This document summarizes key concepts from Chapter 5 and 6 of a textbook on cellular communications fundamentals. It discusses hexagonal cell geometry, co-channel interference ratios, and how directional antennas and cell splitting can improve signal-to-interference ratios. It also covers multiple access techniques like FDMA, TDMA, and DS-CDMA, comparing their spectral efficiencies and advantages/disadvantages. DS-CDMA is noted as able to accept interfering signals better than FDMA and TDMA, simplifying frequency band assignment.

1. Fundamentals of Cellular
Communications
Chapter 5
By
Julie Hsieh
and
Donavon M. Norwood

2. Agenda
1) Hexagonal Geometry – 5.3
2) Cochannel Interference Ratio S/I – 5.4
3) Cellular Design with Omni directional Antenna – 5.5
4) Directional Antenna - 5.7
5) Cell Splitting - 5.8
6) Adjacent Channel Interference – 5.9
7) Segmentation - 5.10
8) Summary
2

3. Hexagonal Cell Geometry
3

4. Hexagonal Cell Geometry (cont.)
Calculated distance between centers of interfering cells
4

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).
5

6. Hexagonal Cell Geometry (cont.)
Relation between
reuse ratio q and cluster size N
For hexagonal
layouts only:
√3*7 ≈ 4.6
By reducing q (D/R), system
capacity is increased, but so is
interference. An increase in
q reduces interference
and system capacity. 6

7. Cellular System Design in worst case Scenario
with Omni directional Antenna
Low S/I = Bad High S/I = Good
When S/I ratio is When S/I ratio is
low, then signal high, then signal
is low with is high with
respect to respect to
interference interference
(which is high by (which is low by
comparison). comparison).
7

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...
8

9. Directional Antennas
Three-Sector Case
Worst-case scenario:
9
S/I = 285 or 24.5 dB

10. Directional Antennas
Six-Sector Case
10

11. Directional Antennas
Six-Sector Case (cont.)
Worst-case scenario:
S/I = 789 or 29 dB 11

12. Directional Antennas
Cell Sectoring Summary
Comparison of Worst-case Scenario S/I
Sectors S/I dB dB_realistic
1 54.3 17.3 14.5
3 285 24.5 18.5
6 789 29 23
12

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. 13

14. Adjacent Channel Interference
Caused by
- imperfect filters
- non-linearity of amplifiers
Reduce by
1) using modulation schemes that
have low out-of-bound radiation
2) carefully designing bandpass filter
3) assigning adjacent channels to
completely different cells
4) using equalizers 14

15. Segmentation
Alternative to
splitting, maybe to fill
a coverage gap.
Divide cell site into
two serving groups to
help minimize new
interference.
15

16. Multiple Access Techniques
Chapter 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
16

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. 17

18. Diagram of
Multiple Access Techniques
18

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).
19

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.
20

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
21

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.
22

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
23

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.
24

25. Time Division Multiple Access (TDMA) – 6.2.3
(cont)
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.
25

26. Time Division Multiple Access (TDMA) – 6.2.3
(cont)
TDMA Frame
26

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) =
27

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
B c =channel spacing
N T =channels covered area
Bw =system bandwidth
28

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
=time slot duration , T f = frame duration , M t =¿ time slots/ frame
TDMA Spectral Efficiency narrowband
Bu =user ' s bandwidth , N u=number of users sharing time slot on different frequencies
29

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 30
interval between hops.

31. Figure 6.5 Direct Sequence spread spectrum
31

32. Figure 6.6 Frequency hopping spread spectrum system
32

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.
33

34. Figure 6.7 Comparison of multiple access methods
34

35. References
Wireless Communications and Networking, Vijay K. Garg
35