1. Wireless Communications

2. Wireless Media
Radio Waves for multicast communications - radio, TV and paging systems
Microwaves for unicast communication - cellular telephones, satellite networks, and wireless LANs
Infrared - short-range communication in a closed area using line-of-sight propagation

3. Electromagnetic Spectrum
Short wave – high frequency (IR, light), no licensing, can’t penetrate walls or curve round obstacles
Microwave – directional, line of sight, licensing needed
Radio – long wave, low frequency, licensing needed; can curve round earth surface, penetrate water, are omnidirectional

4. Bands – spectral radio frequencies
Range
Propagation
Application
VLF
3–30 KHz
Ground
Long-range radio navigation LF
30–300 KHz
Radio beacons and navigational locators MF
300 KHz–3 MHz
Sky
AM radio HF
3–30 MHz
Citizens band (CB), ship/aircraft communication
VHF
30–300 MHz
Sky and line-of-sight
VHF TV, FM radio
UHF
300 MHz–3 GHz
Line-of-sight
UHF TV, cellular phones, paging, satellite
SHF
3–30 GHz
Satellite communication
EHF
30–300 GHz
(Super)
(Extremely)

5. Wavelength and Frequency
Speed of light c = 3 x 108 m/sec =
Wavelength in metres = λ
Frequency in Hz = ν
c = λν
Example: microwave 2.5 GHz, or 2,500,000,000 Hz
Wavelength = 300,000,000 / 2,500,000,000 = 0.12 m

6. Propagation methods

7. Omnidirectional antennas - RW

8. Unidirectional antennas - MW

9. Physical transmission
Attenuation with distance
Blocking, reflection
Refraction – atmosphere, water
Scattering, diffraction – small obstacles
Multipath propagation – multiple signals, intersymbol interference
Doppler shift – moving transmitter/receiver

10. Multiplexing & Modulation
Space, e.g. cellular systems
TDM, FDM - widely used
Code Division Multiplexing (CDMA)
Military use originally
Different channels are assigned different codes
Needs precise control of transmission power and code assignments
An analogy to the problem of multiple access is a room (channel) in which people communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different directions (spatial division). In CDMA they would speak different languages.
Modulation
frequency, amplitude & phase

11. Advanced encoding, modulation and multiplexing schemes
MCM – multicarrier modulation. Data is split into several components, and each is transmitted over separate carrier signals.
OFDM – orthogonal (independent / non-overlapping) FDM, with PSK & QAM
FDM: (frequency division multiplexing) several signals combined on single communications channel. Each is assigned a different frequency (subchannel) within the main channel.
In OFDM priority is given to minimizing the crosstalk, among the channels.
PSK: the phase of a signal is varied to convey information. The simplest technique is binary phase-shift keying (BPSK): codes bits using 2 phases (0/180 degrees).
QAM: (quadrature amplitude modulation) - a method of combining two amplitude modulated signals into single channel; doubling effective bandwidth. QAM signal has 2 carriers, each have same frequency but phase differs by 90 degrees (1/4 cycle, i.e. ‘quadrature’).

12. Advanced encoding, modulation and multiplexing schemes (Continued)
COFDM – coded OFDM - mod scheme that divides single digital signal across 1,000 or more signal carriers. The signals are sent in frequency ranges that do not overlap (orthogonal) so they do not interfere with each other.
Spread Spectrum (to overcome localised interference) – Direct sequence (DSSS) and Frequency hopping (FHSS) – used with CDMA
FHSS divides 2.4 GHz band into 75 1-MHz channels. Transmitters and receivers hop rapidly around these channels at an-agreed-upon interval to avoid collisions. FHSS operates at a maximum rate of 2 Mbps (depends on coding technique).
DSSS divides 2.4 GHz band into overlapping channels, each 22 MHz wide. Instead of frequency-hopping, DSSS uses a technique called "chipping“- spreads modulated data across the spectrum in a fashion that makes it possible to tolerate some signal loss. A DSSS radio chipping with the ‘Barker Code’ modulates data using BPSK or quadrature phase shift keying, operating at 1 or 2 Mbps, respectively.

13. Cellular Telephony Connect mobile & stationary phones
Find/track the mobile, assign to base station
Handoff between base stations
Cells of mile radius, depending on area
Transmission power must be low enough to avoid interference
Different frequency sets in adjacent cells

14. Cellular system

15. Frequency reuse patterns

16. Basic Operation
Sending – via BS to MSC, for connection and frequency allocation for the call
Receiving – mobile paged by BS, and assigned a channel frequency
Handoff – one BS to another, hard or soft
Soft: BS coverage overlaps so that every cell phone set is always well within range of at least one BS. In some cases, mobile sets communicate with more than one BS at a time.
Hard: Link to the prior BS terminated before user transferred to new cell’s BS. The mobile is linked to no more than one BS at a time.
Roaming – between ISPs, according to contract

17. Evolution
Analogue AMPS (advanced mobile phone system) 1983 onwards, 2.4kb/s. As of February 18, 2008, carriers in USA are no longer required to support it.
Second Generation: D-AMPS, GSM (1991 on), IS-95, 64kb/s.
►D-AMPS (digital version of AMPS) adds TDMA to AMPS to get three channels for each AMPS channel, tripling the number of calls that can be handled on a channel.
►GSM (Global System for Mobile communication) uses a variation of TDMA. GSM digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot.
►Interim Standard 95 (IS-95) - a CDMA-based 2G mobile telecommunications standard.
Third generation: International Mobile Telecommunications-2000 (IMT-2000) - the global standard for 3G wireless comms, 2mb/s. IMT-2000 provides a framework for worldwide wireless access by linking the diverse systems of terrestrial and/or satellite based networks.
Fourth generation: 4G LTE (Long Term Evolution), True Mobile broadband, designed primarily for data. IP-based protocols. 100mb/s. LTE services launched by North American carriers (with Samsung SCH-r900 being first LTE Mobile phone) September, 2010.

18. Satellite Networks

19. Satellites - features Microwave, line of sight; GHz range
Footprint – area of coverage (GEO can cover 40% of earth’s surface).
Van Allen belt – charged particles – avoid!
Uplink and downlink – different frequencies

20. Satellite orbit altitudes
Clarke belt
(in honour of
Arthur.C.Clarke)
Equatorial
orbit
Typically
polar
orbit
Latency (RTT) LEO ms
MEO ms
GEO 238 ms at equator (nearly ¼ second)
278 ms at edge of coverage (over ¼ second)

21. Satellite frequency bands
Downlink, GHz
Uplink, GHz
Bandwidth, MHz L
1.5
1.6 15 S
1.9
2.2 70 C 4 6
500 Ku 11 14 Ka 20 30
3500
For more details, see:

22. Satellites in geosynchronous orbit Telephony, broadcast TV, Internet backbone

23. MEOs – used for GPS 18000 km altitude 24 to cover the earth
6 hrs to orbit
GPS based on ‘triangulation’ – need distance to (ideally) 4 satellites (can work with two in some conditions, and if you know you are on the earth’s surface then three suffice).
Used widely by all sorts of users

24. GPS – Global Positioning System

25. LEOS ISL Inter Satellite Link GWL – Gateway Link
UML – User Mobile Link

26. LEO – global telephony Polar orbits, 500-2000 km 90-120 min to orbit
20000 – km/hr
8000 km diameter footprint
System of satellites = network of switches
Little Leos - < 1GHz, low data rate messaging
Big Leos (1-3 GHz) – Globalstar, Iridium
Broadband Leos (like fibre) - Teledesic

27. Iridium constellation 1998
66 satellites, six orbits
altitude 750 km.
Global voice, data, fax, paging, navigation
kbps,
transmission at 1.6 G, ISL 23 G spectrum
66 x 48 spot beams
2000 in operation to cover the earth

28. Globalstar 48 Satellites, 6 orbits 1400 km
Relaying uses earth stations as well as satellites.
Ground stations can create stronger signals

29. Teledesic (Planned for 2005, but too ambitious, construction halted in 2002)
288 satellites, 12 polar orbits,1350 km
F/O like BB channels – Internet in the sky
8 satellites form a unit, earth stations are also used
Earth  several 10k’s cells, each assigned a time slot to transmit
User terminals communicate directly
155 M/1.2G up/down links – Ka band