Monday, 23 December 2013

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
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
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
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
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
Adapt sequence to avoid crowded frequencies (Bluetooth)


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