Channel Models Spring 2017 ELE 492 FUNDAMENTALS OF WIRELESS COMMUNICATIONS 1
Narrowband Channel Models Statistical Approach: Impulse response modeling: A narrowband channel can be represented by an impulse response which is a delta function with time-varying attenuation, i.e. We have seen that variations in amplitude over a small area are typically modeled as a random process, with an autocorrelation function that is determined by the Doppler spectrum. Rayleigh or Rician distribution For a larger area, the SSA amplitude F follows a lognormal distribution with std σ F (4 to 10 db). Shadowing Spring 2017 ELE 492 FUNDAMENTALS OF WIRELESS COMMUNICATIONS 2
Narrowband Channel Models Deterministic Approach: Impulse response modeling: Stored channel impulse responses: very realistic, scenario dependent Path Loss Models: We may also model the received field strength averaged over both small-scale and large-scale fading. i. Free-space path loss model ii. Breakpoint model (n = 2 valid for distances up to d < d break, and n = 4 beyond that) iii. Okumura-Hata model (only for large cells with BS being placed higher than the surrounding rooftops). Path loss (in db) is written as f c : carrier freq., function a(.) and factor C depend on the environment Spring 2017 ELE 492 FUNDAMENTALS OF WIRELESS COMMUNICATIONS 3
Narrowband Channel Models iii. Okumura-Hata model Urban areas (small and medium-size cities): Metropolitan areas: Suburban environments: a(h MS ): same as urban area Rural areas: a(h MS ): same as urban area Spring 2017 ELE 492 FUNDAMENTALS OF WIRELESS COMMUNICATIONS 4
Narrowband Channel Models iv. COST 231 Walfish Ikegami Model (for microcells and small macrocells) v. Motley Keenan Model (for indoor environments) (in db) F wall : sum of attenuations by the walls that a MPC has to penetrate on its way TX-to-RX Depending on the material F wall α 1 20dB for 300MHz-5GHz F floor : sum of attenuation of the floors that are located between TX and RX Spring 2017 ELE 492 FUNDAMENTALS OF WIRELESS COMMUNICATIONS 5
Wideband Channel Models Tapped-Delay Line Models (t) (t-t 1 ) (t-t 2 ) (t-t N ) D D D D c 0 (t) c 1 (t) c 2 (t) c N-1 (t) c N (t) v(t) y(t) time invariant! Rayleigh fading, i.e. c i (t): zero-mean comple Gaussian ACF of c i (t) is determined by the Doppler Spectrum (Jakes) Several variations of the gains a 0, c i (t) i. LOS with rich scattering at the same delay as LOS τ 0 = τ 1 and a 0 c i (t): time varying, Rician fading ii. No LOS, N = 2 Two-path channel, Simplest stochastic fading channel with delay dispersion iii. LOS, N=1 Satellite channel, LOS with delay τ 0, reflection from buildings near RX with delay τ 1. If τ 0 = τ 1 flat-fading Rician channel. Spring 2017 ELE 492 FUNDAMENTALS OF WIRELESS COMMUNICATIONS 6
Wideband Channel Models Model for the Arrival Times of Rays and Clusters: With a narrow signal bandwidth all MPCs add up together, MPCs can not be resolved With a large signal bandwidth MPCs can be resolved. Depending on the BW and channel topology they get grouped (clustered) together. Saleh Valenzuela model: Cluster inter-arrival times, T l, of the clusters follow a Poisson distribution mean arrival rate inter-arrival time bw. two clusters Inter-Arrival times of MPCs in a cluster, τ k,l, follow another Poisson pdf. mean arrival rate inter-arrival time bw. two MPCs Spring 2017 ELE 492 FUNDAMENTALS OF WIRELESS COMMUNICATIONS 7
Wideband Channel Models Models for the Power Delay Profile: Measurements reveal that in most cases PDP can be approimated by the eponential function: single cluster rms delay spread characterizes delay dispersion. A more generalized model includes clusters: rms delay spread Power, delay spread and delay of the l th cluster Typical rms delay spread values: Indoor residential buildings: typically 5 10 ns, up to 30 ns Indoor office environments: typically 10 100 ns, up to 300 ns Factories and airport halls: typically 50 200 ns Microcells: typically 5 100 ns (with LOS), 100 500 ns (without LOS) Tunnels and mines: typically empty tunnels 20 ns, car-filled tunnels up to 100 ns Typical urban and suburban environments: typically 100 800 ns, up to 3 μs Bad Urban (BU) and Hilly Terrain (HT) environments: typically 20 50 μs, mountainous terrain up to 100 μs Spring 2017 ELE 492 FUNDAMENTALS OF WIRELESS COMMUNICATIONS 8
Standardized Channel Models Depending on the systems parameters (bandwith, modulation type, etc) and also on the environment (indoor, outdoor, cable, etc.) there are numerous standard channel models in the literature. Eample: GSM system: COST 207 model Define PDPs, tap weights and Doppler spectra for four typical environments. Rural area (RA) Typical urban (TU) Bad urban (BU) Hilly terrain (HT) Valid for a bandwidth < 200 khz. There are other models for 3G, LTE-A defined by ITU (International Telecommunications Union) There are also models for wireline comm. systems. Spring 2017 ELE 492 FUNDAMENTALS OF WIRELESS COMMUNICATIONS 9