References A. Goldsmith, Wireless Communications, Cambridge University Press, 2005. D. Tse and D. Vaswanth, Fundamentals of Wireless Communications, Cambridge University Press, 2005. T. Rappaport, Wireless Communications, Principles and Practice, 2nd Edition, Prentice Hall. J. Fayyaz, Radio Design of Cellular Networks, Naghoos Press, 2011.

Contents Background and Preview Wireless Propagation Channel Multiple Access Methods Cellular Systems Diversity Issues Information Transmission Capacity Multiple Antenna Technologies Cooperative Communications

Wired Vs. Wireless Communications Each cable is a different channel One media (cable) shared by all Signal attenuation is low High signal attenuation No interference High interference noise; co-channel interference; adjacent channel interference

Why Wireless? Advantages Limitations Sometimes it is impractical to lay cables User mobility Cost Limitations Bandwidth Fidelity Power Security

Wireless ≈ Waves Electromagnetic radiation Emitted by sinusoidal current running through a wire (transmitting antenna) Creates propagating sinusoidal magnetic and electric fields according to Maxwell’s equations: Fields induce current in receiving antenna.

Propagation Principle electric field propagation direction magnetic field

Propagation Mechanisms Line-of-Sight S D Non Line-of-Sight Reflection λ << D Diffraction λ  D Scattering λ >> D

Propagation in the “Real World” a wave can be absorbed penetrate LIGHT ANALOGIES (BENT STICK, LIGHT IN CANYON, BLACK PAINT, WINDOW) reflect bend

The Cluttered World of Radio Waves hills girders rain hallways windows vehicles trees walls

Electromagnetic Spectrum LF HF VHF UHF SHF EHF MF AM radio S/W radio FM radio TV cellular  902 – 928 Mhz 2.4 – 2.4835 Ghz 5.725 – 5.785 Ghz ISM band  30kHz 300kHz 3MHz 30MHz 300MHz 30GHz 300GHz 10km 1km 100m 10m 1m 10cm 1cm 1mm 3GHz X rays Gamma rays  infrared visible UV 1 kHz 1 MHz 1 GHz 1 THz 1 PHz 1 EHz Propagation characteristics are different in each frequency band.

Evaluating Frequencies 50 MHz-250 MHz: Good for outdoor range, large antenna size, bending and penetrating. No foliage problems. “Sees” metallic building structures, doesn’t pass through windows or down corridors. 450 MHz to 2 GHz: - Good compromise for cellular-type systems. Small antenna, but big enough for outdoor range. Minor foliage effects. OK for windows walls and corridors. 5-20 GHz: Antenna too small for outdoor range. Foliage and rain effects. Indoor microcells, Point-to-point, and Satellites to ground stations. hallways, leaves, walls, rain, absorption, diffraction (black), 50mhz good for range (bending, penetrating) but not for windows, hallways, and long antenna (2 meter) 450 would be good for range, windows, corridors, walls, and antenna (6 inches) 900 - 2 GHz still OK, range diminishing but short antenna good for portables (but 900 better) 5+ GHz not good for range, penetration, leaves and even rain we don’t really need the antenna to be smaller (in buildings?) must consider cost (technology, volume) and availability of spectrum

Unlicensed Radio Spectrum (ISM: Industrial, Science, Medicine)  33cm 12cm 5cm 26 Mhz 83.5 Mhz 125 Mhz 902 Mhz 2.4 Ghz 5.725 Ghz 928 Mhz 2.4835 Ghz 5.850 Ghz cordless phones baby monitors WaveLan 802.11b Bluetooth Microwave oven 802.11a

Free-space Path-loss Power of wireless transmission reduces with square of distance (due to surface area increase of sphere). Reduction also depends on wavelength: Long wave length (low frequency) has less loss Short wave length (high frequency) has more loss

Path-loss Models L(d) = L(d0)(d/d0)n Path-Loss Exponent Depends on environment: L(d) = L(d0)(d/d0)n Free space n = 2 Urban area cellular n = 2.7 to 3.5 Shadowed urban cell n = 3 to 5 In building LOS n = 1.6 to 1.8 Obstructed in building n = 4 to 6 Obstructed in factories n = 2 to 3

Multi-path Propagation Electromagnetic waves bounce off of conductive (metal) objects. Reflected waves received along with direct wave.

Multi-Path Effect Multi-path components are delayed depending on path length, causes delay spread. Phase shift causes frequency dependent constructive / destructive interference.

Multi-transmitter Interference Similar to multi-path Two transmitting stations will constructively/destructively interfere with each other at the receiver. Receiver will “hear” the sum of the two signals, which usually means garbage.

Modulation Modulation allows the wave to carry information by adjusting its properties (Amplitude, Frequency, and Phase) in a time varying way. Digital modulation using discrete “steps” so that information can be recovered well despite noise/interference. 8VSB - US HDTV BFSK - Mote Sensor Networks QPSK - 2 Mbps 802.11 & CMDA(IS-95)

Symbol Rate & Bandwidth Modulation allows transmission of one of several possible symbols (two or more). Data stream is encoded by transmitting several symbols in succession. Symbol rate ≈ Bandwidth Symbol Rate or Baud Rate (symbols/sec) Bit Rate or Throughput (bits/sec) Spectrum Usage or Bandwidth (Hz) Inter-symbol interference (ISI) occurs unless delay spread << symbol time.

Thermal Noise Ever-present thermal noise in wireless medium. Sums with any wireless transmission. Potentially causes errors in reception (digital) or degradation of quality (analog). Effectively limits transmission range when transmitting signal strength falls below a threshold.

Thermal Noise Calculation Noise power depends on channel bandwidth: Noise Power = -174dBm/Hz + 10log(BW in Hz) So for 802.11 BW = 25 MHz for 802.11b or 802.11a channel Thus noise power is about -100 dBm -100 dBm = 10-10 mW

Physical Channel Properties Review Received signal power depends on: Transmit power Loss over distance (falls off by dn) Shadowing (e.g. absorption by walls) Multi-path (e.g. bouncing off of metal objects) Noise power depends on: Thermal noise Environmental noise (e.g. microwave ovens) Channel Quality Related to Signal to Noise Ratio

Current Wireless Technologies Cellular Telephony (GSM, CDMA2000) Fixed Wireless Access (WiMax) Wireless Local Area Networks Local Networks (Bluetooth, UWB) Satellite Communications

Cellular Telephony (1)

Cellular Telephony (2)

Cellular Telephony (3) Data is bursty, whereas voice is continuous. 3G widens the data pipe 384Kbps to few Mbps Standard based of WCDMA Packet-based switching for both voice and data New generations HSDPA, HSPA+, LTE WiMAx added to 3G 4G systems start to come up Mostly based on OFDM

Fixed Wireless Networks (WiMax) Provide broadband wireless access to homes/offices in a few Km range. A potential replacement of ADSL Provide high speed Internet and VOD

Wireless Local Area Networks (1) WLANs connect local computers (100m to a few Km range) Breaks data into packets Channel is shared (random access) Backbone internet provides best-effort service Poor performance in some applications (e.g. video)

Wireless Local Area Networks (2) 802.11b (oldest) Standard for 2.4GHz ISM band (80MHz BW) Frequency hopped spread spectrum 1.6-10Mbps, few hundred meter range 802.11a (old) Standard for 5.7GHz NII band (300MHz BW) OFDM with time division 20-70Mbps, variable range 802.11g (newer) Standard for 2.4GHz and 5.7GHz bands OFDM Up to 54Mbps, few hundred meter range 802.11n (current) Up to 140Mbps Uses smart antenna technology

Ultra Wide-Band Also known as “Impulse Radio”. Use very high bandwidth to decrease power level. Hard synchronization No license problem, but seems to interfere with GPS Now mainly targeted to small distance applications. (home networks to replace Bluetooth) Smaller delay and longer range than Bluetooth Emerging as competitor for 802.11

Satellite Communications Cover very large areas. Different orbit heights GEO (36000 Km), MEO and LEO (<2000 Km) Optimized for one-way transmission Radio (DAB) and TV (DVB-S) broadcast Two-way systems Expensive alternative to terrestrial systems

Evolution of Current Technologies Link: Modulation, Coding, Adaptivity, Smart Antennas Network: Dynamic resource allocation, Mobility support Hardware: Better batteries, Circuits/Processors Applications: Soft and adaptive QoS (main issue in real-time multi-media services)

Moving toward Real Multimedia Voice Data Video Delay < 100 mSec - Packet loss < 1% <1% BER 10-3 10-6 Data Rate 8-32Kbps 1-100 Mbps 1-20 Mbps Traffic Continuous Bursty

Design Challenges Wireless channels are capacity-limited broadcast communication medium. Two main problems in wireless media: Fading Interference Traffic patterns, user locations, and network conditions are constantly changing. Energy and delay constraints change design principles across all layers of the protocol stack.

Emerging Systems Ad-hoc Wireless Networks Wireless Sensor Networks Distributed Control Networks Cooperative Networks Cognitive Radio Networks

Ad-hoc Wireless Networks (1)

Ad-hoc Wireless Networks (2) Peer-to-peer communications No backbone infrastructure Multi-hop routing Dynamic topology Fully connected with different link SNRs

Ad-hoc Wireless Networks (3) Ad-hoc Wireless Networks provide a flexible network infrastructure for any emerging application. The capacity of such a network is generally unknown. Transmission, access, and routing strategies are generally ad-hoc Energy constraints impose interesting design tradeoffs for communication and networking.

Wireless Sensor Networks (1) 40

Wireless Sensor Networks (2) Energy is the driving constraint. Nodes powered by non-rechargeable batteries. Data flows to central location. Low per-node rates but up to 100000 nodes. Data highly correlated in transmission. Nodes can cooperate in transmission, reception, compression, and signal processing.

Wireless Sensor Networks (3) (Energy-constrained nodes) Short-range networks must consider transmit, circuit, and processing energy. Sleep modes save energy but complicate networking. Changes everything about the network design: Optimization of bit allocation across all protocols Delay vs. throughput vs. node/network lifetime tradeoffs Optimization of nodes cooperation Efficient MAC layer communication and Scheduling

Distributed Control (1) Packet loss and/or delay impacts controller performance. Controller design should be robust to network faults. Joint application and communication network delay. Interesting ideas in packet-based communication and file transfer.

Distributed Control (2) There is no methodology to incorporate random delays or packet losses into control system design. The best rate/delay tradeoff for a communication system in distributed control can not be determined. Current autonomous vehicle platoon controllers are not string stable with any communication delay. What are the best routing technologies?

Cooperative Networks (1)

Cooperative Networks (2) Increase coverage area Reduce number of “blind spots” Reduce transmit power per node Increase in transceiver complexity More complex synchronization problems More interference to be handled properly Higher end-to-end delays Additional delay to be handled in real-time applications

Cognitive Radio (1) Available spectrum looks scarce. Measurements show the allocated spectrum is vastly underutilized.

Cognitive Radio (2)

Cognitive Radio (3) Sense, learn, and exploit the environment One “simple” option: Use “un-used” spectrum Agile Radios Give priority to ”primary” users But not receiving a signal in wireless environment does not mean that no signal is actually transmitted at that frequency! Even in simplest form is very challenging. Many ideas such as: Game Theory Near-Noise Level Signal detection BW transactions Trust theories (how to identify users with “bad” intensions?)

Summary exciting systems and applications. The wireless vision encompasses many exciting systems and applications. Technical challenges go across all layers of the system design. Wireless systems today still have limited performance and interoperability.