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)