Uplink Power Control Recommendations for IEEE 802.16m Document Number: IEEE C802.16m-08/666 Date Submitted: 07-07-2008 Source: Ali Taha Koc, Shilpa Talwar, Apostolos Papathanassiou, ali.t.koc@intel.com, shilpa.talwar@intel.com Rongzhen Yang, Nageen Himayat, Hujun Yin Venue: IEEE 802.16m-08/016r1 Call for System Description Document (SDD) Comments and Contributions, on the topic of “Uplink power control”. Base Contribution: None Purpose: Discussion and approval of the proposal into the IEEE 802.16m System Description Document Notice: This document does not represent the agreed views of the IEEE 802.16 Working Group or any of its subgroups. It represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material contained herein. 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Uplink Power Control IEEE 802.16m system requirements from SRD [1] • • Average User Throughput Cell edge Throughput Our view, IEEE 802.16m requirements on uplink power control can be improved by • Open Loop Power Control (OLPC) – – – • Closed Loop Power Control (CLPC) – – – • Slow power control Provide balance between cell edge and sector throughput Limit the level of IoT Fast power control Limit the MAC overhead of the signaling Effective estimation of uplink interference Coupling of CLPC and OLPC – Provides high efficiency with reduced signaling overhead Open Loop Power Control • Path loss and shadowing variation can be compensated – Received signal level from different users does not have very large dynamic range • Average IoT (Interference over Thermal) levels can be controlled • Using an Open Loop Power control saves on continuous signaling of power control commands • Using fractional open loop power control provides flexibility to balance the cell edge vs. sector throughput Closed Loop Power Control • In a full frequency reuse system, inter-cell interference dominates the system performance – Controls the power of interference sources individually at the expense of signaling overhead – Requires estimating uplink interference per user – Requires measurement of uplink received signal strength (for example, HARQ ACK/NACK can be used) • Track and compensate the effect of user’s fast fading channel OLPC and CLPC Comparison Open Loop Power Control Signaling Overhead Low Power Control Accuracy Closed Loop Power Control High • Only broadcast algorithm initial parameters • No signaling for power control commands • Broadcast algorithm initial parameters • Signaling power control commands Low High •Algorithm depends on the estimated path loss value that is based on reciprocity • Limited information on MS make it difficult to achieve better adaptation • Accurate uplink signal measurement and interference estimation. • Better control of interference by BS information exchanging Common Open Loop Schemes • Full Power User Thrpoughput CDF with different SNR Targets 1 – Bad for the cell edge users – High SINR variation 0.9 SNR Target=0dB SNR Target=8dB SNR Target=15dB SNR Target=23dB Full Power 0.8 • SNR based • Low SNR target: bad for the center edge user (low spectral efficiency) • High SNR target: bad for the cell edge users (high interference) 0.6 F(x) – Each user transmits enough power to meet receive SNR target at BS – BS broadcasts the SNR target – Tradeoff 0.7 0.5 0.4 0.3 0.2 0.1 0 0 0.5 1 1.5 User Throughput 2 2.5 3 x 10 6 Proposed Open Loop Scheme • Takes interference into account – Set the SNR target depending on the DL SIR estimate Users Throughput CDFs with SIR based power control 1 = 0dB, = 8dB, = 8dB, = 8dB, 0.9 = 0.75 = 0.5 = 0.75 =1 0.8 dB dB SNRtarget SIRnearestBS 0.7 • Base station broadcasts gamma and beta values • Automatic ‘Soft reuse’ – Assign high SNR target for cell-interior users – Assign low SNR target for celledge users F(x) 0.6 0.5 0.4 0.3 0.2 0.1 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Users Throughput 1.6 1.8 2 2.2 x 10 6 Justification of Proposed Method We can write SNR as SNR (1 INR) * SINR (1 INR ) * SIR in a interference limited system We set SNR target as function of SIR (dB) SNRdB 10 log 10(1 INR) 10 log 10( SIR ) SNRd B * SIRd B • By changing gamma, the scheme can control the IoT level • By changing beta, the balance of cell edge versus sector throughput can be adjusted IoT Curves for Different Gamma cdf of IoT 1 =0,=3/4 0.9 =8,=3/4 0.8 0.7 F(x) 0.6 0.5 0.4 0.3 0.2 0.1 0 -2 0 2 4 6 x 8 10 12 Open Loop and Closed Loop Coupling Data Traffic Fast control Slow control Fast control Slow control Fast control • Open Loop is going to be the default scheme – Open Loop Power Control coarsely adjust uplink transmit power with low signaling overhead • When there is data packets, Closed loop will the main power control scheme – Closed Loop Power Control signals can be piggyback to the data traffic for the fast control (for example, only a few bits combined with resource allocation IE (DL/UL MAP IE) for refined power adjustment) Minimize CLPC Signaling Overhead by Piggyback Proposed CL scheme BS 1. Received signal strength and interference measurement 2. CLPC algorithm decides the offset ACK/NACK for DL HARQ CQICH Uplink Data Burst Resource Allocation IE with Power control command MS Example for using existing loop – DL HARQ for CLPC MAP New Data MAP Retrans MAP New Data MAP Retrans MAP DL UL NACK ACK DL HARQ IE NACK Power Control Command Bit Field (1~2 bits) ACK New Data Text Proposal to IEEE 802.16m SDD Insert the following text into Physical Layer clause (Chapter xx in [IEEE 802.16m-08/003r1]) ------------------------------- Text Start ------------------------------11.x.x.x Uplink Power Control A power control algorithm is supported for in the uplink channels with both initial calibration and periodic adjustment procedure. The parameters of power control algorithm are optimized on system-wide basis by the BS, and broadcast periodically. 11.x.x.x.1 Open Loop Power Control Uplink open loop power control compensates all or a fraction of the pathloss and shadowing. Uplink open loop power control uses channel and interference knowledge to operate at optimum power control settings. Mobile stations can derive their initial transmission power according to the path loss, interference measurements obtained from the downlink preambles and pilots. 11.x.x.x.2 Closed Loop Power Control Closed loop power control compensates for fast variations in channel and interference on a per subscriber basis, while minimizing MAC signaling overhead. Closed loop power control couples with open loop power control for efficient operations. Closed loop power control is active with data transmission. Close loop power control measures uplink power using uplink data and/or control channel transmissions and sends control command in unicast service control channel (USCCH). ------------------------------- Text End ------------------------------- References [1] IEEE 802.16m-07/002r4, “TGm System Requirements Document (SRD)” [2] IEEE 802.16m-08/004r1, “Project 802.16m Evaluation Methodology Document (EMD)” Backup Uplink Power Control in Legacy System • Open Loop – – – • Maintain same transmitted power density unless the maximum power reached Normalized CNR values are determined in the standard Full path loss compensation Closed Loop – – – – In the initial ranging phase, the mobile users derives its initial transmission power according to the path loss measurement from the downlink pilot channel. After that the mobile users adjusts its transmission power according closed-loop power control messages signalled by the base station. Signalling overhead is too much for the 802.16e system. BS -> MS: Send Offset information Pnew Plast (C N new C N last ) (10 log 10 ( Rnew ) 10 log 10 ( Rlast )) offset • • • FPC (Fast Power Control) Message Power Control IE RNG-RSP for periodic ranging IoT Curves cdf of IoT 1 • SNR Target=0dB SNR Target=5dB SNR Target=8dB SNR Target=13dB SNR Target=18dB Full Power 0.9 0.8 0.7 • • F(x) 0.6 0.5 0.4 0.3 0.2 0.1 0 -10 0 10 20 30 x 40 50 60 Common assumptions for IoT = 7dB SNR Target = 8dB provides IoT target Changing SNR target has shifting affect on IoT values SINR Curves cdf of SINR 1 0.9 SNR Target=0dB SNR Target=5dB SNR Target=8dB SNR Target=13dB SNR Target=18dB Full Power 0.8 0.7 F(x) 0.6 0.5 0.4 0.3 0.2 0.1 0 -60 -40 -20 0 x 20 40 60 SINR Curves cdf of SINR 1 • 0.9 0.8 Gamma=0,Alpha=3/4 Gamma=8,Alpha=1/2 Gamma=8,Alpha=3/4 Gamma=8,Alpha=1 – 0.7 F(x) 0.6 • 0.5 0.4 0.3 0.2 0.1 0 -30 -20 -10 Increasing Gamma has minimal affect on SINR 0 x 10 20 30 Increasing gamma increase signal strength and interference at same time After finding the optimum gamma, alpha is important to have cell edge and center cell balance