Advanced Topics in Next- Generation Wireless Networks Qian Zhang Department of Computer Science HKUST Wireless Radio.

Advanced Topics in Next-Generation Wireless Networks

Qian ZhangDepartment of Computer Science

HKUST

Wireless Radio

Characteristics of Wireless Medium

• Comparison to wired media– Unreliable– Low bandwidth– Untethered: supports mobility– Broadcast nature– Shared medium– Capacity limitation

• Frequency of operation and legality of access differentiates a variety of alternatives for wireless networking

Frequencies for Communication

VLF = Very Low Frequency UHF = Ultra High FrequencyLF = Low Frequency SHF = Super High FrequencyMF = Medium Frequency EHF = Extra High FrequencyHF = High Frequency UV = Ultraviolet LightVHF = Very High Frequency

The Radio Spectrum(300Hz – 300GHz)

Frequencies for Mobile Communication

• VHF-/UHF-ranges for mobile radio– Simple, small antenna for cars– Deterministic propagation characteristics, reliable connections

• SHF and higher for directed radio links, satellite communication– Small antenna, focusing– Large bandwidth available

• Wireless LANs use frequencies in UHF to SHF spectrum– Some systems planned up to EHF– Limitations due to absorption by water and oxygen molecules

(resonance frequencies)• Weather dependent fading, signal loss caused by heavy

rainfall etc.

Licensed and Unlicensed Bands

• Licensed– Cellular/PCS– Expensive (PCS bands in US were sold for around $20B)– Time consuming to deploy new applications rapidly at low

costs

• Unlicensed– Industrial, Medical, and Scientific (ISM) Bands– Free, component costs are also low– New applications such as WLAN, Bluetooth are easily

developed

• With the increase in frequency and data rate, the hardware cost increases, and the ability to penetrate walls also decreases

Bandwidth Allocation

• In the U.S., the FCC is responsible for allocating radio frequencies

• Why allocate the radio spectrum?– Prevent interference between different devices

– It would be unfortunate if the local TV station interfered with police radio

• Generally, any transmitter is limited to a certain bandwidth– E.g., a single 802.11 channel is 30MHz “wide”

• FCC also regulates the power and placement of transmitters– Consumer devices generally limited to transmitting < 1W of

power

– Can’t have two TV stations on channel 5 next to each other

ISM Band

• Interference in unlicensed bands

• ISM: Industrial, Scientific, and Medical 2450 ± 50MHz

Radio Propagation

• Three most important radio propagation characteristics used in the design, analysis, and installation of wireless networks are:– Achievable signal coverage

– Maximum data rate that can be supported by the channel

– Rate of fluctuations in the channel

Signal

• Signal - physical representation of data– Function of time and location

– Signal parameters to represent the value of data• Frequency, amplitude, phase shift

• Noise– Thermal noise

– Other transmission (e.g., microwaves, cordless phones)

• SNR – Signal to Noise Ratio– Needs to be high enough for a receiver to correctly

receive the information

Signal Propagation Ranges

• Transmission range– Communication possible– Low error rate

• Detection range– Detection of the signal

possible– No communication possible

• Interference range– Signal may not be detected– Signal adds to the

background noise

sender

transmission

detection

interference distance

Radio Environment

Path Loss(clear, unobstructed LOS path)

Shadowing

Multi-path Fading

A. Path Loss Model

• Different, often complicated, models are used for different environments

• A simple model for path loss, L, is

where is the local mean received signal power

Pt is the transmitted power

d is the transmitter-receiver distance, f is frequency

K is a transmission constant (transmitter and receiver antennae gain)

The path loss exponent a = 2 in free space

rP

B. Shadow Fading

• Received signal is shadowed by obstructions– Such as hills and buildings

• The received signal strength for the same distance from the transmitter will be different– Depending on the environment and the surroundings,

and the location of objects

– This variation of signal strength due to location is referred to as shadow fading

• This results in variations in the local mean received signal power

C. Multipath Propagation

• Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction

Signal at Sender

Signal at Receiver

Reflection(surface of the earth, building,

wall)

Scattering (foliage,

street sign, lamp posts,

etc.)

Diffraction (shape edge,

towers, peak)

Multipath Fading

• Multipath fading – Fluctuations of the signal amplitude because of

the addition of signals arriving in different phases (paths)

• Multipath fading results in high BER– Can be mitigated by FEC, diversity schemes,

and using directional antennae

Effects of Mobility

• Channel characteristics change over time and location

• Radio propagation is very complex– Multipath scattering from nearby objects– Shadowing from dominant objects– Attenuation effects

• Results in rapid fluctuations of received power

Less variation the slower you move

What is an antenna?

• Is an electrical conductor used either for radiating or collecting electromagnetic (EM) energy– Antennas generally designed for a certain range of

frequencies– Lots of types...

Antenna Technology

• Omnidirectional antenna– Having an essentially nondirectional pattern in a given

plane

• Directional antenna– Having the property of radiating or receiving EM energy

more effectively in some directions other than others

• Smart antenna– An array of antenna elements connected to DSP

– Pros: enhance wireless link capacity using antenna diversity and interference suppression

– Cons: more expensive and standardization takes time