1. MITP 413: Wireless Technologies Week 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 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

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

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

12. 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

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

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
in phase
out of phase
Frequency= 1/50
Frequency= 1/45

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. 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-

23. 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

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