1. Basics of Small Scale Fading: Towards choice of PHY

2. Basic Questions Tx Desert Metro Street Indoor Rx
What will happen if the transmitter
changes transmit power ?
changes frequency ?
operates at higher speed ?
Transmit power, data rate,
signal bandwidth, frequency
tradeoff
What will happen if we conduct
this experiment in different types
of environments?
Channel effects
Desert
Metro
Street
Indoor
Effect of mobility Rx
What will happen if
the receiver moves?

3. Review of basic concepts
Channel Impulse response
Power delay profile
Inter Symbol Interference
Coherence bandwidth
Coherence time

4. Channel Impulse Response

5. Power delay Profile RMS Delay Spread () = 46.4 ns
-90
RMS Delay Spread () = 46.4 ns
-90
Maximum Excess delay < 10 dB = 110 ns
Mean Excess delay () = 45 ns
-95
Received Signal Level (dBm)
-100
Noise threshold
-105 50
100
150
200
250
300
350
400
450
Excess Delay (ns)

6. Example (Power delay profile)
4.38 µs
1.37 µs
0 dB
-10 dB
-20 dB
-30 dB  1 2 5
(µs)

7. RMS Delay Spread: Typical values
Delay spread is a good measure of Multipath
Manhattan
San Francisco
Suburban
Office building 2
SFO
Office building 1
10ns
50ns
150ns
500ns
1µs
2µs
5µs
10µs
25µs 3m
15m
45m
150m
300m
600m
3Km
7.5Km

8. Inter Symbol Interference
Symbol time
Pr()
4.38 µs
1.37 µs
0 dB
-10 dB
-20 dB 
-30 dB  1 2 5
(µs) 1 2 5
(µs)
4.38
Symbol time > 10*  No equalization required
Symbol time < 10*  Equalization will be required to
deal with ISI
In the above example, symbol time should be more than 14µs to avoid ISI.
This means that link speed must be less than 70Kbps (approx)

9. Coherence Bandwidth  delay spread Bc Freq. domain view
Time domain view
Range of freq over
which response is flat Bc
 delay spread
High correlation of amplitude
between two different freq.
components

10. RMS delay spread and coherence b/w
RMS delay spread and coherence b/w (Bc) are inversely proportional
For 0.9 correlation
For 0.5 correlation

11. Time dispersive nature of channel
Delay spread and coherence bandwidth are parameters which
describe the time dispersive nature of the channel.
Time domain view
Freq domain view
Symbol Time (Ts)
Signal bandwidth (Bs)
signal 1
Signal
signal 2
channel 1
channel 2
Channel
channel 3
RMS delay spread ()
Coherence b/w (Bc)

12. Revisit Example (Power delay profile)
0 dB
-10 dB
-20 dB
-30 dB  1 2 5
(µs)
Signal bandwidth for Analog Cellular = 30 KHz
Signal bandwidth for GSM = 200 KHz

13. Doppler Shift  v Example
Carrier frequency fc = 1850 MHz (i.e.  = 16.2 cm)
Vehicle speed v = 60 mph = m/s
If the vehicle is moving directly towards the transmitter
If the vehicle is moving perpendicular to the angle of arrival of the transmitted signal

14. Coherence Time Tc signal bandwidth symbol time fc-fd fc+fd
Time domain view
Frequency domain view
signal bandwidth
symbol time
fc+fd
fc-fd
Coherence Time: Time interval
over which channel impulse
responses are highly
correlated Tc

15. Doppler spread and coherence time
Doppler spread and coherence time (Tc) are inversely proportional
fm is the max doppler shift
For 0.5 correlation
Rule of thumb

16. Time varying nature of channel
Doppler spread and coherence time are parameters which
describe the time varying nature of the channel.
Time domain view
Freq domain view
signal 1
Signal
signal 2
Symbol Time (TS)
Signal bandwidth (BS)
channel 1
Channel
channel 2
channel 3
Coherence Time (TC)
Doppler spread (BD)

17. Small scale fading BS BC BS BC TS TC TS TC Flat fading
Multi path time delay
Frequency selective fading BC BS
fading
Fast fading TC TS
Doppler spread TC TS
Slow fading

18. PHY Layer Design Choices ?
Required Data Rates
Determines channel : frequency selective or flat fading; fast or slow fading
Required QoS at the PHY: bit-error-rate (BER), packet-error-rate (PER), Frame-error-rate (FER)
May be determined by application needs (higher layers)
Affected by Interference and Noise levels
PHY layer choices include selection of
Modulation/Demodulation
Techniques to mitigate fading: diversity, equalization, OFDM, MIMO
Techniques to mitigate interference (if necessary)
Error correction Coding