Wireless Technologies - Erlang B Curves and Channel Allocation

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

2. Erlang B Curves

3. 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.)

4. SIR vs. Frequency Reuse

5. 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

6. FCA vs. DCA FCA DCA • 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 • 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

7. T T Radio Channels Troposcatter Microwave LOS Mobile radio Indoor radio

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

9. Sinusoid Addition (Constructive) + =

10. Sinusoid Addition (Destructive) + = Signal is faded.

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

12. Path Loss Exponents

13. Large-Scale Path Loss (Scatter Plot)

14. 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

15. 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

16. Small-Scale Fading

17. 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

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

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

20. 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).

21. Power-Delay Profile delay spread

22. 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 Small-Scale Fading Based on Doppler spread Fast Fading 1. High Doppler spread 2. Coherence time< Symbol period 3. Channel variationsfaster than base- band signal variations Slow Fading 1. Low Doppler spread 2. Coherence time > Symbol period 3. Channel variations slowerthan base- band signal variations Types of Small-Scale Fading

23. Types of Small-Scale Signal Fading as a Function ofSymbol Period and Signal Bandwidth Ts Symbol Period Relative to delay spread 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

24. 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