CSI-SF: Estimating Wireless Channel State Using CSI Sampling & Fusion

1. SymbolMapping Symbol Mapping FFT RX FFT Channel (H) Encoding SpatialMapping(Q) MIMO Decoding FFT RF AGC Demap Chan Estimation Demap AGC AGC Bitstream RF RF Chan Estimation RF Chan Estimation RF CSI-SF: Estimating Wireless Channel State Using CSI Sampling & Fusion Q2 Q1† Packet 2 (Tx: B, Rx: ABC) Packet 1 (Tx: A, Rx: ABC) Q1† 2 Riccardo Crepaldi∗, Jeongkeun Lee†, Raul Etkin†, Sung-Ju Lee†, Robin Kravets ∗University of Illinois at Urbana-Champaign †Hewlett-Packard Laboratories 3 CSI2 CSI1 56 3 3 Power scalefactor 56 56 1 1 EstimatedCSI Improving Performance in High Speed Wireless Channel Quality Metrics • How do systems describe the channel quality? • Pros • Finer granularity compared to SNR • Fading and phase shift information for each subcarrier • Tighter bound with channel performance [Halperin’10] • Cons • CSI is significantly larger than SNR • CSI Describes a specific MIMO configuration • CSI for multiple MIMO configurations must be measured for a complete channel characterization PHY Layer enhancements: MIMO technology OFDM Channel Bonding Adaptive protocols for: Rate adaptation Channel selection Optimal antenna scheme selection Best AP selection Choices based on: Channel quality [N Input streams] [M spatial streams] • Signal-to-Noise Ratio (SNR): • One scalar value per packet • Accurate only for narrow-band channel • Channel State Information (CSI): • M x N x W complex matrix • W= Number of OFDM subcarriers • Current 802.11n devices: • M=3, N=3 • W=56 (20MHz) or W=114 (40MHz) What MIMO configuration should I choose? The Channel quality is… Why CSI? TX: M parallel data streams Quality of CSI-SF Estimate RX: N receivers Application of CSI-SF CSI Matrix Combining CSI-SF enables estimation of uninvestigated channel configurations Z(t) X(t) Y(t) • A real networking problem: Rate Adaptation • Metrics: SNR, effective SNR, estimated effective SNR • What’s a good metric? • Small or no gaps • Monotonic behavior CSI Ex: Estimate of a 2x3 configuration using two 1x3 received packets 2x3 – 20MHz Large gaps Non-monotonic beavior 2x3 – 40MHz 3x3 – 20MHz eSNR (effective SNR based on real samples) e2SNR (effective SNR based on CSI-SF estimates) SNR is not a good metric, eSNR and e2SNR are better Compared to standard eSNR, e2SNR uses estimates instead of real samples thus limiting the overhead Example of CSI-SF combining two 1x3 packets in a 2x3 packets Packet 1: 1x3 – Ant A Packet 1: 1x3 – Ant B CSI-SF Reduces Overhead SNR [db] Hypoyetical channel variation Overhead required to rate control to converge using different metrics SNR [db] Packet 3: 2x3 – Ant AB Real CSI OFDM subcarrier index Estimated CSI Hewlett Packard LaboratoriesUniversity of Illinois @ Urbana-Champaign, Department of Computer Science – Mobicom 2011