Physical layer overview

Physical Layer Overview

Physical-Layer Architecture

• The physical layer also incorporates a clear channel assessment (CCA) function to indicate to the MAC when a signal is detected

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) PHY2. 802.11b: High-Rate Direct Sequence (HR/DS or HR/DSSS) PHY3. 802.11g: Extended Rate PHY (ERP)4. The future 802.11n, which is colloquially called the MIMO PHY or

the High-Throughput PHY

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

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

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

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

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.

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.


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


• 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.

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

RF Propagation with 802.11Signal Reception and Performance


• 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)


• The theorem expresses the mathematical limit of the capacity of a

communications channel

The Shannon limit

Shannon limit as a function of SNR


• 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


• 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


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.


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

RF Propagation with 802.11Inter-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.

Physical layer overview

Engineering

Transcript of Physical layer overview