ChSim A wireless channel simulator for OMNeT++ Simulation workshop TKN, TU Berlin September 08, 2006 Computer Networks Group Universität Paderborn
Outline Introduction Example scenario, results & modeling PHY interface & program structure Integration to joint simulator framework Conclusion & wishlist for joint simulator 2
Introduction Wireless channels may vary vs. time in mobile scenarios vs. frequency in multipath environments ChSim generates channel state statistics for multiple links Leads to transmission rate and error statistics 3
Scenario example A mobile wireless network: relative distance relative speed At runtime: Definition of relative speed and distance by mobility model Included: Manhattan grid, (Random waypoint) Fixed setup is possible Mobility parameters are passed to channel models Included: Clarke s fading model (Rayleigh), path loss, shadowing, and correlation 4
Modeling fading Time selective fading: Frequency of each fading path ist shifted according to mobility speed Doppler shift: f d ~ v rel Paths overlap at receiver Clarke s model 1. Simplification: There are k discrete fading paths 2. Simplification: Isotropic antenna gain pattern I.e., scatterers are located in a ring around the mobile receiver No ray-trace 3D models needed Frequency selective fading Parameterized by delay spread Delay variance of paths 5
Fading result example Channel states with various speeds 6
Correlation example 10 Channel gain of two uncorrelated links H [db] H [db] 0-10 -20-30 10-10 -20-30 0 100 200 300 400 500 600 700 800 900 1000 Samples 0 Channel gain of two correlated links ρ=0.9-40 0 100 200 300 400 500 600 700 800 900 1000 Samples Useful for multiple channel systems, e.g. MIMO, OFDM Chsim supports link and frequency correlation 7
Interface to physical layer Output of Chsim: Attenuation factor h(t) One h per time-sample, frequency-band, and link With transmission power & noise the PHY: Calculates: SNR = h(t) E s /N 0 Selects: Transmission rate using rate thresholds Calculates: Error statistics using coding gain and thresholds Drawback compared to G.-E.: All PHY calculations have to be done per channel sample However, with interleaving block fading can be assumed Limits sample frequency to one sample per MAC frame E s /N 0 at receiver [db] 25 20 15 10 5 0-5 SNR per symbol, =-9dBm, P 50m distance and single 2ms block fading channel with 1m/s motion speed tx -10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 chosen tx-rate [Bits/s] 6 x Ideal 107 rate allocation for the 8 IEEE 802.11a rates, 2ms adaptation cycle, and 2 different bound sets 5 4 3 2 1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time [s] Bounds: L=13500Bytes Bounds: L=1500Bytes Bounds: L=1500Bytes (min. G code Bounds: Proxim MP.11a default PHY not included in ChSim, 802.11a/g PHY implementations exist 8
Program structure Basic structure: MobileStation[n]: Channel[m]: channelstatecalculator: Channel modeling methods errorcalculator: Empty: no error model in ChSim 9
Integration to joint simulator framework TriggerGen and Filewriter: Natural connection to higher layers Integrate these (and other channel models) as a: Single compound module btw. terminals or OMNeT++ NED connection Preferred but requires OMNeT++ changes Wish list for joint simulator: Separation and clean interfaces for/btw.: Mobility model (Mobility framework implementations should be used) Channel PHY Arbitrarily amounts of channels per node and MAC link E.g. for MIMO Different time resolutions for Channel and PHY/MAC modules 10
Conclusion & future work Introduced: ChSim, our contribution to the joint simulator framework Provides channel statistics Analog channel models close to physical scenario Advantage vs. G.-E.: Physical parameters can be used directly or considered as factors E.g. Analyze performance of MIMO system with channel correlation Disadvantage vs. G.-E.: Computational overhead 4 G.-E. Future work: Revise correlation model, and add further methods Release 802.11a/g PHY Documentation For joint simulator: Include more, e.g. digital, channel models PHY, channel, mobility models should be clearly separated and connected using clean interfaces 11
Thank you Questions? Download Chsim at http://wwwcs.upb.de/cs/cn/projects.html 12
Frequency selective fading 10 5 Delay spread = 10ns = 150ns = 5µs 0-5 H [db] -10-15 -20-25 -30 0 5 10 15 20 25 30 35 40 45 50 Frequency band 13
Rayleigh fading 8000 Histogram of a slow fading channel generated with ChSim 7000 6000 5000 4000 3000 2000 1000 0-70 -60-50 -40-30 -20-10 0 10 14