1. Wireless Networks

2. Radio Environment Path loss Shadow Fading
Multipath interference
Limit the bit-rate and/or area coverage
8C Cimini-7/98

3. Path Loss There are many, complicated path loss models.
Here is a simple model for loss:
d = distance from TX to RX; f = frequency; K = a transmission constant
a is 2 for free-space; typically

4. Shadow Fading
The received signal is shadowed by obstructions, such as hills and buildings
Results in variations in local mean received signal power, typically:
Implications:
Non-uniform coverage
Increases required transmit power
8C Cimini-7/98

5. Multipath Leads to: Constructive and destructive
Received
Power
Delay Spread t
Leads to: Constructive and destructive
interference of arriving rays
dB With Respect
to RMS Value
0.5
0.5l
1.5
-30
-20
-10 10 1
t, in seconds 30 20
x, in wavelength
8C Cimini-7/98

6. Delay Spread (Time Domain)
Two-ray model
t = rms delay spread 2t
Delay
Received
Power
Channel Output
Channel Input T 2T 1 1 T 2T T 2T
8C Cimini-7/98

7. Mitigating ISI Two common techniques:
Direct Sequence Spread Spectrum (DSSS)
Orthogonal Frequency Division Multiplexing (OFDM)

8. Spread Spectrum
A technique to spread the transmitted signal power over a broad spectrum
Reduces power transmitted at any one frequency: reduces interference to others; makes detection/tapping difficult.
Less susceptible to interference at any one frequency: reduces interference from others; makes jamming difficult.
Two main types: DSSS (e.g. CDMA) and frequency-hopping.
Both techniques originally developed by the military.

9. DSSS Modulator Transmitter Channel Spreading (PN) Code Tc << T
Carrier
Recovery
Demod
Receiver
Data
(T)
Synch
Interference
Original
Data Signal
Narrowband
Filter
Other
SS Users
Demodulator
Filtering
ISI
Modulated
with Spreading
Input
8C Cimini-7/98

10. OFDM S Transmitter: x x Breaks data into N substreams
R/N bps x
R bps w1
Serial To
Parallel
Converter S
Modulate f1
R/N bps x wN
Modulate fN
Breaks data into N substreams
Each substream modulated onto different carrier
Slower bit-rate per substream means less fading.
Fading flat on each substream, so less variation.

11. Wireless Ethernet IEEE 802.11b
Ethernet frames
Aka:
IEEE b
Wireless LAN
WiFi
WaveLan
CSMA/CA MAC Protocol
DSSS
Physical Layer
Medium (air)

12. Wireless Ethernet Network
Access
Point
Access
Point
Router
Outside
world
Wired network
Uses 2.4GHz unlicensed spectrum.
Hosts take it in turns to send/receive packets at up to 11Mb/s.
Ad-hoc networks are possible too (without an Access Point).

13. Medium Access Control CSMA/CA
Problem: Collision detection is hard/impossible. Transmitters
don’t reliably know if there is a collision at the receiver.
“Hidden terminal”: Two or more senders might not receive from each other.
Signal strength might be weaker at the receiver due to loss, interference (from other transmitters, or from self, via multipath.
Sender 1
Sender 2
Hidden Terminal:
Metal partition
collision
Signal strength
Access
Point

14. Medium Access Control CSMA/CA
Instead of detecting collisions, CSMA/CA tries to avoid them. Transmitter:
Listens to see if medium is idle (“carrier sense”).
If idle, wait an additional random backoff time.
If line is still idle, transmit.
Wait for receiver acknowledgement.
Retransmit if necessary.

15. Medium Access Control CSMA/CA
Length field is sent, so other senders have hint about when
the medium will be idle.
Sender
Data
Ack
Receiver
Defer sending
Others

16. Medium Access Control CSMA/CA
Sender
RTS
CTS
Data
Ack
Receiver
Request to send: RTS
Clear to send: CTS
Defer sending
Others
Collision period is just time of RTS/CTS
RTS/CTS is optional in b

17. Wireless Ethernet DSSS Physical Layer1 1
Data: 10100
Pseudo-random sequence:
XOR:
Direct Sequence Spread Spectrum:
Data is XOR-ed with a pseudo-random n-bit “Chip” (or chipping code).
Spreads the spectrum by a factor of n.
All transmitters and receivers use same chipping code.
(In CDMA, multiple transmitters and receivers talk simultaneously using different chipping codes).
To other receivers, signal looks like low-level white noise.

18. Wireless LANs
802.11b
Standard for 2.4GHz ISM band (80 MHz)
DSSS, 1.6 Mbps, 500 ft range
Star or peer-to-peer architecture
802.11a
Standard for 5GHz NII band (300 MHz)
OFDM with time division
20-70 Mbps (adapt. modulation/coding), variable range
Aloha access, Peer-to-peer architecture
802.11g
Same as a but in the 2.4 GHz ISM band
802.11n, e
Standards being developed to include MIMO or QoS

19. Other Wireless Networks
3G Cellular mobile
Uses CDMA; 2Mb/s indoor, 384kb/s outdoor.
Technology developed and deployed.
Spectrum auctioned at over $1,000 per subscriber.
Some think this will make it too expensive.
Bluetooth
For short-range communication within a small system.
E.g.: headset to cell-phone; computer to printer.
Uses 2.45GHz unlicensed spectrum and frequency-hopping.
Up to 721kb/s data rate and about 10m.

20. WiMax Emerging standard for long-range wireless LANS.
Utilizes multiple antennas to get high data rates
40 Mbps fixed, 15 Mbps mobile
Projected range of several kilometers
Fixed standard finalized and being certified
Could be a competitor to cellular
Intel heavily invested.

21. Ad-Hoc Networks Peer-to-peer communications.
No backbone infrastructure.
Routing can be multihop.
Topology is dynamic.
-No backbone: nodes must self-configure into a network.
-In principle all nodes can communicate with all other nodes, but multihop routing can reduce the interference associated with direct transmission.
-Topology dynamic since nodes move around and link characteristics change.
-Applications: appliances and entertainment units in the home, community networks that bypass the Internet. Military networks for robust flexible easily-deployed network (every soldier is a node).