MITP 413: Wireless Technologies Week 3 Michael L. Honig Department of ECE Northwestern University April 2004

Erlang B Curves

Trunking Efficiency Refers to the traffic intensity (Erlangs) that can be supported given a fixed number of channels and a target blocking probability. For a fixed blocking probability: Trunking efficiency improves with the number of channels. Best to pool as many channels as possible. Sectorization reduces trunking efficiency. (Does not apply to CDMA.)

SIR vs. Frequency Reuse

Channel Allocation Objective: equalize grade of service (blocking probability) over coverage area  Allows increase in subscriber pool. Fixed Channel Assignment (FCA): channels assigned to each cell are predetermined. Separate channels within a cell to avoid adjacent-channel interference Nonuniform FCA: distribute channels among cells to match averaged traffic load over time. Channel borrowing: borrow channels from neighboring cell Temporary: high-traffic cells return borrowed channels Static: channels are non-uniformly distributed and changed in a predictive manner to match anticipated traffic Dynamic Channel Assignment (DCA): channels are assigned to each call from the complete set of available channels Must satisfy S/I constraint Channels returned to pool after call is completed Can be centralized (supervised by MSC) or distributed (supervised by BS) Distributed DCA used in DECT

FCA vs. DCA FCA DCA Low complexity Better under heavy traffic Sensitive to changes in traffic Variable grade of service Higher probability of outage Suitable for macro-cellular systems (e.g., cellular) Low call setup delay Requires careful frequency planning Centralized assignment Moderate/High complexity Must monitor channel occupancy, traffic distribution, S/I (centralized) Better under light/moderate traffic Insensitive to changes in traffic Stable grade of service Low probability of outage (call termination) Suitable for micro-cellular systems (e.g., cordless) Moderate/high call setup delay No frequency planning Assignment can be centralized or distributed

Radio Channels Troposcatter Microwave LOS Mobile radio Indoor radio T

Sinusoidal Signal Time delay = 12, Phase shift = 12/50 cycle = 86.4 degrees Amplitude= Period= 50 sec, frequency= 1/50 cycle/sec Time (seconds)

Sinusoid Addition (Constructive) + =

Sinusoid Addition (Destructive) + = Signal is faded.

Indoor Propagation Measurements Hypothetical large indoor environment Ceiling Normalized received power vs. distance

Path Loss Exponents ENVIRONMENT PATH LOSS EXPONENT, n Free space 2 Urban cellular radio 2.7 to 3.5 Shadowed urban cellular radio 3 to 5 In building line-of-site 1.6 to 1.8 Obstructed in building 4 to 6 Obstructed in factories 2 to 3

Large-Scale Path Loss (Scatter Plot)

Empirical Path Loss Models Propagation studies must take into account: Environment (rural, suburban, urban) Building characteristics (high-rise, houses, shopping malls) Vegetation density Terrain (mountainous, hilly, flat) Okumura’s model (based on measurements in and around Tokyo) Median path loss = free-space loss + urban loss + antenna gains + corrections Obtained from graphs Additional corrections for street orientation, irregular terrain Numerous indoor propagation studies for 802.11

Link Budget How much power is required to achieve target S/I? dBs add: Target S/I (dB) + path loss (dB) + other losses (components) (dB) - antenna gains (dB) Total Power needed at transmitter (dB) Actual power depends on noise level. With 1 microwatt noise power, 60 dB transmit power  1 Watt

Small-Scale Fading

Short- vs. Long-Term Fading Short-term fading Long-term fading Signal Strength (dB) T T Time (t) Long-term fading: Distance attenuation Shadowing (blocked Line of Sight (LOS)) Variations of signal strength over distances on the order of a wavelength

Doppler (Frequency) Shift in phase out of phase Frequency= 1/50 Frequency= 1/45

Rayleigh Fading deep fade phase shift Received waveform Amplitude (dB)

Channel Coherence Time Coherence Time: Amplitude and phase are nearly constant. Rate of time variations depends on Doppler shift: (velocity X carrier frequency)/(speed of light) Coherence Time varies as 1/(Doppler shift).

Power-Delay Profile delay spread

Types of Small-Scale Fading Based on multipath time delay spread Flat Fading 1. BW of signal < BW of channel 2. Delay spread < Symbol period Frequency Selective Fading 1. BW of signal > BW of channel 2. Delay spread > Symbol period Based on Doppler spread Fast Fading 1. High Doppler spread 2. Coherence time < Symbol period 3. Channel variations faster than base- band signal variations Slow Fading 1. Low Doppler spread 2. Coherence time > Symbol period 3. Channel variations slower than base-

Types of Small-Scale Signal Fading as a Function of Symbol Period and Signal Bandwidth Relative to delay spread Ts Flat Slow Fading Flat Fast Fading delay spread Frequency-Selective Slow Fading Frequency-Selective Fast Fading Ts Tc (coherence time) Symbol Period relative to coherence time. Signal BW relative to channel BW Bs Frequency Selective Fast Fading Frequency Selective Slow Fading coherence BW Bc Flat Fast Fading Flat Slow Fading Bs Bd = fd (Doppler shift) Signal bandwidth relative to Doppler shift

Fading Experienced by Wireless Systems Standard Flat/Freq.-Sel. Fast/Slow AMPS Flat Fast IS-136 Flat Fast GSM F-S Slow IS-95 (CDMA) F-S Fast 3G F-S Slow to Fast (depends on rate) 802.11 F-S Slow Bluetooth F-S Slow