergence of the tx power of HNBs SINR value of the MUE during the power control process of the HNBs

Conclusion This work is an extension of the mathematical framework in the authors’ previous work: –L. Qian, X. Li, J. Attia and Z. Gajic, “Power Control for Cognitive Radio Ad Hoc Networks,” in Proc. of 15 th IEEE LANMAN, June A downlink power control problem is formulated for HNBs that operates simultaneously in the same frequency band with MNBs. Both centralized and distributed solutions are given.