Radio Link Layer tuning in HSPA Evolution Laura Kneckt Supervisor : Professor Jyri Hämäläinen Instructor: M. Sc. Stefan Wager

Thesis target To study flexible RLC PDU sizes with high and low bit rates and to find an optimal max RLC PDU size which could be changed dynamically Adaptation algorithm for changing the max RLC PDU size –Input alternatives: CQI, HARQ or RLC block error rate, scheduled TF –Delay in TN (transport network) –Hysteresis 2

RLC + Flexible RLC RLC is designed to improve the performance of data traffic over the air interface so that higher layer protocols such as TCP can function effectively Large enough RLC PDU size to avoid window stalling Small enough RLC PDU size for retransmission efficiency RLC PDU TB RLC PDU TB 3

Protocol termination points for HS-DSCH HS-DSCH – High Speed Downlink Shared Channel PDCP – Packet Data Convergence Protocol RLC – Radio Link Control Protocol MAC – Medium Access Control UE – User Equipment Node B – Base station SRNC – Serving Radio Network Controller 4

Max RLC PDU size An example of how a RLC SDU can be segmented into five consecutive RLC PDUs. The first four RLC PDUs are of the maximum size and the fifth is smaller. In MAC-hs the segmentation and multiplexing of incoming MAC-d PDUs is supported in order to have optimal size Transport Formats (TF) 5 RLC SDU RLC PDU RLC PDU RLC PDU RLC PDU RLC PDU Max RLC PDU size is an upper limit for the RLC PDU payload size. All incoming packets that exceed the limit would be segmented to RLC PDUs with payloads sized the maximum limit, and the last segment being possibly smaller than the maximum limit.

Simulations Simplified MIMO model in use –TF table sizes doubled –Largest TB size in Layer 1: 42196, in Layer 2: –Scheduler selects ”real TF” but in L1 the TB size is faked to be max bits. –Theoretical peak bit rate up to 42Mbps Studied scenarios Peak bit rate Low bit rate –Handovers –User speed –User distance from the BS –Power limitation –Attenuation –Nrof codes / max nrof UE bits Low bitrates hard to produce without losing the status message in the UL and losing the whole connection 6

Simulation parameters One user Downloading large files ~ 34MB HS/EUL channels Modulation up to 64QAM + 2x2 MIMO 7

Peak bit rates One cell, one user near the base station, 15 codes, HARQ target BLER 10% With MIMO the peak bit rate is about 34Mbps and without MIMO about 16Mbps. The window stalling affects the transmission rate when max RLC PDU size is smaller than 1600 bits with MIMO and 800 bits without MIMO. These are the upper limits for max RLC PDU size with given RLC timer values. 8

Low bitrates Low bitrates are studied to find out whether the RLC retranmissions have an impact to the DL transmission rate Different scenarios Mobility –One cell, moving to coverage border –Two cells, moving from basestation to another BS Poor channel quality with limited HS power –Total cell power 43dBm, of which 41dBm reserved for non HS services –Increasing the attenuation 21dBm, 31dBm, 36dBm (max attenuation 41dBm) Additional 21dBm attenuation with limited HS power –Power limitations 41.5, 41.7, 42dBm (max power limitation 43dBm) –Remaining power 5,8W; 5,2W; 4,1W for HS-DSCH and HS-SCCH Limited codes, 1; 2; 15 –Attenuation 21 dBm (of 41dBm) –Power limited to 4,1W When trying to get lower throughputs the UL becomes the limiting factor –Not “enough” RLC retransmissions because the status messages in UL are lost and due to that, the whole connection is lost 9

Handover scenario User moves from one cell to another Figure shows the throughput in time with different max RLC PDU sizes: 400, 800, 1200, 1600 and 2000 bits 20 iterations; plotted mean values and confidence intervals No significant differencies between max RLC PDU sizes, mostly the curves are inside confidence intervals of another curves The values are average values over one second. The lowest throughput is 1M which is good in handover. 10

One cell Cell radius 5 kms User starts next to basestation and moves to cell border Three max RLC PDU sizes: 400, 1000 and 2800 Window stalling limits the throughput for sizes 400 and 1000 at the beginning After user has moved 1km the throughputs are almost the same The significant changes in thoughput are because the user starts downloading another object. Moving to coverage border 11

Low channel power, increasing attenuation The remaining power for HS channels 2 dBm, the attenuation with blue line is 21dBm, with pink line 31dBm, with green line 36dBm (max attenuation 41dBm) Number of RLC retransmissions is about 0,2% of RLC PDU transmission rate The small differences in throughput are not due to RLC retransmissions 12

Power limitation Attenuation 21dBm Limited HS power Small changes in curves, but mostly inside confidence intervals RLC retransmission rate lower than 0,2% of all RLC PDU transmissions 13

Decreasing codes Throughput decreases when max RLC PDU size increases Maximum RLC retransmission rate 0,2% doesn’t cause the slight decrase of throughput The flow control is optimized for small RLC PDU sizes and could be the reason that causes the decrease 14

Conclusions With given RLC timer settings, the optimal max RLC PDU size with MIMO is 1600 and 800 without MIMO to avoid window stalling in high bit rates In low bitrate scenarios, RLC retransmission rate is lower than 0,2% –No effect on max RLC PDU size No RLC PDU size adaptation algorithm needed for the studied scenarios 15