Lecture 6

Network Deployment (2)

Basics of Transmission Schemes (1)

+ 2

Capacity Expansion

n Main investment in deploying a cellular network is

the cost of infrastructure, land, base station
equipment, switches installation, interconnection,
etc.
n Income is proportional to subscriber base
n Initial
installment may not be able to support
increasing subscriber demand
n How can capacity be increased without replicating
deployment?
+ 3

Techniques to expand capacity

n Additional spectrum
n Very hard to obtain – also expensive
n 1900 MHz bands for PCS; 700 MHz bands from TV

n Architectural approaches
n Cell splitting
n Cell sectorization
n Reuse partitioning
n Lee’s microcell zone technique

n Changing to digital – TDMA or CDMA

n Dynamic channel allocation

+ 4

Cell Splitting

n Hotspots are created in

certain areas
n Introduce a smaller cell of
half the size midway
between two co-channel
cells

n Interference problems

n Channels must be split a

between the larger and A-a
smaller cells
+ 5

The Overlaid Cell Concept

)
DL
, (1 n Channels are divided between a
larger macro-cell that co-exists
with a smaller micro-cell that is
completely contained within the
macro-cell

DL
,(2
n D2/R2 is larger than D1/R1

)
R1
n Split-band analog systems

n Reuse partitioning
n Used in LTE (Revisit)

R2
+ 6

Cell Sectoring

n Use directional antennas to

reduce interference

n Radio propagation is focused in

certain directions
n Antenna coverage is restricted
to part of a cell called a sector

n By reducing interference, the

cluster size can be reduced (Js is
reduced, and so we can reduce Nc)
+ 7

Three-sector cells and a cluster size

of Nc= 4 B

B C G

C G A
• 120o directional antennas are
employed
A D F

• Channels allocated to a cell

D F E B

E B C G

B C G A
are further divided into three
C G A D F
parts
A D F E • Without directional antennas,
D F E B Sr = 13.8 dB which is
E B C G inadequate
C G A
• With directional antennas, Sr =
A D F
18.5 dB
D F E

E 4
R −4 R −4 1 # D & 9 2
Sr ≈ −4
= −4
= % ( = Nc
Cell Sector Under Interfering Non-Interfering Js D 2D 2$ R ' 2
Consideration Sectors Sectors
+ 8

Sectored Frequency Planning

n Example: Allocate frequencies for an AMPS operator in cellular B-block
who uses a 7 cell frequency reuse pattern with 3 sectors per cell

n Use a Frequency Chart – available from FCC web site

n Groups frequencies into 21 categories Cells 1-7 and sectors A-B-C in each cell
+ Sectored Frequency Planning
9

n Example: Allocate frequencies for a

GSM operator in U.S. PCS B-block
who uses a 7 cell frequency reuse
pattern with 3 sectors per cell

n Use a Frequency Chart – available

from FCC web site

n Groups frequencies into 21

categories Cells A-G and sectors 1-3
in each cell
+ 10

Summary: Cell sectoring

n The cluster size can be reduced by employing directional antennas

n The capacity increase is 1.67 times for N = 4 and 2.3 times for N = 3
compared to N = 7

n Sectoring is better than splitting

n No new base station has to be set up
n No new planning efforts are needed to maintain interference levels

n Sectoring leads to handoff between sectors which increases signaling

load and some loss of call quality

n A cell cannot be ideally sectored and the signal to interference values

obtained here are optimistic
+ 11

Channel Allocation Techniques

n Idea:
n During the day on weekdays, downtown areas have a lot of
demand for wireless channels
n In weekends and evenings, suburban areas have a larger
demand and downtown areas have very little demand
n Instead of allocating channels statically to cells, allocate
channels on demand while maintaining signal-to-interference
ratio requirements

n The (voice) user does not care how the channels are
allocated as long as
n He/she gets access to the channel whenever required
n The quality of the signal is acceptable
+ 12

Channel Allocation Techniques (2)

n Fixed channel allocation (FCA)

n Channel borrowing

n Dynamic channel allocation (DCA)

n Centralized DCA
n Distributed DCA
n Cell-based
n Measurement-based

n Hybrid channel allocation (HCA)

+ 13

Channel borrowing
Affected Cells
(Locked Channels)

B C G n Idea: Borrow channels from

C G A low loaded cells and return
A D F them whenever required
D F E B
n Temporary channel
E B C G borrowing
B C G A
n Return channel after call is
completed
Borrow
C G A Channels D F

A D F E n Locks channel in co-channel

D F E B cells
E B C G n Static channel borrowing
C G A n Distribute channels non-
A D F
uniformly but change them in
a predictable way
D F E

E
+ 14

Dynamic Channel Allocation

n All channels are placed in a pool

n When a new call comes in, a channel is selected based on the overall
SIR in the cell
n Selection of the channel in this way is costly
n Needs a search and computation of SIR values

n Centralized
n A central entity selects channels for use and returns it to the pool after
completion of calls
n Distributed
n Base stations locally compute the channels that can be used
n Cell-based – BSs communicate with each other on the wired backbone to
determine the best way to select channels
n Measurement-based – BSs measure RSS or receive RSS reports from MSs
that they use in their decisions
+ 15

Comparison of FCA and DCA

Attribute Fixed Channel Allocation Dynamic Channel Allocation

Traffic Load Better under heavy traffic load Better under light/moderate traffic load

Flexibility in channel allocation Low High

Reusability of channels Maximum possible Limited
Temporal and spatial changes Very sensitive Insensitive
Grade of service Fluctuating Stable
Forced Call Termination Large probability Low/moderate probability
Suitability of cell size Macro-cellular Micro-cellular
Radio equipment Covers only the channels Has to cover all possible channels that
allocated to the cell could be assigned to the cell
Computational effort Low High
Call set up delay Low Moderate/High
Implementation complexity Low Moderate/High
Frequency planning Laborious and complex None
Signaling load Low Moderate/High
Control Centralized Centralized, decentralized or distributed
+ 16

Interference Management in LTE-

OFDMA
n Borrows ideas from Reuse Partitioning and Dynamic Channel Allocation

n Aims for a frequency reuse of 1

n Sub-carriers and “resource blocks” (RBs) may not be universally reused

n Base stations talk with each other to manage interference and also
scheduling RBs to users
+ 17

Strict and Soft Fractional

Frequency Reuse in LTE
Power

Power
f1 f2 f3 f4 frequency f2 f3 f4 frequency

Power distribution
in this cell
f2
+f4
f2
+f3

f4
+f3

(a) Strict FFR with reuse of 3+1 (b) Soft FFR with reuse of 3
+ 18

Femtocells

n Initial Idea
n Coverage challenged areas with good Internet connectivity
n Progressive Benefits
n High spectrum efficiencies
n Typically indoor!
n High data rates are possible
n Reducing subscriber churn
n Reducing CAPEX and OPEX costs for service providers
n Backhaul capacity and capital expenditures are reduced
+ 19

Issues with Femtocell deployment

n Femtocellbase station cannot transmit at high power

nor at low power
n Should not swamp users that do not belong to femtocell
n Should not deny coverage to someone who installs the
femtocell

n Femtocell base station reception has to be dynamic

n A mobile that is near should not swamp it because of its
minimum transmit power
n A mobile far away should not be forced to transmit at high
power to reach the femtocell
n This may interfere with transmissions in a macrocell
+ 20

Design Issues in Local Area Wireless Data

n IEEE 802.11
n Initial deployments were based on the 915 MHz bands
n There was only one channel
n In the 2.4 GHz bands
n There are three non-overlapping channels à frequency reuse is
possible
n Thresholds!
n In the 5 GHz bands, there are eleven non-overlapping channels

n Three dimensional planning is required

n Antenna patterns and building architecture

n There are three levels of transmit power at the AP

n Not clear what can be done at the MS
+ 21

Overlapping channels in the 802.11

specifications

1 2 3 4 5 6 7 8 9 10 11

2.412 2.462
5 MHz

Use three non-overlapping channels

+ 22

Using overlapping channels

1 2 3 4 5 6 7 8 9 10 11

2.412 2.462
5 MHz
n It is possible to use Channels 1, 4, 7 and 11 instead of 1, 6 and 11
n There is a drop in throughput
n There are some results of actual performance but they are
inconclusive
n It is not clear whether the drop in throughput is due to backoff or
packet loss
+ 23

SIRs in 802.11 WLANs (@2Mbps)

n Reports of measurements and models of 802.11 RSS

and throughputs are vendor specific
n One report says that a minimum SIR of 15 dB is required for
good throughput
n Used UDP streams and estimated the SIR using a path loss
model
n Throughput falls from 1.8 Mbps to 1 Mbps as the SIR reduces
from 15 dB to 10 dB
n Reuse issues are then simulated
n Unlike voice, data is bursty – so the design and deployment
issues are different
n Most real deployments design the network for coverage rather than
specific QoS goals
+ 24

WLAN Deployment Methods

n Random deployment by users

n Arrange access points in a grid

n Optimally place access points for coverage/interference

+ 25

Coexistence?

n Interference
n Two wireless technologies interfere if co-location
causes significant performance degradation
n Coexistence
n Two wireless technologies coexist if there is no
significant impact on the performance
n Interoperable
n Devices belonging to two different wireless
technologies are interoperable if they can
communicate and exchange data between them
+ 26

Coexistence between HomeRF and IEEE

802.11

n HomeRF uses very slow frequency hopping

n 50 hops/s – frame is 20 ms long
n Compare with Bluetooth – 1600 hops/s and 625 µs
n Also operates in the 2.4 GHz bands

n Experiments
on studying the impact of
HomeRF on 802.11 throughput
n HomeRF is very detrimental to 802.11 throughput
n HomeRF is an “interference”
+ 27

Bluetooth and 802.11 (1)

n Impact of BT on 802.11
n At large RSS, the
throughput is fairly good
n As the RSS falls, the
throughput falls drastically
n BT causes substantial
interference, but there is
some kind of capture
when the RSS is good

Source: J. Lansford et al., IEEE Network, September 2001

+ 28

Bluetooth and IEEE 802.11 (2)

n Impact of IEEE 802.11 on BT

n 802.11 signal is like a
wideband jammer
n As the RSS from the AP falls,
the throughput improves
n As the RSS from the AP
increases, voice packets are
dropped randomly
n The transition occurs
suddenly

n Short ACKs are less likely to

cause errors than long
frames

Source: J. Lansford et al., IEEE Network, September 2001

+ 29

Communication Issues and Radio

Propagation
Fading
Channels

Large Small
Scale Scale
Fading Fading

Path-Loss
Time Time Angular
&
Variation Dispersion Dispersion
Shadowing

Impacts
Coverage Impacts signal design,
receiver design, coding, BER
+ 30

Before we get into small-scale

fading…
nWhat is the best we can do when there is
NO fading?
nWhat are the tradeoffs between bit errors,
power, noise, and bandwidth?
+ 31

Digital Modulation (Revisited)

n Changing the parameters of a sinusoid is called “shift keying” if

information is digital
n Types
n Amplitude-shift keying (ASK)
n Amplitude difference of carrier
n Frequency-shift keying (FSK)
n Frequency difference near carrier frequency
n Phase-shift keying (PSK)
n Phase of carrier signal shifted
n Quadrature amplitude modulation (QAM)
n Both amplitude and phase of the carrier carry data

n Bits/Symbol
n Binary (one bit in one symbol => two symbols)
n M-ary (log2M bits in one symbol => M symbols)
+ 32

Binary Amplitude-Shift Keying

n Idea
n One binary digit represented by the presence of the carrier, at
constant amplitude
n The other binary digit is represented by the absence of the carrier

Average Power
in Signal =
n Remarks: A2/4
n The carrier signal is A cos(2πfct) Average
n The symbol duration is T seconds Energy per bit
Eb = A2T/4
n Also called On-Off keying or OOK
+ 33

Amplitude-Shift Keying
n Susceptible to sudden gain changes

n Inefficient modulation technique (what do we mean by this?)

n Used on voice-grade lines up to 1200 bps

n Used to transmit digital data over optical fiber and in IR systems

ON OFF ON
+ 34

Binary Frequency-Shift Keying

(BFSK)
n Two binary digits represented by two
different frequencies near the carrier
frequency

Average
Power in
Signal =
A2/2
n f1
and f2 are offset from carrier frequency fc Eb = A2T/2
by equal but opposite amounts
+ 35

Frequency-Shift Keying (FSK)

n Less susceptible to error than ASK

n On voice-grade lines, used up to 1200bps

n Used for high-frequency (3 to 30 MHz) radio transmission

n Can be used at higher frequencies on LANs that use coaxial cable

0 1 0
+ 36

Binary Phase-Shift Keying (PSK)

n Uses two phases to represent binary digits

n OR

n We revisit BPSK later

Average Power in Signal = A2/2

Eb = A2T/2
+ 37

M-ary Modulation Schemes

n M-ary => M symbols

n The symbols are a1, a2,…, aM

n Each symbol carries k = log2M bits

n Example: M = 4 => the symbols are a1, a2, a3, a4
n Let a1
= 00, a2 = 01, a3 = 10, and a4 = 11
n We have k = 2 bits/symbol

n The symbols can once again be represented by the

amplitude, phase, or frequency of the carrier
+ Average Power in Signal =
38

Example: 4-ASK 7A2/4

Eb = 7A2T/8

nIn 4-ASK, we need 4 different amplitudes

of the carrier to represent 4 symbols
nLet the amplitudes be 0,1,2 and 3
nThe symbols
8 will be:
>
> ↵0 ! 00 : 0, 0  t  T
>
<↵ ! 01 :
1 cos(2⇡fc t), 0  t  T
s(t) =
>
> ↵2 ! 11 : 2 cos(2⇡fc t), 0  t  T
>
:
↵3 ! 10 : 3 cos(2⇡fc t), 0  t  T
+ 39

More on M-ary modulation

nM-ASK (also called PAM) is not common

nMore common are
n MPSK – There are M phases of the carrier to
represent the M symbols
n MFSK – There are M frequencies around fc to
represent the M symbols
nQuadrature amplitude modulation (QAM)
n Usesa combination of amplitude and phase
n M-QAM
+ 40

Advanced Modulation Schemes

n Variations on ASK, FSK and PSK possible

n Attempt to improve performance
n Increase data for a fixed bandwidth
n Remove requirement for phase synchronization
n Differential modulation and detection
n Improve BER performance

n Main schemes for wireless systems are based

on FSK and PSK because they are more robust
to noise
+ 41

Orthogonal signaling with codes

What is
orthogonality?

Show in time and

frequency

Show in space

Bandwidth
+ 42

Modulation schemes used in wireless

networks
n GMSK n BPSK, QPSK, 16-QAM, 64-
n GSM, CDPD, Mobitex, GPRS, QAM
HIPERLAN/1 n HIPERLAN/2, IEEE 802.11a
(with OFDM), LTE
n p/4 – DQPSK
n BPSK, QPSK
n Tetra, IS-136
n IS-95, IEEE 802.11 (with
DSSS)
n OQPSK
n IS-95, cdma2000 n Pulse Position Modulation
n IEEE 802.11 IR
n FSK
n ARDIS, 802.11 FHSS, n Orthogonal Modulation
Bluetooth n IS-95, cdma2000
+ 43

Communication Issues

n Noise (unwanted interfering signals) is not necessarily additive,

white or Gaussian
n Examples: Inter-symbol interference (ISI), Adjacent channel
interference (ACI), Co-channel interference (CCI)
n In CDMA interference from users etc.

n Noise affects the Bit Error Rate (BER)

n Fraction of bits that are inverted at the receiver

n Also, the radio channel has multiplicative components that

degrade the performance
n The behavior of the radio channel can increase ISI, reduce the
signal strength, and increase the bit error rate
+ 44

Performance in General

n What determines how successful a receiver will be in

interpreting an incoming signal?
n Signal-to-noise ratio => power
n Data rate
n Bandwidth

n Typical trends
n An increase in data rate increases bit error rate
n An increase in SNR decreases bit error rate
n An increase in bandwidth allows an increase in data rate

n In mobile wireless systems both bandwidth and power are in

short supply
+ 45

Wireless Performance Considerations

n In wireless communications, the primary issues are

n Spectrum
n Power
n Effects of the radio channel

n When we look at modulation schemes, we are interested in

the following
n Performance in AWGN channels
n Provides a baseline performance
n Performance in multipath fading channels
n Expected performance in realistic channels
n Bandwidth efficiency
n Cost and complexity
+ 46

Performance in AWGN Channels

n Suppose the communications channel is only affected by

AWGN (thermal noise) Bit Error Rate
or BER is a
n This is the most ideal conditions you may get
function of
n Similar to a wire

n Provides a benchmark or baseline performance Eb

n Can get some insight into whether one modulation scheme is = b
better than another N0
n Ideally we want
n Very low bit error rates at small signal-to-noise ratio
n Ensures we can conserve battery power by transmitting at low
powers
n Yet the information can be recovered reliably
+ 47

Performance in AWGN channels (2)

n AWGN = Additive White Gaussian

Noise Binary Modulation Schemes
n This has a “flat” noise
spectrum with average
power spectral density of N0

n The probability of bit error (bit

error rate) is measured as a
function of ratio of the “energy
per bit” – Eb to the average noise
PSD value
n BER or Pe variation with
Eb/N0
n Eb/N0 is a measure of the
“power requirements”

n Tradeoffs!
+ 48

Signal Constellation (1)

+ 49

Signal Constellation (2)

n Given any modulation scheme, it is possible to

obtain its signal constellation.
n Represent each possible signal as a vector in 8-PSK
a Euclidean space.

n In symbol detection – decode incoming signal

as closest symbol in the signal constellation
space

n If we know the signal constellation, we can

estimate the performance in terms of the
probability of symbol error given the noise
parameters

n Probability of error depends on the minimum 16-QAM

distance between the constellation points
+ 50

Probability of Error
+ 51

Performance in AWGN(2)
+ 52

M-ary modulation schemes

phase shift keying or QAM

n Bits per symbol = log2M

n Phase shift keying
n Signal points are on a circle
n More bits/sec/Hz but larger Pe increasing M
for given Eb/N0
n Orthogonal keying orthogonal keying
n M-dimensional constellation
n FSK
n Pulse position modulation
n Orthogonal keying/signaling
n Less bits/sec/Hz but much
smaller Pe for given Eb/N0
n QAM
n Works mostly like PSK increasing M
+ 53

Bandwidth Efficiency and Complexity

n Bandwidth efficiency (related to spectral efficiency)

n For a given bit error rate what is the required bandwidth for a specified data rate?
n Recall discussion of capacity
n Example – At a BER of 10-5, BPSK requires 2 MHz for a data rate of 2 Mbps
n Ideally our goal is to stuff as many bits as possible in a given bandwidth
n Bandwidth (spectrum) efficiency is measured in terms of the data rate supported over a
given bandwidth
n Units: bits/sec/Hz.

n Cost/Complexity
n In achieving good performance and bandwidth efficiency, the modulation scheme should
not be too expensive or complex to implement
n Circuitry should be simple to implement and inexpensive (e.g. detection, amplifiers)
+ 54

Bandwidth of modulation schemes

GMSK
+ 55

Tradeoffs between BER, power and

bandwidth
n (1) Trade BER performance for
power – fixed data rate

n (2) Trade data rate for power –

fixed BER

n (3) Trade BER for data rate – fixed

power
+ 56

Next Week

nSmall Scale Fading