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

2. 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

3. 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

4. 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

5. 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

6. 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 by We have rm = .
Dr. Xinbing Wang

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

8. 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

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

10. 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

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

12. 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

13. 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)
In our daily lives, wireless communication technology is used everywhere, from VCR remote control, to satellite weather forecast. The common characteristics of wireless communication systems is that there is no physical (visible) lines between two communication parties.
Therefore, a wireless system is able to support user roaming. For example, we do not have to use a remote control in a particular position to.., we can use our cellular phones almost everywhere.
However, there are many impairments to a wireless channel, causing a lot of limitations to wireless communications system such as geographical.. (signal fading, additional noise, cochannel interference. Wireless systems also suffers from limit usable spectral width, so that the transmission rate is relatively low.
Specifically, wireless cellular systems based on radio propagation has been evolving from narrow band (1G, late 170s) to wide-band(3G).
With their geographical coverage limitation, wireless systems need a backbone network to extend their geographical coverage to enable global communications.
The interoworking of a wireless network as the front-end and the Internet as the backbone has received much attention in recent years.
So we will first take a look at the network architecture of current wireless systems,…, Then we will talk about the evolution from 2G to 3G systems.
Dr. Xinbing Wang

14. 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
In our daily lives, wireless communication technology is used everywhere, from VCR remote control, to satellite weather forecast. The common characteristics of wireless communication systems is that there is no physical (visible) lines between two communication parties.
Therefore, a wireless system is able to support user roaming. For example, we do not have to use a remote control in a particular position to.., we can use our cellular phones almost everywhere.
However, there are many impairments to a wireless channel, causing a lot of limitations to wireless communications system such as geographical.. (signal fading, additional noise, cochannel interference. Wireless systems also suffers from limit usable spectral width, so that the transmission rate is relatively low.
Specifically, wireless cellular systems based on radio propagation has been evolving from narrow band (1G, late 170s) to wide-band(3G).
With their geographical coverage limitation, wireless systems need a backbone network to extend their geographical coverage to enable global communications.
The interoworking of a wireless network as the front-end and the Internet as the backbone has received much attention in recent years.
So we will first take a look at the network architecture of current wireless systems,…, Then we will talk about the evolution from 2G to 3G systems.
Dr. Xinbing Wang

15. 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

16. 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

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

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

19. 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

20. 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

21. 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

22. 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

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

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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. After Class Reading materials Exercises Chapter 7 (optional) Chapter 8
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