Introduction

1. Introduction

2. A Brief History • Wireless communications originated with the demonstration by Tesla in 1893, followed by the invention of wireless telegraph by Marconi in 1896 • Advances in wireless communications have led to radio, television, mobile telephones, and communication satellites • Development of wireless networks: LAN, MAN, and WAN • Trend continues: Voice – Data – Broadband • Need to support mobility

3. Trends • Revenue for wireless communication industry has surpassed that of wired telephony industry • Wireless applications span both local area and wide area for: • voice-oriented services, and • data-oriented services • Global cellular networks are providing very convenient communication infrastructure • Broadband wireless networks are evolving: Wireless LANs are very popular

4. Evolution of Personal Communication Systems • Integrating the network intelligence of today’s PSTN (public switched telephone network) with modern digital signal processing and RF technology • Standards for facilitating ubiquitous communication support are on the way • Personal communication system using wireless components are more cost-effective than their wired counterpart • Advances in communication technology and networking protocols are necessary to facilitate the realization of the “anytime-anywhere” systems

5. Applications of Mobile and Wireless Networks • Low-cost LAN or MAN infrastructure • Replacement of wired networks • Anytime, anywhere access to data/information in wireless mode • Commercial and business applications • Sensors, RFID • Short-range communications • Location-dependent services • Emergency and disaster recovery services • Vehicular communications

6. Mobile and Wireless Devices • Notebook/Laptop computers • PDAs • Mobile phones and pagers • Sensors • Embedded controllers in appliances • Switching infrastructure (APs)

7. Wireless Communication Fundamentals

8. Characteristics of Wireless Medium • Comparison to wired media: • Unguided link • 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

9. Operational Ranges • 1 GHz (cellular) • 2 GHz (PCS and WLAN) • 5 GHz (WLANs) • 28-60 GHz (local multipoint distribution services (LMDS) and point-to-point base-station connections) • IR frequencies for optical communications

10. 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

11. Radio Propagation - 1 • An understanding of radio propagation is essential for appropriate design, deployment, and management strategies for any wireless network • The characteristics of the radio channel has a significant impact on the performance of wireless networks and communications • Radio propagation is predominantly site-specific and can vary significantly depending on the terrain, frequency of operation, velocity of the mobile terminal, interference sources, and other dynamic environmental conditions • Radio propagation in open areas in much different than indoor and urban areas • Non line-of-sight (NLOS), obstructed LOS (OLOS), variable strength signals and multipath delay spread affect the reception of data

12. Radio Propagation - 2 • The radio frequency of operation also affects radio propagation characteristics and system design • Signals from the transmitter arrive at the receiver via multiple paths resulting in multipath delay spread that affects the reception of data • In free-space the signal strength is inversely proportional to the square of the distance

13. Radio Propagation - 3 • 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

14. Radio Propagation - 4 • The achievable signal coverage for a given transmission power determines the size of a cell in a cellular topology and the range of operations in a base station transmitter. This is obtained via empirical path-loss models • Data rate limitations are influenced by the multipath structure of the channel and the fading characteristics of the multipath components. This also influences the signaling scheme and receiver design • Rate of fluctuations in the channel is caused by the movement of transmitter, receiver, or objects in between. It impacts the design of the adaptive parts of the receiver such as timing, carrier synchronization, and phase recovery.

15. Radio Propagation Mechanisms • Reflection and Transmission: • Occurs when electromagnetic waves impinge on obstructions larger than the wavelength • Upon reflection or transmission, the radio signals attenuates by factors that depend on the frequency, angle of incidence, and the nature of medium • Diffraction: • Diffracted fields are generated by secondary wave sources formed at the edges of the buildings, walls, and other large objects. • Diffraction facilitates the reachability of signals that are not in line of sight of the transmitter. However, the losses are more than that of reflection and transmission • Scattering: • Irregular surfaces scatter signals in all directions in the form of spherical waves. Propagation in many directions results in reduced power levels.

16. Signal Coverage • Signal coverage is influenced predominantly by radio frequency of operation and the terrain • Several models are necessary for a variety of environments to enable system design • Core of the signal coverage calculations for any environment is the path-loss model which relates the loss of signal strength to the distance between the two terminals • The radio signal strength falls as some power a of the distance, called the power-distance gradient or path-loss gradient

17. Power-Distance Gradient • If the transmitted power is Pt, after a distance d in meters, the signal strength is proportional to Ptd-a. • In general, the relationship between the transmitted power Pt and the received power Pr in free space is given by: Pr/Pt = GtGr(l/4pd)2 Gt and Gr: transmitter and receiver antennae gain d: distance between transmitter and receiver l = wavelength of the carrier = c/f

18. Two-path Model • The distance-power relationship observed for free-space does not hold for all environments. In most realistic environments, signals reaches the receiver through several different paths • Pr/Pt = GtGr(hb2hm2/d4) LOS Path hb hm d Ground Reflection

19. Shadow Fading • Depending on the environment and the surroundings, and the location of objects, the received signal strength for the same distance from the transmitter will be different. This variation of signal strength due to location is referred to as shadow fading • To overcome the shadow fading effects, a fade margin is added to the path loss or received signal strength. The fade margin is the additional signal power that can provide a certain fraction of the locations with the required signal strength

20. Path Loss Models • Megacellular areas • Macrocellular areas • Microcellular areas • Picocellular areas • Femtocellular areas

21. Multipath Fading and Doppler Spectrum • Fluctuations of the signal amplitude because of the addition of signals arriving in different phases (paths) is called multipath fading • Multipath fading results in high BER, and can be mitigated by ECC, diversity schemes, and using directional antennae • Doppler spectrum is the spectrum of fluctuations of the received signal strength caused by the movement of mobile terminals.