Wireless Networks

Radio Environment Path loss Shadow Fading Multipath interference Limit the bit-rate and/or area coverage 8C32810.83-Cimini-7/98

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

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 8C32810.84-Cimini-7/98

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 8C32810.85-Cimini-7/98

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 8C32810.88-Cimini-7/98

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

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.

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 8C32810.117-Cimini-7/98

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.

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

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).

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

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.

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

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 802.11b

Wireless Ethernet DSSS Physical Layer 1 1 Data: 10100 Pseudo-random sequence: 10100001001100101000 XOR: 0101001110000101000 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.

802.11 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 802.11a but in the 2.4 GHz ISM band 802.11n, 802.11e Standards being developed to include MIMO or QoS

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.

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.

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).