ChSim A wireless channel simulator for OMNeT++

Transcription

1 ChSim A wireless channel simulator for OMNeT++ Simulation workshop TKN, TU Berlin September 08, 2006 Computer Networks Group Universität Paderborn

2 Outline Introduction Example scenario, results & modeling PHY interface & program structure Integration to joint simulator framework Conclusion & wishlist for joint simulator 2

3 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

4 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

5 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

6 Fading result example Channel states with various speeds 6

7 Correlation example 10 Channel gain of two uncorrelated links H [db] H [db] Samples 0 Channel gain of two correlated links ρ= Samples Useful for multiple channel systems, e.g. MIMO, OFDM Chsim supports link and frequency correlation 7

8 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] SNR per symbol, =-9dBm, P 50m distance and single 2ms block fading channel with 1m/s motion speed tx chosen tx-rate [Bits/s] 6 x Ideal 107 rate allocation for the 8 IEEE a rates, 2ms adaptation cycle, and 2 different bound sets Time [s] Bounds: L=13500Bytes Bounds: L=1500Bytes Bounds: L=1500Bytes (min. G code Bounds: Proxim MP.11a default PHY not included in ChSim, a/g PHY implementations exist 8

9 Program structure Basic structure: MobileStation[n]: Channel[m]: channelstatecalculator: Channel modeling methods errorcalculator: Empty: no error model in ChSim 9

10 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

11 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 a/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

12 Thank you Questions? Download Chsim at 12

13 Frequency selective fading 10 5 Delay spread = 10ns = 150ns = 5µs 0-5 H [db] Frequency band 13

14 Rayleigh fading 8000 Histogram of a slow fading channel generated with ChSim