BBN Throughput Scaling in Dense Enterprise WLANs with
BBN: Throughput Scaling in Dense Enterprise WLANs with Blind Beamforming and Nulling Wenjie Zhou (Co-Primary Author), Tarun Bansal (Co-Primary Author), Prasun Sinha and Kannan Srinivasan The Ohio State University
Changes in Uplink Traffic Traditionally, WLAN traffic: • downlink heavy • less attention to uplink traffic Recently, uplink traffic increased rapidly : • mobile applications Cloud Computing Code Offloading Online Gaming Vo. IP, Video Chat Sensor Data Upload 2
Can we scale the uplink throughput with the number of clients?
Network MIMO Exchange raw samples AP 1 C 1 AP 2 C 2 AP 3 C 3 Huge bandwidth consumption
Mega. MIMO 1 Does not apply to uplink : Clients do not share a backbone network [1] Rahul, H. , Kumar, S. , and Katabi, D. Mega. MIMO: Scaling Wireless Capacity with User Demand. In Proc. of ACM SIGCOMM 2012.
Interference Alignment 1 • • C 1 AP 1 C 2 AP 2 C 3 AP 3 4 packet, 3 slots Enough time slots, everyone gets half the cake Exponential slots of transmissions, not suitable for mobile clients Heavy coordination between clients [1] Cadambe, V. R. , and Jafar, S. A. Interference Alignment and the Degrees of Freedom for the K User Interference Channel. IEEE Transactions on Information Theory (2008).
Existing interference alignment and beamforming techniques are not suitable to mobile uplink traffic. How can we bring the benefits of beamforming to uplink traffic?
AP Density in Enterprise WLANs 1 CDF 0. 75 0. 5 (140, 0. 5) 0. 25 0 50 100 150 Number of Access Points (APs) 200 BBN leverages the high density of access points 8
Omniscient TDMA Single Collision Domain AP 2 AP 3 AP 4 Switch AP 1 Time Slot: 3 1 2 x 1 x 2 C 1 x 3 C 2 C 3 Three Packets received in Three Slots Only one AP is in use 9
Blind Beamforming and Nulling Single Collision Domain h(1) 11 x 1 + h(1) 21 x 2 + h(1) 31 x 3 12 x 1 + AP 1 22 x 2 + h(1) 32 x 3 Time Slot: 1 AP 2 AP 3 Switch h(1)14 x 1 + h(1)24 x 2 + h(1)34 x 3 h(1)13 x 1 + h(1)23 x 2 + h(1)33 x 3 h(1) AP 4 13 x 1 C 1 h(1)33 23 x 2 C 2 x 3 C 3 10
Blind Beamforming and Nulling Single Collision Domain Receives: a 11 x 1 + s 1 h(1)21 x 2 + s 1 h(1)31 x 3 Receives: a 12 x 1 + a 22 x 2 + a 32 x 3 AP 2 AP 3 AP 4 Switch AP 1 Time Slot: 2 Transmits: v 3 (h(1)13 x 1 + h(1)23 x 2 + h(1)33 x 3) Transmits: v 4 (h(1)14 x 1 + h(1)24 x 2 + h(1)34 x 3) 11
Blind Beamforming and Nulling Single Collision Domain Slot 1: h(1)11 x 1 + h(1)21 x 2 + h(1)31 x 3 Slot 1: h(1)12 x 1 + h(1)22 x 2 + h(1)32 x 3 Slot 2: a 11 x 1 + s 1 h(1)21 x 2 + s 1 h(1)31 x 3 Slot 2: a 12 x 1 + a 22 x 2 + a 32 x 3 (s 1 h(1)11 -a 11)x 1 AP 2 AP 3 AP 4 Switch AP 1 Three Packets received in Two Slots 12
Number of APs Required • In a network with APs, APs in BBN can receive N uplink packets in two slots • 3 clients, 4 APs • 4 clients, 7 APs • 10 clients, 46 APs 13
Throughput Improvement • Previous Example Topology – APs in BBN receive three packets in two slots: an improvement of 50% • General Topology – Uplink throughput in BBN scales with the number of clients (N/2 packets per slot). – Half of the cake as in Interference Alignment • Always two slots • No coordination between clients 14
BBN Highlights • Leverages the high density of access points • All computation and design complexity shifted to APs • APs only need to exchange decoded packets over the backbone instead of raw samples 15
Further Optimizations to Improve SNR Receivers AP 2 AP 1 Switch Transmitters x 2, x 3 x 1 AP 4 AP 3 C 1 C 2 C 3 • Which subset of APs act as transmitters and which subset as receivers? • Which AP decodes which packet? BBN Approach: xi is decoded at the APj where it is expected to have highest SNR 16
Challenge 1/4: Synchronization of APs • To perform accurate beamforming, APs need to be tightly synchronized with each other • Solution: – Source. Sync (Rahul et al. , SIGCOMM 2010): synchronizes APs within a single collision domain – Vidyut (Yenamandra et al. , SIGCOMM 2014): uses power line to synchronize APs in the same building 17
Challenge 2/4 : Multi. Collision Domain • Not all APs may be able to hear each other directly • Solution: Make smaller groups where all APs in a single group can hear each other. 18
Distributed System Group Head • Within a group, all APs can hear each other • When one group is communicating, neighboring groups remain silent 19
Challenge 3/4 : Inconsistency in the AP density • Number of APs may be less than • Solution: Appropriate MAC layer algorithm that restricts the number of participating clients 20
MAC Timeline Uplink Keep Silent – Allow neighboring groups to transmit Time Slot 1 Approve A, B and C Notification Period Downlink Uplink Time Slot 2. . . . Poll Uplink . . . Time Compute pre-coding vectors in the background 21
Challenge 4/4 : Robustness • Nulling is not always perfect. Can’t Subtract x 1 Decoding Error x 1 , x 2 , x 3 AP 2 AP 4 AP 5 x 1 x 2 C 1 C 2 Switch AP 1 x 3 C 3 22
Challenge 4/4 : Robustness • What if we have extra APs x 1 , x 2 , x 3 AP 2 AP 3 AP 4 AP 5 Switch AP 1 x 1 x 2 C 1 C 2 AP 6 AP 7 x 3 C 3 23
Experiments Intended Signal = x 1 Interference from x 2, x 3 AP 2 AP 3 AP 4 USRP N 210 Switch AP 1 x 2, x 3 x 2 x 1 C 2 x 3 C 2 24
Throughput 1. 48 X BBN provides 1. 48 x throughput compared to TDMA 25
Trace-Driven Simulation • Over multiple collision domains (divided into groups) • Field Size: 500 m X 500 m • Number of clients: 1000 • Vary the number of APs • Residual interference distribution from experiment • Other algorithms simulated – Omniscient TDMA – IEEE 802. 11 26
Throughput BBN • • • 2000 APs 4. 6 X throughput gain ~76 APs near each client 27
Fairness BBN achieves higher fairness • Beamforming increased SINR of clients that are far away 28
Summary and Future Work • BBN leverages the high density of APs to scale the uplink throughput for single antenna systems – Throughput scales linearly with the number of clients – All computational and design complexity shifted to APs • Future Work – Coexist with legacy network – Data rate selection Thank you 29
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