Wirless Networks

Sharing the channel: TDMA

Time Division Multiple Access

: Share same carrier frequency

by dividing signal into different time slots (used in GSM cell

phone networks)

 

Sharing the channel: FDMA, SDMA

 

Frequency Division Multiple Access: Divide

frequency channel into subbands

–

Different users assigned to different subbands

–

Each subband has its own carrier

–

Use different frequencies for transmit and receive

(frequency-division duplexing). Eg, using FDD, adjacent

cell towers don’t “hear” each other.

–

TDMA/FDMA/FDD used in GSM cell phone networks

 

Space Division Multiple Access: adjust antenna

radiation pattern (eg, using phased arrays)

depending on location of the user

–

Focus transmitter power in required direction

–

On receive, eliminate noise from other sources

 

Sharing the channel: CDMA

•

Two vectors are orthogonal if their dot products are 0.

Here’s a set of 4 mutually orthogonal vectors:

–

V1: (1, 1, 1, 1)

V2: (1, 1, -1, -1)

V3: (1, -1, 1, -1)

V4: (1, -1, -1, 1)

•

Assign each transmitter a particular one of the orthogonal

vectors (Vi) to use to encode its transmissions (called the

“chip code”). With vectors shown above we can support 4

transmitters.

–

If message bit is 0, transmit –Vi

–

If message bit is 1, transmit Vi

•

Channel will sum the transmitted values:

–

send 00 using V1: -1 -1 -1 -1 -1 -1 -1 -1

send 01 using V2: -1 -1 1 1 1 1 -1 -1

send 11 using V3: 1 -1 1 -1 1 -1 1 -1

send 10 using V4: 1 -1 -1 1 -1 1 1 -1

channel: 0 -4 0 0 0 0 0 -4

 

CDMA Receiver

At receiver take groups of len(V) bits and form

dot product with Vi for desired channel.

–

If result is negative, message bit is 0

–

If result is positive, message bit is 1

channel: 0 -4 0 0 0 0 0 -4

receive using V1: 1 1 1 1 1 1 1 1

dot product:

-4 -4

message bits:

0 0

receive using V2: 1 1 -1 -1 1 1 -1 -1

dot product:

-4 4

message bits:

0 1

receive using V4: 1 -1 -1 1 1 -1 -1 1

dot product:

4 -4

message bits:

1 0

 

 

Asynchronous CDMA:

 

Use N orthogonal vectors to multiplex N transmitters (e.g.,

use a NxN Walsh Matrix)

•

Scheme described above works for

synchronous CDMA

when

all symbols are transmitted starting at same moment. For

example this works fine for a cell tower transmitting to

mobile phones.

•

But hard to synchronize mobile phone transmissions, so use

asynchronous CDMA

:

–

Can’t create transmissions that are truly orthogonal if they

start at different times

–

Approximate orthogonality with longer uncorrelated pseudo-

random sequences (called pseudo-noise or PN). “pseudo” implies

that sequence can be reconstructed at receiver given a known

starting point.

–

Assuming equal signal strengths from each transmitter at

receiver, if we decode bits

using a particular PN sequence

synchronized with desired transm

itter, we’ll get desired signal

plus some uncorrelated noise from other transmitters.

 

Sharing the channel: CSMA

 

Carrier Sense Multiple Access

–

“Carrier Sense” – transmitter listens to ensure no other carrier

is present on channel before transmitting

–

“Multiple Access” – multiple transmitters share the channel,

transmissions received by everyone (not quite true for wireless)

–

If two transmitters start at the same time (both having

detected no other carrier), their

transmissions collide and entire

packet is lost (looks like other

errors that cause packet to be

discarded). Recover via retransmission.

•

CSMA with collision avoidance (CSMA/CA)

–

Transmitter informs others of intent to transmit (costs

bandwidth); collisions still

possible as in pure CSMA

–

Used by 802.11, 802.15

•

CSMA with collision detection (CSMA/CD)

–

Transmitter detects collision, stops, and retries after random

interval.

–

Used by original Ethernet, doesn’

t work with radio where range

effects cause some receivers not to hear certain transmitters

 

Transmitting Information: OFDM

 

Orthogonal Frequency Division Multiplexing:

•

Bit stream split into lower-bandwidth parallel bit streams

each transmitted on a separate channel

–

Symbol times long relative to propagation time

–

Add guard interval to reduce in

ter-symbol interference (multi-

path echoes die away during guard interval)

•

Channels are orthogonal

–

No cross-talk

–

Don’t need filters for each subchannel

–

Don’t need inter-carrier guard bands

•

Robust, makes good use of channel capacity

–

ADSL,VDSL (telephone line data transmission)

–

802.11a, 802.11g, 802.16, 802.15.3 (ultra wide band)

–

Terrestrial Digital TV

–

Power line networking

 

Transmitting Information: Spread Spectrum

Direct Sequence Spread Spectrum (DSSS)

–

Used in asynchronous CDMA where each “chip” from

pseudo-random “chip vector” is used to change phase of

transmission.

–

Chip rate is many times higher than message rate (i.e.,

we transmit many chips per message symbol), so message

is spread out over large spectrum

•

Highest freq is twice the chip rate

–

Longer PN sequences and higher chip rate increase

rejection of uncorrelated transmissions (“process gain”)

–

Used in 802.11b, CDMA phones

•

Frequency-Hopping Spread Spectrum (FHSS)

–

Like DSSS except adjust carrier frequency instead of

phase.

–

Adapt sequence to avoid crowded frequencies (Bluetooth)