ECE/CSC 575 – Section 1 Introduction to Wireless Networking

ECE/CSC 575 – Section 1 Introduction to Wireless Networking Lecture 6Dr. Xinbing Wang

Received Signal • The received signal power: where Gr is the receiver antenna gain, L is the propagation loss in the channel, i.e., L = LP LS LF Fast fading Slow fading Path loss Dr. Xinbing Wang

Radio Propagation and Antenna (Ch. 5) • Wired and wireless medium • Radio propagation mechanism • Antenna and antenna gain • Path-loss modeling • Free Space model • Two-Ray model • Shadow fading • Different environments • Effect of Multipath and Doppler • Multipath fading • Doppler spectrum Dr. Xinbing Wang

Effect of Multipath and Doppler • Small-scale fading: The received signal is rapidly fluctuating due to the mobility of the terminal causing changes in multiple signal components arriving via different paths. • There are two effects which contribute to the rapid fluctuation of the signal amplitude. • Multipath fading: caused by the addition of signals arriving via different paths. • Doppler: caused by the movement of the mobile terminal toward or away from the base station transmitter. • Small-scale fading results in very high bit error rates. It is not possible to simply increase the transmit power to overcome the problem • Error control coding, diversity schemes, directional antennas. Dr. Xinbing Wang

Modeling of Multipath Fading • Results in fluctuations of the signal amplitude because of the addition of signals arriving with different phases. • This phase difference is caused due to the fact that signals have traveled different distances by traveling along different paths. • Because of the phases of the arriving paths are changing rapidly, the received signal amplitude undergoes rapid fluctuation that is often modeled as a random variable. Dr. Xinbing Wang

Modeling of Multipath Fading (2) • Rayleigh distribution (NLOS) • Most commonly used distribution for multipath fading (the envelope distribution of received signal) is Rayleigh distribution with pdf • Assume that all signals suffer nearly the same attenuation, but arrive with different phases. • r is the random variable corresponding to the signal amplitude, and 2 is the variance. • Used to determine what fraction of area receives signals with the requisite strength. • Middle value rm of envelope signal within sample range to be satisfied byWe have rm = 1.777 . Dr. Xinbing Wang

P(r) 1.0 0.8 =1 0.6 =2 0.4 =3 0.2 r 0 2 6 10 4 8 Raleigh Distribution The pdf of the envelope variation Dr. Xinbing Wang

Modeling of Multipath Fading (3) • Ricean distribution (LOS – transmitter is close) • When a strong LOS signal component also exists, the pdf is given by •  is a factor that determines how strong the LOS component is relative to the rest of the multipath signals. If =0, then it becomes Rayleigh distribution. • I0(x) is the zero-order modified Bessel function of the first kind. Dr. Xinbing Wang

Rician Distribution = 0 (Rayleigh)  = 1  = 2  = 3 Pdf p(r)  = 1 r The pdf of the envelope variation Dr. Xinbing Wang

Doppler Shift • BS transmits a single frequency f, the received signal at the MT at time t has a frequency of f+v(t). • v(t) is the Doppler shift and is given by (t) V Dr. Xinbing Wang

Moving Speed Effect V1 V2 V3 V4 Signal strength Time Dr. Xinbing Wang

Summary: Wireless Communications • Comparison of wired and wireless medium • Frequency bands and licenses • Radio propagation mechanism • Reflection and transmission • Diffraction • Scattering • Indoor and outdoor radio propagation • Distance-power relationship and received signal • Path-loss modeling • Free space propagation • Two-ray model • Shadow fading • Different environments: Marocell and Microcell environments • Multipath and Doppler spectrum • Rayleigh and Ricean models • Doppler shift Dr. Xinbing Wang

Overview of the Course • Part 1: Wireless communication systems (Chapter 1) • Flexibility to support roaming • Limitations: Geographical coverage, transmission rate, and transmission errors • Part 2: Wireless communication technology • Radio propagation (Chapter 5) • Spread spectrum (Chapter 7 -- Self reading (ECE 791W)) • Coding and error control (Chapter 8 -- Self reading (ECE/CSC 570)) • Part 3: Current wireless systems • Cellular network architecture: UMTS (Chapter 10) • Mobile IP (Chapter 12) • Wireless LAN (Chapters 11/13/14) • Part 4: Other wireless networks • Ad hoc networks (Reading materials) • Wireless PAN (Chapter 15) • Satellite systems (Chapter 9) • Sensor networks (Reading materials) Dr. Xinbing Wang

Current Wireless Systems: Cellular Systems--UMTS • Fundamentals of cellular communications • System capacity • frequency reuse • Cell splitting • Admission control • handoff • Universal mobile telecommunication system (UMTS) • Network architecture • Functional units • Quality of service • Mobility management Dr. Xinbing Wang

System Capacity • System capacity is the largest number of users that can be supported. • Transmitter power • High power is required to support a large number of users in one cell. All users share the same set of frequencies, or radio channels. There is an upper limit. • Lower power transmitter for each small cell, but increase the number of cells, reuse of frequencies, and antenna sectoring. • The geographical regions that use the same set of radio frequencies must be physically separated from each other to avoid interference. • Cellular communication: Way of replicating identically structured geographical regions. Dr. Xinbing Wang

Frequency Reuse • If two cells are far away from enough that the same set of frequencies can be used in both cells, it is called frequency reuse. • With frequency reuse, a large area can be divided into small areas, each uses a subset of frequencies and covers a small area. • With frequency reuse, the system capacity can be expanded without employing high power transmitters. • Cellular architecture: each cell may be modeled as square, hexagonal, circular and so on for performance analysis. • Handoff: when mobile hosts (users) move out of the coverage of its serving base station. Dr. Xinbing Wang

Cell Shape R R R Cell R R (c) Different cell models (a) Ideal cell (b) Actual cell Dr. Xinbing Wang

Impact of Cell Shape and Radius on System Characteristics Dr. Xinbing Wang

Cell Cluster Concept • The total number of channels available in a cellular system is finite. • The system capacity is a function of the total number of available channels and how these channels are deployed. • In wireless communication systems, the channels used in forward and reverse are separated for duplexing. • Cells which use the same set of frequencies are referred to as cochannel cells, may result in cochannel interferences. • A group of cells that use a different set of frequencies in each cell is called a cell cluster. Dr. Xinbing Wang

Cellular Concept - Example • Consider a high-power transmitter that can support 35 voice channels over an area of 100 km2 with the available spectrum • If 7 lower power transmitters are used so that they support 30% of the channels over an area of 14.3 km2 each. • Then a total 7*30% * 35 = 80 channels are available instead of 35. 2 3 1 7 4 6 5 Dr. Xinbing Wang

Capacity Expansion by Frequency Reuse • Assume each cell is allocated J channels (J<=K). If the K channels are divided among the N cells into unique and disjoint channel groups, each with J channels, then K = J N • The N cells in a cluster use the complete set of available frequencies. • The cluster can be replicated many times. Let M be the number of replicated clusters and C be the total number of channels, then C = M J N Dr. Xinbing Wang

System Capacity - Example • Suppose there are 1001 radio channels, and each cell is 6 km2 and the entire system covers an area of 2100km2. • Calculate the system capacity if the cluster size is 7. • How many times would the cluster of size 4 have to be replicated in order to approximately cover the entire cellular area? • Calculate the system capacity if the cluster size is 4. • Does decreasing the cluster size increase the system capacity? Solution: Dr. Xinbing Wang

Nearest Cochannel Neighbors • The cluster size, N, N = i2+ij+j2 3 2 1 4 1 2 3 2 1 Dr. Xinbing Wang

Geometry of Hexagonal Cells • Planning for cochannel cells • Geo-location for accurate positioning j D i 30o R 0 Dr. Xinbing Wang

Geometry of Hexagonal Cells (2) • Let Dnorm be the distance from the center of a candidate cell to the center of a nearest cochannel cell, normalized to • Let D be the actual distance between two centers of adjacent cochannel cells. • From the geometry, we have Considering the cluster size, N, N = i2+ij+j2, we have Dr. Xinbing Wang

Number of Cells in A Cluster • A candidate cell has 6 nearest cochannel cells. Each of them in turn has 6 neighbors. So we can have a large hexagon. • The area of a hexagon is proportional to the square of its radius, (let =2.598), R D Dr. Xinbing Wang

Frequency Reuse Ratio • The frequency reuse ratio, q, is defined as q = D/R which is also referred to as the cochannel reuse ratio. • Tradeoff • q increases with N. • A smaller value of N has the effect of increasing the capacity of the cellular system and increasing cochannel interference • Tradeoff between q and N Dr. Xinbing Wang

Cochannel Interference • Intracell Interference: interferences from other mobile terminals in the same cell. • Duplex systems • Background white noise • Intercell interference: interferences from other cells. • More evident in the downlink than uplink for reception • Can be reduced by using different set of frequencies • Design issue • Frequency reuse • interference • System capacity Dr. Xinbing Wang

Cochannel Interference (2) • For simplicity, we consider only the average channel quality as a function of the distance dependant path loss. • Signal-to-cochannel inference ratio, (S/I), at the mobile receiver is defined by • S and I denote the power of the desired signal and cochannel interference. • NI is the number of cochannel interfering cells • Ii is the interference power caused by transmissions from the ith interfereing cochannel cell base station. Dr. Xinbing Wang

Cochannel Interference (3) • When the mobile is located at the cell boundary, the worst case cochannel interference occurs as the power of the desired signal is minimum • With the hexagon shape cellular systems, there are always six cochannel interfering cells in the first tier, i.e., NI =6. • How many cells in the second and third tiers? • If r=R and assume Di=D, we have • If we substitute q=D/R, the frequency reuse ratio can be expressed as Dr. Xinbing Wang

Worst Case Cochannel Interference • We need to modify our assumption, i.e., assume Di=D. • The S/I ratio can be expressed as D+R R D+R D D D-R D-R Dr. Xinbing Wang

Example: Worst Case Cochannel Interference (2) • A cellular system that requires an S/I ratio of 18dB. (a) if frequency reuse ratio is 7, what is the worst-case S/I? (b) Is a frequency reuse factor of 7 acceptable in terms of cochannel interference? If not, what would be a better choice of frequency reuse ratio? • Solution: Dr. Xinbing Wang

After Class • Reading materials • Chapter 7 (optional) • Chapter 8 • Chapter 10 • Exercises • Why cellular architecture is used for wireless communications? • What is cochannel interference? • What is the relationship between system capacity, cluster size, and total number of channels? Dr. Xinbing Wang

ECE/CSC 575 – Section 1 Introduction to Wireless Networking