Wireless Multiple Access

Digital Radio

Started by providing voice services, successor to analog radio telephone systems.

Digital radio has advantages over analog systems, due to the advantages of digital versus analog signals

integration of voice, data, fax, paging, messaging etc possible (the same as ISDN)
less bandwidth per voice channel needed by using digital compression algorithms

GSM

GSM operates at 900MHz and 1800MHz. The spec for GSM is 5000 pages long. (!)

200 full duplex channels per cell maximum. Each channel is composed of a downlink and an uplink frequency with 200kHz per link. Each channel is divided by TDM into 8 slots for individual connections.

TDM scheme

Each slot is composed of a 148 bit data frame. Frames are marked with a starting/ending pattern of three 0 bits.

Each frame has 2, 57 bit data payload fields (information fields). Control bits for these fields indicate whether the information is voice of data.

Each frame carries a 26 bit synch field for the receiver to establish synch with the sender.

Each data frame is 547 usec, for a gross signalling data rate of 270,567 bps.

Each TDM frame has 8 data frames (8 slots) plus some wasted guard time between slots. The net data rate (not counting overhead, synch, guard time) is 9600 bps per connection (sufficient for compressed voice).

26 TDM frames make up a multiframe. Two of these slots are used for control.

Control

Another control channel, the common control channel, is used to for paging (announcing incoming calls), random access (phones request dedicated slot on control channel), acknowledgment (granting a control channel slot to a phone).

The random access channel uses a slotted ALOHA scheme for access control.

GSM PCS 1900 systems (the GSM Standard for 1900 MHz)

UMTS

http://www.imt-2000.com/wcdma/index.htm

Wideband CDMA (WCDMA) technology in the 2GHz region to provide 2Mbps connection to mobile. This is seen as the 3rd generation (GSM being the 2nd) of digital wireless technology.

Builds on to GSM for some flexibility of new and old service, preservation of infrastructure.

GSM will max out at 384kbps. Fixed or slow moving WCDMA will go to 2Mbps.

Selected in Jan 98 by some European standards body as technology to implement UMTS (Universal Mobile Telecommunications System)

Wireless LANS

http://www.mouse.demon.nl/ckp/lanwan/iee80211.htm
http://www.wlana.com/index.html
http://www.breezecom.com/80211.html

One way to look at wireless LANS is as celluar systems with low-powered base stations in each room. Then a building has multiple cells and allows mobility and wireless connections for portable computing devices.

One difference is that there is only a single channel in each cell (like a LAN) rather than subdivided multiple channels.

CSMA doesn't work because the collisions may take place at the receiver (the base station) and not just the transmitter. This happens because of the short ranges involved. A and B may both be close enough to the same basestation to transmit to it, but not close enough to each other to sense collisions. The requirement that everyone has knowledge of the state of the channel simultaneously (modulo the collision window) isn't met due to limited range.

Multiple Accesss Collision Avoidance

Used in wireless LANs, where cells with base stations are setup to receive transmissions and relay them through a cable to another base station for transmission to the destination. The cells are small so that one covers a single room, and so that they don't overlap. Each cell uses only a single channel, which is what makes this more like a LAN than a cellular phone system.

MACA is used in the IEEE 802.11 wireless LAN standard. Stations first broadcast a short request to send (RTS) to the base station, which is thus stimulated into sending an equally short clear to send (CTS) broadcast. The CTS alerts all stations within range of the base station that the channel is taken. The length of the data frame is included in the RTS/CTS so nearby stations know how long the channel will be busy for. After receiving the CTS the requesting station transmits the data frame.

Stations hearing the CTS (close to the receiving base station) must be silent for the duration of the data frame. Stations hearing the RTS (close to the sender) need only be silent until the CTS is received (which they don't hear, but can deduce how long to wait). During the subsequent data frame such a station is free to broadcast, since it's energy won't reach the receiver.

D hears CTS from B, but not RTS from A.
D must be silent until the data frame is done (which is why the CTS/RTS contain the data frame length).

Since collisions are still possible, just like with CSMAs collision window, a scheme for detection (no received CTS) and re-transmission is used. The random delay uses the same exponential binary backoff algorithm as in 802.3 to de-synchronize colliding stations.

Studies and simulation of MACA revealed some potential for improvements. These resulted in MACAW

The window of collision in MACA is the time during which an RTS is being sent. Adding CSMA improved performance since two stations wouldn't collide on sending RTSs as frequently.

IEEE 802.11 Wireless Ethernet

References

"IEEE 802.11 Tutorial", Jim Zyren and Al Petrick, www.wirelessethernet.org/

"Anatomy of IEEE 802.11b Wireless", Joel Conover, August 7, 2000, www.networkcomputing.com/

Status

First adopted June 1997 with slower data rates. 1999 saw commercial interest explode with standardization of 11 Mbps.

802.11 working group task forces currently (2002) working on 54 Mbps standard.

Architecture

Ad-hoc networks

Basic Service Set (BSS) consists of a set of stations communicating in a peer-to-peer fashion. Infrastructure

All communication is through the access point.

The AP bridges wireless nodes into a wired distribution medium.

Physical layer

Finding available, unlicensed operation spectrum suitable for high speed communication and available all over the world was tricky.

The Industrial Scientific Medical band at 2.4GHz filled the bill.

80 MHz is available in this band.

Spread spectrum multiplexing was selected to avoid interference with other users of the ISM band. The maximum power transmission is 1 watt.

Lower speeds have standards for both frequency hop and direct-sequence spread spectrum techniques. The 11 Mbps standard only uses direct sequence.

Direct-sequence Spread Spectrum (DSSS)

The data stream is modulated with an 11 bit Barker code (11 chips/bit) to spread the energy, leaving the same total power but a much lower peak power. Figure from Zyren and Petrick.

A pseudo random number sequence at 11 Mchips/sec is XORd with the data. The receiver needs to use the same sequence to pull the signal out of the noise. Figure from Zyren and Petrick.

The 11M standard uses more sophisticated encoding technique with the same basic modulation technique to encode more bits per symbol.

Complementary Code Keying (Lucent & Harris, 1988)

64 unique symbols in the code, hence 6 bits/symbol

modulated on top of the QPSK 2 Mbps standard results in additional 6 bits per symbol

QPSK: 2 bits/symbol * 1 M symbols/sec * 11 chips/bit = 22 MHz

receivers need 64 different correlators, one for each possible code

22 MHz required regardless of data rate, so ISM band will accomodate 3 non-overlapping channels

Data Link layer

802.11 uses a CSMA/CA strategy known as DCF (Distributed Coordination Function). Since collisions are still possible and go undetected, the transport layer (TCP, say) could determine the loss of packet and re-transmit. However, this isn't efficient for noisy channels so an ACK frame is used in the data link layer.

ACKs have higher priority and can be transmitted immediately without any delay, unlike the next data frame.

Collision detection would require more expensive analog radio circuitry, increasing power consumption.

Transmitters sense the channel for some time, if the channel is free, they transmit. If the channel is not free, they wait for a DIFS (DCS interframe space), then wait for a random backoff time. When the random delay is complete they can transmit. If while waiting they hear another station transmit they freeze their timer and resume it when the channel is idle again. Figure from Zyren and Petrick.

Note how different this is than CSMA/CD. The backoff happens before collision, in the hopes of avoiding it.

In situations with higher probability of collisions, such as multiple overlapping BSSs and many stations, an RTS/CTS scheme can be used to decrease collisions and solve the "hidden node" problem.

Addressing

802.11 uses standard 802.3 48 bit addresses.

Timing

All stations within a BSS are synchronized to within 4 us, plus propagation delay. Time stamped beacons sent periodically by AP to achieve this.

Frame format

The physical frame format is shown below.

The 144 bit preamble is used to synchronize radio receiver circuits, determine radio gain, and determine whether the channel is clear.

The preamble ends with a 16 bit delimiter field that marks the start of the actual data frame.

The PLCP header has four fields:

For compatibility reasons, the PLCP header is always transmitted at 1 Mbps.

Note the heavy overhead (24 bytes) when compared to wired Ethernet (8 byte preamble), and the fact that even with faster data rates, this header is always transmitted slowly.

This frame overhead doesn't even count the addressing, checksums and other link layer overhead.

The payload shown in the figure above consists of the 802.11 data frame. The format of that frame, along with the size of the fields, is given below:

Frame control, 2 bytes Duration, 2 bytes Dest address, 6 bytes Source address, 6 bytes BSSID, 6 bytes Sequence #, 2 bytes Data payload, 0-2313 bytes Checksum, 4 bytes The total overhead of a 100 byte data payload frame is therefore

Power management

Two modes:

Roaming

Left up to network equipment vendors, i.e. not in the standard. Client probes for APs, client makes decision based on signal strength following a "make, then break" strategy.