fundamentals of cellular communications

Fundamentals of Cellular Communications

Chapter 5

ByJulie Hsieh

andDonavon M. Norwood

2

Agenda

1) Hexagonal Geometry – 5.32) Cochannel Interference Ratio S/I – 5.43) Cellular Design with Omni directional Antenna – 5.54) Directional Antenna - 5.75) Cell Splitting - 5.86) Adjacent Channel Interference – 5.97) Segmentation - 5.108) Summary

3

Hexagonal Cell Geometry

4

Hexagonal Cell Geometry (cont.)Calculated distance between centers of interfering cells

5

Hexagonal Cell Geometry (cont.)Distance between centers of interfering cells

Taking Δu = 2 and Δv = 1, D = √7 ≈ 2.646. So, in a hexagonal tessellation (N=7), centers of first-tier interfering cells are 2.65 units apart—where the unit is the distance between two adjacent cells (which do not interfere since they use different channels).

6

Hexagonal Cell Geometry (cont.)Relation between

reuse ratio q and cluster size N

By reducing q (D/R), system capacity is increased, but so is interference. An increase inq reduces interferenceand system capacity.

For hexagonal layouts only:

√3*7 ≈ 4.6

7

Cellular System Design in worst case Scenario with Omni directional Antenna

Low S/I = Bad

When S/I ratio is low, then signal is low with respect to interference (which is high by comparison).

High S/I = Good

When S/I ratio is high, then signal is high with respect to interference (which is low by comparison).

8

Cellular System Design in worst case Scenario with Omni directional Antenna

6 first-tier interfering channel sources.

S/I = 54.3 = 17.3 dB, or in reality about only 14 dB, which is not high enough.

S/I can be increased by sectoring each cell using directional antennas...

9

Directional AntennasThree-Sector Case

Worst-case scenario:

S/I = 285 or 24.5 dB

10

Directional AntennasSix-Sector Case

11

Directional AntennasSix-Sector Case (cont.)

Worst-case scenario:

S/I = 789 or 29 dB

12

Directional AntennasCell Sectoring Summary

Comparison of Worst-case Scenario S/I

Sectors S/I dB dB_realistic1 54.3 17.3 14.53 285 24.5 18.56 789 29 23

13

Cell Splitting

Done to increase system capacity.

Ratio of power of large cell transmitter to small cell transmitter (where r = R/2) is

For path loss slope γ = 4, this equals 16 or 12 dB.

14

Adjacent Channel Interference

Caused by- imperfect filters - non-linearity of amplifiers

Reduce by1) using modulation schemes that have low out-of-bound radiation2) carefully designing bandpass filter3) assigning adjacent channels to completely different cells4) using equalizers

15

Segmentation

Alternative to splitting, maybe to fill a coverage gap. Divide cell site into two serving groups to help minimize new interference.

16

Multiple Access TechniquesChapter 6-1 thru 6.5

Agenda Introduction – 6.1 Narrowband Channelized Systems - 6.2 Spectral Efficiency - 6.3 Wideband Systems - 6.4 Comparisons of FDMA, TDMA and DS-CDMA - 6.5

17

Introduction

• The goal in the design of a cellular system is to be able to be able to handle as many calls as possible in a given bandwidth with the specified blocking probability.

• Multiplexing – deals with the division of of resources to create multiple channels. Multiplexing can create channels in frequency, time, etc. The corresponding terms are frequency division multiplexing (FDM), time division multiplexing (TDM), etc. Since the sprectrum available is limited, we need to find ways for multiple users to share the spectrum at the same time.

• Multiple Acess Schemes – allow may users to share the radio spectrum at the same time. These Multiple Access Schemes can be classified as a reservation-based multiple access and random-based multiple access.

18

Diagram ofMultiple Access Techniques

19

Narrowband Channelized Systems - 6.2

• Traditional architectures for analog and digital systems are channelized.

• A channelized systems divides the spectrum into a large number of relatively narrow radio channels that are defined by the carrier.

• The frequency used to transmit from the base station to the mobile station is called the forward channel (downlink channel).

• The frequency used to transmit from the mobile station to the base station is called the reverse channel (uplink channel).

20

Frequency Division Duplex (FDD) and

Time Division Duplex Systems (TDD) - 6.2.1

• Frequency Division Duplex (FDD) – separation is provided between the downlink and uplink on different frequencies. Separation can also be provided by using two antennas or one antenna through the use of a RF filter.

• Time Division Duplex (TDD) – separation is achieved by automatically altering in time the direction of transmission on a single frequency. One direction either the uplink or the downlink frequency most be off in order for data transmission.

• Both TDD and FDD require the same amount of spectrum, but the difference between the two is that FDD is divided by two different frequencies by the required bandwidth, versus TDD which uses one frequency and it uses twice the bandwidth.

21

Frequency Division Multiple Access (FDMA) - 6.2.2

• FDMA is the simplest scheme to use for multiple access. It separates the different users by assigning a different carrier frequency. Multiple users are separated by using bandpass filters.

FDMA/FDD channel architecture

22

Frequency Division Multiple Access (FDMA) – 6.2.2(cont)

Advantages:

1) The capacity can be increased by reducing the the information bit rate and using an efficient speech coding scheme.

2) Technological advances for implementation are simple.

3) Hardware simplicity, because multiple users are isolated by using bandpass filters.

Disadvantages:

1) The architecture implemented in first generation analog systems.

2) The maximum bit-rate per channel is fixed and small.

3) Inefficient use of spectrum; when a channel is not in use it remains idle and can not be used to enhance system capacity.

4) Crosstalk occurs from adjacent channels and interference by non-linear effects.

23

Time Division Multiple Access (TDMA) - 6.2.3

• In a TDMA system, each user uses the whole channel bandwidth for a fraction of time. Time is divided into equal time intervals called slots where user data is transmitted.

TDMA/FDD channel architecture

24

Time Division Multiple Access (TDMA) – 6.2.3(cont)

Advantages:

a) Permits a flexible bit rate.

b) Offers the opportunity for frame-by-frame monitoring of signal strength/bit error rates to enable mobile devices/base stations to execute and initiate handoffs.

c) When not used with FDMA uses the bandwidth more effectively because no frequency guard band is required between the channels.

d) Transmit each signal with efficient guard time between time slots to accommodate time inaccuracies because of clock instability, delay spread, transmission delay because of propagation distance and the tails of signal pulse because of transient responses.

25


Disadvantages:

a) For mobiles and hand-sets, the uplink demands high peak power in transmit mode which shortens battery life.

b) Requires a large amount of signal processing for matched filtering and correlation detection for synchronization with time slot.

c) Requires synchronization. If time is not synchronized, the channels may collide with each other.

d) Propagation from the mobile station to base station varies with its distance from base station.

26


TDMA Frame

27

Spectral Efficiency - 6.3

Spectral efficiency of a mobile systems shows how efficiently the spectrum is being used by the system and depends on the choice of multiple access scheme. Spectral efficiency measurement allows one to estimate the capacity of a mobile system.

Spectral Efficiency of Modulation(Total # Channels Available in Cluster)/

(Bandwidth)(Cluster Coverage Area) =

28

FDMA Spectral Efficiency – 6.3.2

Multiple Access Spectral Frequency is defined as the ratio of the total time or frequency dedicated for traffic transmission to the total time or frequency available to the system. In FDMA users share the radio

spectrum in the frequency domain.

FDMA Spectral Efficiency ≤ 1:

where

Bc=channel spacing

N T=channels covered area

Bw=systembandwidth

29

TDMA Spectral Efficiency – 6.3.2

Multiple Access Spectral Frequency is defined as the ratio of the total time or frequency dedicated for traffic transmission to the total time or frequency

available to the system. TDMA can operate as wideband or narrowband. In wideband TDMA the entire spectrum is used by the individual user.

TDMA Spectral Efficiency wideband

TDMA Spectral Efficiency narrowband

=time slot duration ,T f= frame duration , M t=¿ time slots / frame

Bu=user ' s bandwidth , N u=number of users sharing time slot ondifferent frequencies

30

Wideband Systems 6.4

• Wideband Systems – a system in which the entire bandwidth is made available to each user and larger than the bandwidth required to send data. They are known as spread systems. There are two types of spread systems, direct sequence spread system (DSSS) and frequency hopping spread system (FHSS).

• Direct sequence spread system (DSSS) – the bandwidth of the baseband information carrying signals from different users is spread by different codes with bandwidth much larger than the baseband signal. The receiver signal is despreaded with the same code.

• Frequency Hopping Spread System (FHSS) – is the periodic change of the frequency or the frequency associated with transmission. If modulation M-ary frequency-shift keying (MFSK), two or more frequencies are in the set that change at each hop. FHSS can be classified as fast frequency hopping in which the hopping rate exceeds symbol rate and slow hopping in which two or more symbols are transmitted in the time interval between hops.

31

Figure 6.5 Direct Sequence spread spectrum

32

Figure 6.6 Frequency hopping spread spectrum system

33

Comparison of FDMA, TDMA, and DS-CDMA 6.5

• The primary advantage of DS-CDMA is its ability to accept a fair amount of interfering signals, therefore the problems with frequency band assignment and adjacent cell are greatly simplified.

• FDMA and TDMA radios must be assigned a frequency or a time slot to assure that there is no interference with other similar radios, therefore sophisticated filtering and guarded protection is needed with both FDMA and TDMA technologies.

• In DS-CDMA adjacent cells can be in the same frequency, however with TDMA and FDMA adjacent cells can not be in the same frequency because of interference.

34

Figure 6.7 Comparison of multiple access methods

35

References

Wireless Communications and Networking, Vijay K. Garg

fundamentals of cellular communications

Documents