Advanced Topics in Next- Generation Wireless Networks Qian Zhang Department of Computer Science HKUST Wireless Radio.
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Advanced Topics in Next- Generation Wireless Networks Qian Zhang Department 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 Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = 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 P t 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
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 Propagation (Cont.) Time dispersion: signal is dispersed over time –Interference with “neighbor” symbols –Inter Symbol Interference (ISI) The signal reaches a receiver directly and phase shifted –Distorted signal depending on the phases of the different parts
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