ECE/TCOM 590

Introduction to Wireless Systems

January 22, 2004

Course Objective

As stated in the introduction to the text:

Study of the operation (to a less extent the design) of wireless systems from the perspective of the radio frequency (RF) or microwave subsystems

Ref.: Rosengrant, Intro. To Telecommunications, 2002

Ref.: Rosengrant, Intro. To Telecommunications, 2002

Ref.: Rosengrant, Intro. To Telecommunications, 2002

Ref.: Rosengrant, Intro. To Telecommunications, 2002

Intro. To Wireless Systems

• 1980 survey by ATT re. Cellular phone use:– <900,000 users by 2000, but– in reality > 60,000,000 in 1998!

• Rapid growth in other wireless technologies:– Direct Broadcast Satellite (DBS)– Wireless Local Area Networks (WLAN)– Global Positioning Satellites (GPS)– Radio Frequency Identification Systems (RFID)

Wireless Communications Options

• Sonic or ultrasonic - low data rates, poor immunity to interference

• Infrared - moderate data rates, but easily blocked by obstructions (use for TV remotes)

• Optical - high data rates, but easily obstructed, requiring line-of-sight

• RF or Microwave systems - wide bandwidth, reasonable propagation

Wireless System Types• Point-to-point

– single transmitter to single receiver– high gain antennas in fixed locations– dedicated data communications

• Point-to-multipoint– connect central station to large number of receivers– AM & FM radio; pagers

• Multipoint-to-multipoint– simultaneous communications between mobile stations– use a grid of base stations, cellular telephones

Wireless System Types

• Simplex systems– one direction communication– radio & TV

• Half-Duplex– two directions, but not simultaneously– “push to talk” - citizen band radios

• Full-Duplex– simultaneous two-way communications– cellular phones using frequency division (separate

bands) or time (separate time) division multiplexing

Cellular Telephone Systems (1)• Division of geographical area into non-

overlapping hexagonal cells, where each has a receiving and transmitting station

• Adjacent cells assigned different sets of channel frequencies, frequencies can be reused if at least one cell away

• Generally use circuit-switched public telephone networks to transfer calls between users

Cellular Telephone Systems (2)• Initially all used analog FM modulation and

divided their allocated frequency bands into several hundred channels, Advanced Mobile Phone Service (AMPS) – both transmit and receive bands have 832, 25

kHz wide bands. [824-849 MHz and 869-894 MHz] using full duplex (with frequency division)

• 2nd generation uses digital or Personal Communication Systems (PCS)

Satellite systems• Large number of users over wide areas

• Geosynchronous orbit (36,000 km above earth)– fixed position relative to the earth– TV and data communications

• Low-earth orbit (500-2000 km)– reduce time-delay of signals– reduce the need for large signal strength– requires more satellites

• Very expensive to maintain & often needs line-of sight

Global Positioning Satellite System (GPS)

• 24 satellites in a medium earth orbit (20km)

• Operates at two bands, L1 at 1575.42 and L2 at 1227.60 MHz , transmitting spread spectrum signals with binary phase shift keying.

• Accurate to better that 100 ft and with differential GPS (with a correcting known base station), better than 10 cm.

Direct Broadcast Satellite

• TV service from two geosynchronous satellites with relatively small 18” diameter antenna

• Uses digital modulation technique – quadrature phase shift keying (QPSK)

• Two satellites, DBS-1 and DBS-2 each provide 16 channels with 120 W radiated power per channel.

Wireless Local Area Networks

• Use of Bluetooth standard - use of small, inexpensive RF transceivers to link a wide variety digital systems over a small distance.

• Operate at 2.45 GHz with max. bit rates of 1 to 2 Mbps.

• Wireless Local loop – small area telephone service – 50 – 100 feet cell size

Frequency choices• availability of spectrum

• noise (increases sharply at freq. below 100 MHz and above 10 GHz)

• antenna gain (increases with freq.)

• bandwidth (max. data rate so higher freq. gives smaller fractional bandwidth)

• transmitter efficiency (decreases with freq.)

• propagation effects (higher freq, line-of sight)

Multiple Access• Frequency Division Multiple Access

(FDMA) [used by AMPS, dividing 25 MHz receive and transmit bands each into 833 channels of 30 kHz bandwidth.

• Time Division Multiple Access (TDMA)

• Code Division Multiple Access (CDMA) - spread spectrum technique, relatively narrowband signal from each user is spread out in frequency using a unique spreading code

Propagation

• Free space power density decreases by 1/R2

• Atmospheric Attenuation

• Reflections with multiple propagation paths cause fading that reduces effective range, data rates and reliability and quality of service

• Techniques to reduce the effects of fading are expensive and complex

Antennas• RF to an electromagnetic wave or the inverse

• Radiation pattern - signal strength as a function of position around the antenna

• Directivity - measure of directionality

• Relationship between frequency, gain, and size of antenna, = c/f– size decreases with frequency– gain proportional to its cross-sectional area \ 2

– phased (or adaptive) array - change direction of beam electronically

Filters

• Low (High) Pass - Rejects signals above (below) a cutoff frequency

• Bandpass - passes frequencies within a pass band

• Insertion loss: attenuation of signals through the passband

• Out-of-band attenuation rate

Amplifiers

• Low-Noise Amplifiers - used in the input stages

• Power amplifiers - used in output stages

• IF amplifiers - used in the IF stages

• Specifications– power gain (dB)– noise figure – intercept points– harmonic generation– saturation

Mixers

• Three Port to produce the sum and difference frequencies from two sinusoidal inputs

• Hence frequency conversion as used in superheterodyne transmitters and receivers

• Typically use nonlinear devices such as diodes or transistors

• Mixer conversion loss (present with diodes)

• LO to RF isolation (particularly important if no low-noise amplifier on the input is present)

Ref: text by Pozar

Oscillators• Provide frequency conversion and

sinusoidal sources for modulation

• Oscillators must always be accurate (few parts per million) and often tunable

• Types– transistor with tunable L-C network (simple,

inexpensive, but not always stable)– quartz crystal (very accurate, not tunable)– phase-locked loops (accurate and tunable

outputs) also called frequency synthesizers

Baseband Processing

• Coherent digital modulation methods– LO synchronized in both frequency and phase

with down-converted carrier signal: carrier synchronization

– use of digital signal processing chips– requires A-D for transmission and D-A for

reception