physical layer overview

1. Physical Layer Overview

2. Physical-Layer Architecture
• The physical layer also incorporates a clear channel assessment (CCA) function
to indicate to the MAC when a signal is detected

3. The Radio Link
• Based on physical medium, three physical layers were
standardized in 802.11 :
1. Frequency-hopping (FH) spread-spectrum radio PHY
2. Direct-sequence (DS) spread-spectrum radio PHY
3. Infrared light (IR) PHY
• 3 further physical layers based on radio technology
1. 802.11a: Orthogonal Frequency Division Multiplexing (OFDM) PHY
2. 802.11b: High-Rate Direct Sequence (HR/DS or HR/DSSS) PHY
3. 802.11g: Extended Rate PHY (ERP)
4. The future 802.11n, which is colloquially called the MIMO PHY or
the High-Throughput PHY

4. The Radio Link - Two frequency bands:
• What is a frequency band?
• Two categories of wireless frequency bands:
1. Licensed frequency bands
2. Unlicensed frequency bands

5. The Radio Link – Licensed frequency bands:
What are licensed frequency bands ?
• licensed frequency bands:
• Users are primary users or narrowband receivers
• Licenses can restrict
i. frequencies
ii. transmission power used ,
iii. area over which radio signals can be transmitted
• E.g. radio broadcast stations must have a license
• Intentional interference may be subject to criminal or civil
penalties

6. The Radio Link - unlicensed frequency bands:
• What are unlicensed frequency bands ?
• No license , users are secondary users
• These bands are commonly referred to as the ISM bands -
industrial, scientific, and medical equipment
• The 2.4-GHz band is available worldwide for use
• E.g. microwave ovens operate at 2.45 GHz
• Building, manufacturing, and designing 802.11 equipment
does require a license

7. The Radio Link - unlicensed frequency bands:
Unlicensed does not mean it doesn’t have any rules and
regulations to be followed.
• unlicensed devices must do is obey limitations on
transmitted power.
• Interference with any device (licensed or Unlicensed) must
be avoided by using spread-spectrum technologies

8. Spread Spectrum
• This technology is a requirement for unlicensed devices.
• Spreading the transmission over a wide band which tries to
eliminate the interference problems with other devices
• But doesn't make the problem go away : As more RF devices
occupy the area that your wireless network covers, you'll see
the noise level go up.

9. Types of spread spectrum
1. Frequency hopping (FH or FHSS)
• FH systems jump from one frequency to another in a
random pattern, transmitting a short burst at each
subchannel
2. Direct sequence (DS or DSSS)
• spread the power out over a wider frequency band using
mathematical coding functions.
• Two direct-sequence layers were specified.
• a 2-Mbps PHY, and
• HR/DSSS PHY.

10. Types of spread spectrum
3. Orthogonal Frequency Division Multiplexing (OFDM)
• OFDM divides an available channel into several sub
channels and encodes a portion of the signal across each
sub channel in parallel.
• Frequency-hopping systems
• are the cheapest to make.
• Precise timing is needed to control the frequency hops,
• But sophisticated signal processing is not required

11. Types of spread spectrum
• Direct-sequence systems
• require more sophisticated signal processing,
• require more specialized hardware
• and higher electrical power consumption.
• But allows a higher data rate than frequency-hopping
systems.

12. RF Propagation with 802.11
• Radio signal on space is mixture of signal and noise.
• Main functionality of radio signal communication is making signal
intelligible over noise.
• Performance of the signal is measured in terms of signal-to-noise ratio
(SNR).
Signal Reception and Performance

13. RF Propagation with 802.11
Signal Reception and Performance

14. RF Propagation with 802.11
• Theoretically there is no limit for the amount of data carried by a radio
channel. But practically there is a limit for radio channel.
• The theorem expresses the mathematical limit of the capacity of a
communications channel. Also called as shannon capacity.
The Shannon limit
S/N = 2 ^ (C/W) - 1 (S/N as power ratio)
SNR = 10 * log10 (2 ^ (C/W) - 1) (SNR as dB)

15. RF Propagation with 802.11
• The theorem expresses the mathematical limit of the capacity of a
communications channel
The Shannon limit
Shannon limit as a function of SNR

16. RF Propagation with 802.11
• Range is the distance of MS from AP
• As range increases, the signal level drops, hence the throughput drops;
• with a constant noise floor, the degraded signal will result in a degraded
signal to noise ratio.
Path Loss, Range, and Throughput
Throughput versus distance
when the signal-to-noise ratio
gets too small to support a high
data rate, the station will fall
back to a lower data rate with
less demanding signal-to-noise
ratio requirements

17. RF Propagation with 802.11
• What is path loss ?
• The loss in free-space is sometimes called the path loss. It is
the minimum loss occurring when signal travels in a path.
Path loss (dB) = 32.5 + 20 log F + log d
• where the frequency F is expressed in GHz,
• and the distance d is expressed in meters
• . Obstacles such as walls and windows will reduce the signal, and
antennas and amplifiers may be used to boost the signal, which
compensates for transmission losses.
Total loss = TX power + TX antenna gain - path loss - obstacle loss - link
margin + RX antenna gain

18. RF Propagation with 802.11
Multiple paths
Multipath Interference
• Waves spread outward from the transmitting antenna in all directions and are
reflected by surfaces in the area
• The wave at the receiver is the sum of all the different components
• multipath interference can be resolved by changing the orientation or position
of the receiver.

19. RF Propagation with 802.11
Multipath Interference
Wave combination by superposition
In (c) ,the two waves are almost exactly the
opposite of each other, so the net result is
almost nothing

20. RF Propagation with 802.11
Inter-Symbol Interference (ISI)
• Waves that take different paths from the transmitter to the receiver will travel
different distances and be delayed with respect to each other
• Once again, the two waves combine by superposition, but the effect is that the
total waveform is garbled.