Wireless & Mobile Communications Chapter 2: Wireless Transmission

Wireless & Mobile CommunicationsChapter 2: Wireless Transmission

Frequencies Signals Antennas Signal propagation Multiplexing Spread spectrum Modulation Cellular systems

Spring 2003ICS 243E - Ch.2 Wireless Transmission 2.2

Spectrum Allocation

VLF = Very Low Frequency UHF = Ultra High FrequencyLF = Low Frequency SHF = Super High FrequencyMF = Medium Frequency EHF = Extra High FrequencyHF = High Frequency UV = Ultraviolet LightVHF = Very High Frequency

Relationship between frequency ‘f’ and wave length ‘’ := c/f

where c is the speed of light 3x108m/s

1 Mm300 Hz

10 km30 kHz

100 m3 MHz

1 m300 MHz

10 mm30 GHz

100 m3 THz

1 m300 THz

visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmissioncoax cabletwisted pair


Frequencies Allocated for Mobile Communication

VHF & UHF ranges for mobile radio allows for simple, small antennas for cars deterministic propagation characteristics less subject to weather conditions –> more reliable connections

SHF and higher for directed radio links, satellite communication small antennas with directed transmission large bandwidths available

Wireless LANs use frequencies in UHF to SHF spectrum some systems planned up to EHF limitations due to absorption by water and oxygen molecules

weather dependent fading, signal loss caused by heavy rainfall, etc.


Allocated Frequencies

ITU-R holds auctions for new frequencies, manages frequency bands worldwide for harmonious usage (WRC - World Radio Conferences)

Europe USA Japan

Mobilephones

NMT 453-457MHz,463-467 MHz;GSM 890-915 MHz,935-960 MHz;1710-1785 MHz,1805-1880 MHz

AMPS, TDMA, CDMA824-849 MHz,869-894 MHz;TDMA, CDMA, GSM1850-1910 MHz,1930-1990 MHz;

PDC810-826 MHz,940-956 MHz;1429-1465 MHz,1477-1513 MHz

Cordlesstelephones

CT1+ 885-887 MHz,930-932 MHz;CT2864-868 MHzDECT1880-1900 MHz

PACS 1850-1910 MHz,1930-1990 MHzPACS-UB 1910-1930 MHz

PHS1895-1918 MHzJCT254-380 MHz

WirelessLANs

IEEE 802.112400-2483 MHzHIPERLAN 15176-5270 MHz

IEEE 802.112400-2483 MHz

IEEE 802.112471-2497 MHz


Signals I

physical representation of data function of time and location signal parameters: parameters representing the value of

data classification

continuous time/discrete time continuous values/discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values

signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift sine wave as special periodic signal for a carrier:

s(t) = At sin(2 ft t + t)


Fourier Representation of Periodic Signals

)2cos()2sin(2

1)(

11

nftbnftactgn

nn

n ∑∑∞

=

∞

=

++=

1

0

1

0

t t

ideal periodic signal real composition(based on harmonics)


Different representations of signals amplitude (amplitude domain) frequency spectrum (frequency domain) phase state diagram (amplitude M and phase in polar

coordinates)

Composite signals mapped into frequency domain using Fourier transformation

Digital signals need infinite frequencies for perfect representation modulation with a carrier frequency for transmission (->analog

signal!)

Signals II

f [Hz]

A [V]

I= M cos

Q = M sin

A [V]

t[s]


Antennas are used to radiate and receive EM waves (energy) Antennas link this energy between the ether and a device

such as a transmission line (e.g., coaxial cable) Antennas consist of one or several radiating elements

through which an electric current circulates Types of antennas:

omnidirectional directional phased arrays adaptive optimal

Principal characteristics used to characterize an antenna are: radiation pattern directivity gain efficiency

Antennas


Isotropic Antennas

Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna

Real antennas always have directive effects (vertical and/or horizontal)

Radiation pattern: measurement of radiation around an antenna

zy

x

idealisotropicradiator


Omnidirectional Antennas: simple dipoles

Real antennas are not isotropic radiators but, e.g., dipoles with lengths /4, or Hertzian dipole: /2 (2 dipoles) shape/size of antenna proportional to wavelength

Example: Radiation pattern of a simple Hertzian dipole

Gain: ratio of the maximum power in the direction of the main lobe to the power of an isotropic radiator (with the same average power)

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

simpledipole

/4 /2


Directional Antennas

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

top view, 3 sector

x

z

top view, 6 sector

x

z

Often used for microwave connections (directed point to point transmission) or base stations for mobile phones (e.g., radio coverage of a valley or sectors for frequency reuse)

directedantenna

sectorizedantenna


Array Antennas

Grouping of 2 or more antennas to obtain radiating characteristics that cannot be obtained from a single element

Antenna diversity switched diversity, selection diversity

receiver chooses antenna with largest output diversity combining

combine output power to produce gain

cophasing needed to avoid cancellation

ground plane

+

/4/2/4/2 /2

/2

+


Signal Propagation Ranges

distance

sender

transmission

detection

interference

Transmission range communication possible low error rate

Detection range detection of the signal

possible no communication

possible, high error rate Interference range

signal may not be detected

signal adds to the background noise


Signal Propagation I

Radio wave propagation is affected by the following mechanisms: reflection at large obstacles scattering at small obstacles diffraction at edges

reflectionscattering diffraction


Signal Propagation II

The signal is also subject to degradation resulting from propagation in the mobile radio environment. The principal phenomena are: pathloss due to distance covered by radio signal (frequency

dependent, less at low frequencies) fading (frequency dependent, related to multipath

propagation) shadowing induced by obstacles in the path between the

transmitted and the receiver

shadowing


Signal Propagation III

Interference from other sources and noise will also impact signal behavior: co-channel (mobile users in adjacent cells using same frequency) and

adjacent (mobile users using frequencies adjacent to transmission/reception frequency) channel interference

ambient noise from the radio transmitter components or other electronic devices,

Propagation characteristics differ with the environment through and over which radio waves travel. Several types of environments can be identified (dense urban, urban, suburban and rural) and are classified according to the following parameters: terrain morphology vegetation density buildings: density and height open areas water surfaces


Pathloss I

Free-space pathloss:To define free-space propagation, consider an isotropic source consisting of a transmitter with a power Pt W. At a distance ‘d’ from this source, the power transmitted is spread uniformly on the surface of a sphere of radius ‘d’. The power density at the distance ‘d’ is then as follows:

Sr = Pt/4d2

The power received by an antenna at a distance ‘d’ from the transmitter is then equal to:

Pr = PtAe/4d2

where Ae

is the effective area of the antenna.


Pathloss II

Noting that Ae = Gr/(4where Gr is the gain of the receiver

And if we replace the isotropic source by a transmitting antenna with a gain Gt the power received at a distance ‘d’ of the transmitter by a receiving antenna of gain Gr becomes:

Pr = PtGrGt/[4d2

In decibels the propagation pathloss (PL) is given by:

PL(db) = -10log10(Pr/Pt) = -10log10(GrGt/[4d2)

This is for the ideal case and can only be applied sensibly to satellite systems and short range LOS propagation.


Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction

Positive effects of multipath: enables communication even when transmitter and receiver

are not in LOS conditions - allows radio waves effectively to go through obstacles by getting around them thereby increasing the radio coverage area

Multipath Propagation I

signal at sendersignal at receiver


Multipath Propagation II

Negative effects of multipath: Time dispersion or delay spread: signal is dispersed over time due signals coming over different paths of different lengths

Causes interference with “neighboring” symbols, this is referred to as Inter Symbol Interference (ISI)

multipath spread (in secs) = (longest1 – shortest2)/c

For a 5s symbol duration a 1s delay spread means about a 20% intersymbol overlap. The signal reaches a receiver directly and phase shifted (due to reflections)

Distorted signal depending on the phases of the different parts, this is referred to as Rayleigh fading, due to the distribution of the fades. It creates fast fluctuations of the received signal (fast fading). Random frequency modulation due to Doppler shifts on the different paths. Doppler shift is caused by the relative velocity of the receiver to the transmitter, leads to a frequency variation of the received signal.


Effects of Mobility

Channel characteristics change over time and location signal paths change different delay variations of different signal parts different phases of signal parts

quick changes in the power received (short term fading)

Additional changes in distance to sender obstacles further away

slow changes in the average power received (long term fading)

short term fading

long termfading

t

power


Multiplexing Techniques

Multiplexing techniques are used to allow many users to share a common transmission resource. In our case the users are mobile and the transmission resource is the radio spectrum. Sharing a common resource requires an access mechanism that will control the multiplexing mechanism.

As in wireline systems, it is desirable to allow the simultaneous transmission of information between two users engaged in a connection. This is called duplexing.

Two types of duplexing exist: Frequency division duplexing (FDD), whereby two frequency

channels are assigned to a connection, one channel for each direction of transmission.

Time division duplexing (TDD), whereby two time slots (closely placed in time for duplex effect) are assigned to a connection, one slot for each direction of transmission.


Multiplexing in 3 dimensions time (t) (TDM) frequency (f) (FDM) code (c) (CDM)

Goal: multiple use of a shared medium

s2

s3

s1

Multiplexing

f

t

c

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels ki


Narrowband versus Wideband

These multiple access schemes can be grouped into two categories: Narrowband systems - the total spectrum is divided into a

large number of narrow radio bands that are shared.

Wideband systems - the total spectrum is used by each mobile unit for both directions of transmission. Only applicable for TDM and CDM.


Frequency Division Multiplexing (FDM)

Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time –

orthogonal system Advantages:

no dynamic coordination necessary, i.e., sync. and

framing works also for analog signals low bit rates – cheaper,

delay spread Disadvantages:

waste of bandwidth if the traffic is distributed unevenly

inflexible guard bands narrow filters

k2 k3 k4 k5 k6k1

f

t

c


f

t

c

k2 k3 k4 k5 k6k1

Time Division Multiplexing (TDM)

A channel gets the whole spectrum for a certain amount of time – orthogonal system

Advantages: only one carrier in the

medium at any time throughput high - supports bursts flexible – multiple slots no guard bands ?!

Disadvantages: Framing and precise

synchronization necessary

high bit rates

at each

Tx/Rx


f

Hybrid TDM/FDM

Combination of both methods A channel gets a certain frequency band for a certain

amount of time (slot). Example: GSM, hops from one band to another each time

slot Advantages:

better protection against tapping (hopping among

frequencies) protection against frequency

selective interference Disadvantages:

Framing and

sync. requiredt

c

k2 k3 k4 k5 k6k1


Code Division Multiplexing (CDM)

Each channel has a unique code

(not necessarily orthogonal) All channels use the same spectrum

at the same time Advantages:

bandwidth efficient no coordination and synchronization

necessary good protection against interference

and tapping Disadvantages:

lower user data rates due to high gains required to reduce interference

more complex signal regeneration

2.19.1

k2 k3 k4 k5 k6k1

f

t

c


Issues with CDM

CDM has a soft capacity. The more users the more codes that are used. However as more codes are used the signal to interference (S/I) ratio will drop and the bit error rate (BER) will go up for all users.

CDM requires tight power control as it suffers from far-near effect. In other words, a user close to the base station transmitting with the same power as a user farther away will drown the latter’s signal. All signals must have more or less equal power at the receiver.

Rake receivers can be used to improve signal reception. Time delayed versions (a chip or more delayed) of the signal (multipath signals) can be collected and used to make bit level decisions.

Soft handoffs can be used. Mobiles can switch base stations without switching carriers. Two base stations receive the mobile signal and the mobile is receiving from two base stations (one of the rake receivers is used to listen to other signals).

Burst transmission - reduces interference


Types of CDM I

Two types exist: Direct Sequence CDM (DS-CDM)

spreads the narrowband user signal (Rbps) over the full spectrum by multiplying it by a very wide bandwidth signal (W). This is done by taking every bit in the user stream and replacing it with a pseudonoise (PN) code (a long bit sequence called the chip rate). The codes are orthogonal (or approx.. orthogonal).

This results in a processing gain G = W/R (chips/bit). The higher G the better the system performance as the lower the interference. G2 indicates the number of possible codes. Not all of the codes are orthogonal.

Frequency

Time

CodeCDMA


Types of CDM II

Frequency hopping CDM (FH-CDM)

FH-CDM is based on a narrowband FDM system in which an individual user’s transmission is spread out over a number of channels over time (the channel choice is varied in a pseudorandom fashion). If the carrier is changed every symbol then it is referred to as a fast FH system, if it is changed every few symbols it is a slow FH system.

A

A

A

A

A

A

A

A

A

B

B

B

B

B

B

B B

B


Orthogonality and Codes

An m-bit PN generator generates N=2m - 1 different codes. Out of these codes only ‘m’ codes are orthogonal -> zero

cross correlation. For example a 3 bit shift register circuit shown below

generates N=7 codes.

+

1 2 3

Mod2 Adder (1+0=1, 0+1=1, 0+0=0, 1+1=0)

Initial State: 1 1 10 1 11 0 10 1

0001

1 0 01 1 0


Orthogonal Codes A pair of codes is said to be orthogonal if the cross correlation is

zero: Rxy(0) = 0 .

For two m-bit codes: x1,x2,x3,...,xm and y1,y2,y3,...,ym:

For example: x = 0011 and y = 0110. Replace 0 with -1, 1 stays as is. Then:

x = -1 -1 1 1

y = -1 1 1 -1

-----------------

Rxy(0) = 1 -1 +1 -1 = 0


Example of an Orthogonal Code: Walsh Codes

In 1923 J.L. Walsh introduced a complete set of orthogonal codes. To generate a Walsh code the following two steps must be followed: Step 1: represent a NxN matrix as four quadrants (start off with

2x2)

Step 2: make the first, second and third quadrants indentical and invert the fourth

b

b

b

b’

= 1 1

1 0or

0 0

0 1

2 codes: 11 and 10 2 codes: 00 and 01

b bb b’

b bb b’

b bb b’

b bb b’

=

Code 1

Code 2

or1 11 0

1 11 0

1 11 0

0 00 1

0 00 10 00 1

0 00 11 11 0

Code 1Code 2Code 3Code 4


Modulation

Digital modulation digital data is translated into an analog signal (baseband) ASK, FSK, PSK - main focus in this chapter differences in spectral efficiency, power efficiency, robustness

Analog modulation shifts center frequency of baseband signal up to the radio

carrier Motivation

smaller antennas (e.g., /4) Frequency Division Multiplexing medium characteristics

Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)


Modulation and Demodulation

synchronizationdecision

digitaldataanalog

demodulation

radiocarrier

analogbasebandsignal

101101001 radio receiver

digitalmodulation

digitaldata analog

modulation

radiocarrier

analogbasebandsignal

101101001 radio transmitter


Digital Modulation

Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK):

very simple low bandwidth requirements very susceptible to interference

Frequency Shift Keying (FSK): needs larger bandwidth

Phase Shift Keying (PSK): more complex robust against interference

1 0 1

t

1 0 1

t

1 0 1

t


Advanced Frequency Shift Keying

bandwidth needed for FSK depends on the distance between the carrier frequencies

special pre-computation avoids sudden phase shifts MSK (Minimum Shift Keying)

bit separated into even and odd bits, the duration of each bit is doubled

depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen

the frequency of one carrier is twice the frequency of the other

even higher bandwidth efficiency using a Gaussian low-pass filter GMSK (Gaussian MSK), used in GSM


Example of MSK

data

even bits

odd bits

1 1 1 1 000

t

low frequency

highfrequency

MSKsignal

bit

even 0 1 0 1

odd 0 0 1 1

signal h n n hvalue - - + +

h: high frequencyn: low frequency+: original signal-: inverted signal

No phase shifts!


Advanced Phase Shift Keying

BPSK (Binary Phase Shift Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK low spectral efficiency robust, used e.g. in satellite systems

QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol symbol determines shift of sine wave needs less bandwidth compared to

BPSK more complex

Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PACS, PHS) 11 10 00 01

Q

I01

Q

I

11

01

10

00

A

t


Quadrature Amplitude Modulation

Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation

it is possible to code n bits using one symbol 2n discrete levels, n=2 identical to QPSK bit error rate increases with n, but less errors compared to

comparable PSK schemes

Example: 16-QAM (4 bits = 1 symbol) Symbols 0011 and 0001 have the

same phase, but different amplitude. 0000 and 1000 have different phase, but same amplitude.

used in standard 9600 bit/s modems

0000

0001

0011

1000

Q

I

0010


Spread spectrum technology: CDM

Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference

Solution: spread the narrow band signal into a broad band signal using a special code protection against narrow band interference

protection against narrowband interference

Side effects: coexistence of several signals without dynamic coordination tap-proof

Alternatives: Direct Sequence, Frequency Hopping

detection atreceiver

interference spread signal

signal

spreadinterference

f f

power power


Effects of spreading and interference

P

fi)

P

fii)

sender

P

fiii)

P

fiv)

receiverf

v)

user signalbroadband interferencenarrowband interference

2.28.1

P


Spreading and frequency selective fading

frequency

channelquality

1 23

4

5 6

narrow bandsignal

guard space

22

22

2

frequency

channelquality

1

spreadspectrum

2.29.1

narrowband channels

spread spectrum channels

Spring 2003ICS 243E - Ch.2 Wireless Transmission 2.452.30.1

DSSS (Direct Sequence Spread Spectrum) I

XOR of the signal with pseudo-random number (chipping sequence) many chips per bit (e.g., 128) result in higher bandwidth of the

signal Advantages

reduces frequency selective fading

in cellular networks base stations can use the same frequency range

several base stations can detect and recover the signal

soft handover

Disadvantages precise power control necessary

user data

chipping sequence

resultingsignal

0 1

0 1 1 0 1 0 1 01 0 0 1 11

XOR

0 1 1 0 0 1 0 11 0 1 0 01

=

tb

tc

tb: bit periodtc: chip period


DSSS (Direct Sequence Spread Spectrum) II

Xuser data

chippingsequence

modulator

radiocarrier

spreadspectrumsignal

transmitsignal

transmitter

demodulator

receivedsignal

radiocarrier

X

chippingsequence

lowpassfilteredsignal

receiver

integrator

products

decisiondata

sampledsums

correlator

2.31.1

Spring 2003ICS 243E - Ch.2 Wireless Transmission 2.472.32.1

FHSS (Frequency Hopping Spread Spectrum) I

Discrete changes of carrier frequency sequence of frequency changes determined via pseudo

random number sequence Two versions

Fast Hopping: several frequencies per user bit

Slow Hopping: several user bits per frequency

Advantages frequency selective fading and interference limited to short

period simple implementation uses only small portion of spectrum at any time

Disadvantages not as robust as DSSS simpler to detect


FHSS (Frequency Hopping Spread Spectrum) II

user data

slowhopping(3 bits/hop)

fasthopping(3 hops/bit)

0 1

tb

0 1 1 t

f

f1

f2

f3

t

td

f

f1

f2

f3

t

td

tb: bit period td: dwell time

2.33.1


FHSS (Frequency Hopping Spread Spectrum) III

modulatoruser data

hoppingsequence

modulator

narrowbandsignal

spreadtransmitsignal

transmitter

receivedsignal

receiver

demodulatordata

frequencysynthesizer

hoppingsequence

demodulator

frequencysynthesizer

narrowbandsignal

2.34.1


Concept of Cellular Communications

In the late 60’s it was proposed to alleviate the problem of spectrum congestion by restructuring the coverage area of mobile radio systems.

The cellular concept does not use broadcasting over large areas. Instead smaller areas called cells are handled by less powerful base stations that use less power for transmission. Now the available spectrum can be re-used from one cell to another thereby increasing the capacity of the system.

However this did give rise to a new problem, as a mobile unit moved it could potentially leave the coverage area (cell) of a base station in which it established the call. This required complex controls that enabled the handing over of a connection (called handoff) to the new cell that the mobile unit moved into.

In summary, the essential elements of a cellular system are:

Low power transmitter and small coverage areas called cells

Spectrum (frequency) re-use

Handoff


Cell structure

Implements space division multiplex: base station covers a certain transmission area (cell)

Mobile stations communicate only via the base station

Advantages of cell structures: higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc.

locally Problems:

fixed network needed for the base stations handover (changing from one cell to another) necessary interference with other cells

Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

2.35.1


Cellular Network

MSCPSTN

CellMSC: Mobile Switching CenterPSTN: Public Switched Telephone Network

BaseStation

Handoff

(Theoretical)

Practical Cell - coverage depends on antenna location and height, transmitter power, terrain, foliage, buildings, etc.

Other MSCs

(IS 41)

F1,F2,..,F6

F7,F8,..,F12

F1,F2,..,F6

F7,F8,..,F12


Some Definitions

Forward path or down link - from base station down to the mobile Reverse path or up link - from the mobile up to the base station The mobile unit - a portable voice and/or data comm. transceiver.

It has a 10 digit telephone number that is represented by a 34 bit mobile identification number -> (215) 684-3201 is divided into two parts: MIN1: 215 translated into 10bits and MIN2: 684-3201 translated into 24bits. In addition each mobile unit is also permanently programmed at the factory with a 32 bit electronic serial number (ESN) which guards against tampering.

The cell - a geographical area covered by Radio Frequency (RF) signals. It is essentially a radio communication center comprising radios, antennas and supporting equipment to enable mobile to land and land to mobile communication. Its shape and size depend on the location, height , gain and directivity of the antenna, the power of the transmitter, the terrain, obstacles such as foliage, buildings, propagation paths, etc. It is a highly irregular shape, its boundaries defined by received signal strength! But for traffic engineering purposes and system planning and design a hexagonal shape is used.


More definitions

The base station (BS) - a transmitter and receiver that relays signals (control and information (voice or data)) from the mobile unit to the MSC and vice versa.

The mobile switching center (MSC) - a switching center that controls a cluster of cells. Base stations are connected to the MSC via wireline links. The MSC is directly connected to the PSTN and is responsible for all calls related to mobiles located within its domain. MSCs intercommunicate using a link protocol specified by IS (International Standard) 41. This enables roaming of mobile units (i.e. obtaining service outside of the home base). The MSC is also responsible for billing, it keeps track of air time, errors, delays, blocking, call dropping (due to handoff failure), etc. It is also responsible for the handoff process, it keeps track of signal strengths and will initiate a handoff when deemed necessary (note to handoff or not to handoff is not a trivial issue!)


The Basic Cellular Communication Protocol I

Every mobile unit whether at home or roaming, has to register with the MSC controlling the area it is in. If it does not register then the MSC does not know of its existence and will not be able to process any of its calls.

The home location register (HLR) is used to keep information regarding a mobile unit/user, it is a database for storing and managing subscriber information. When roaming, a mobile unit registers with a foreign MSC and data from its HRL is relayed to the visitor location register (VLR). The VLR is a dynamic database used to store roaming mobile subscriber information. The HLR and VLR communicate via the MSCs using IS 41.

The cellular system uses out of band signalling. Most of the control information is sent over different channels from the user information (voice or data) channels. Inband signalling is used for control during the connection (disconnect, handoff, etc.)


The Basic Cellular Communication Protocol II

A mobile unit when enabled (power on) scans the control channels and tunes to the one with the strongest signal. The control channels are known and carry signals pertaining to the cell sites, e.g. transmission power to be used by the mobile unit in a particular cell. This process is called initialization.

If the mobile wants to initiate a call, it sends in a service request on the reverse path control link. The service request contains the destination phone number and identification information (MIN1, MIN2, and ESN) of the source mobile unit to verify the originator.

When the base station receives the request, it relays it to the MSC. The MSC then checks to see it is it a number of another mobile or of a fixed user. If the latter the call is forwarded to the PSTN. If the former, it checks to see if the destination mobile unit is a subscriber (local or visitor/roamer). If not it relays the call to the PSTN to forward to the appropriate MSC.


The Basic Cellular Communication Protocol III

If the destination is within its cluster it sends out a paging message to all the base stations. Every base station then relays this message by broadcasting it on its control channel. If the destination mobile unit is enabled (power on) it will detect this message and respond to the base station.

The base station relays this response to the MSC. The MSC then allocates channels to both the source mobile unit and the destination mobile unit. The corresponding base stations pass this information on to the respective mobile units. The mobile units then tune to the correct channels and the communication link is established.


Spectrum and Capacity Issues

Spectrum is limited

Allocated SpectrumF1 F2 F3 F4 F5 F6 F7 F8 F9

FDMF1,F2,...F9: frequency channels


Frequency Re-use I

To be able to increase the capacity of the system, frequencies must be re-used in the cellular layout (unless we are using spread spectrum techniques).

Frequencies cannot be re-used in adjacent cells because of co-channel interference. The cells using the same frequencies must be dispersed across the cellular layout. The closer the spacing the more efficient the scheme!

F1F1

CochannelInterference

F2

F2

MinimumRe-use distance

Fx:subset offrequenciesused in a cell


Frequency Re-use II

For an omni-directional antenna, with constant signal power, each cell site coverage area would be circular (barring any terrain irregularities or obstacles).

To achieve full coverage without dead spots, a series of regular polygons for cell sites are required.

The hexagonal was chosen as it comes the closest to the shape of a circle, and a hexagonal layout requires fewer cells (when compared to triangles or rectangles, it has the largest surface area given the same radius R) -> less cells.

Goal is to find the minimum distance between cells using same frequencies.


Frequency re-use distance I

AA

i j

i,j - integers -> intercell distance

D - min. dist.

D

along cell centers60%

i,j: multiples of 31/2R

R

R: cell radius

R

D

(u,v)

D=31/2R[i2+j2+ij]1/2R

R = radius of hexagonal

300

uv

u2-u1=31/2Ri

v2-v1=31/2Rj

i,j are integers

(0,0)

31/2R 12

31

31/2R


Frequency re-use distance II

For two adjacent cells: D=31/2R The closest we can place the same frequencies is called the

first tier around the center cell (minimal re-use distance -> lower -> more capacity!).

For simplicity we only take the first tier of cells into account for co-channel interference (i.e., we ignore 2nd, 3rd, etc. tiers, cause much less interference, negligible!).

Original cell

First tier of interferersSecond tier of interferers

They are all equidistantaway from each other (D)

Cluster of “N” cells with different frequencies

Each cell has exactly six equidistant interfering cells


Frequency re-use distance III

D

R

Radius

Radius = DFirst Tier(all use samefrequencies ascenter cell)

Cluster of “N” cells withfrequencies different

(large hexagon)from center cell


Frequency re-use distance III

Radius = dist. between two co-channel cells = (3R2[i2+j2+ij])1/2 = D

Since the area of a hexagon is proportional to the square of the distance between its center and a vertex (i.e., its radius), the area of the large hexagon is:

Alarge = k[Radius]2 = k[3R2[i2+j2+ij]]

where k is a constant. Similarly the area of each cell (i.e., small hexagon) is:

Asmall = k[R2]

Comparing these expressions we find that:

Alarge/Asmall = 3[i2+j2+ij] = D2/R2


Frequency re-use distance IV

From symmetry we can see that the large hexagon encloses the center cluster of N cells plus 1/3 the number of the cells associated with 6 other peripheral hexagons. Thus the total number of cells enclosed by the first tier is:

N+6(1/3N) = 3N

Since the area of a hexagon is proportional to the number of cells contained within it:

Alarge/Asmall = 3N/1 = 3N

Substituting we get:

3N = 3[i2+j2+ij] = D2/R2

Or:

D/R = q =(3N)1/2

“q” is referred to as the reuse ratio!


Co-channel Interference I

The co-channel interference ratio S/I is given as:

S = desired signal power in a cell (note that many texts use “C” instead of S), Ik = interference signal power from the kth cell, Ni = number of interfering cells.

If we only assume the first tier of interfering cells, then Ni=6,and all cells interfere equally (they are all equidistant!).

The signal power at any point is inversely proportional to the inverse of the distance from the source raised to the power. (2<<5)

SI--- S

Ik⟨⟩

1k =

Ni∑

------------------------=


Co-channel Interference II

Ik is proportional to D , and S is proportional to R , where is the propagation path loss and is dependent upon terrain environment. For cellular systems it is often taken as = 4.

Therefore:

The relationship between SNR (signal to noise ratio - Eb/No) and S/I for cellular systems with Rayleigh fading channels: SNR = S/I(db) – 9db.

SI--- R

–

6D–⋅

-----------------1

6q–⋅

----------------q

6-----===


For a given S/I how to get N

Recall that: D/R = q =(3N)1/2

An S/I = 18db (decibels=10logS/I) = 63.1, gives an acceptable voice quality.

Therefore q = [6x63.1]1/4 = 4.41 when = 4 Substituting for N we get N = (4.41)2/3 equals approx. 7 This means that if we have 49 frequency channels

available, each cell gets 49/7 = 7 frequency channels. If we have 82 available then 82/7 = 11.714 -> which means

that 5 cells will have 12 and 2 cells will have 11! How does that translate to “i and j” for a cell layout?

N = [i2+j2+ij], find i,j that satisfy the equation!


Calculating i, j, and D from N

111

2

2

2

34

5

6

777

D

D = 4.41R

ij

N=7 -> i=2, j=1

f4

f5

f1

f3

f2

f6

f7

f3

f2

f4

f5

f1

f3

f5f6

f7f2

f2


Frequency planning

Frequency reuse only with a certain distance between the base stations

Standard model using 7 frequencies:

Fixed frequency assignment: certain frequencies are assigned to a certain cell problem: different traffic load in different cells

Dynamic frequency assignment: base station chooses frequencies depending on the

frequencies already used in neighbor cells more capacity in cells with more traffic assignment can also be based on interference measurements

f4

f5

f1

f3

f2

f6

f7

f3

f2

f4

f5

f1

2.36.1


Increasing Capacity

We can see that by reducing the area of a cell we can increase capacity as we will have more cells each with its own set of frequencies.

What is drawback of shrinking the size of the cells (cell splitting)? Increase in the number of handoffs -> increased load on the system! Also need more infrastrucutre -> base stations (each cell needs a BS).

An easier solution exists, sectorization. It does not reduce handoffs, its advantage: it does not require more infrastructure.


Sectorization I

We can also increase the capacity by using sectors in cells. Directional antennas instead of being omnidirectional, will

only beam over a certain angle.

120% 60%F1

F2F3

F1+F2+F3=Fa

F1F2

F3

F4F5

F6

F1+F2+F3+F4+F5+F6=Fa

3 sectors 6 sectorsFa: A cell’s set of frequencies

f1

f2

f3

f2

f1

f1

f2

f3

f2

f3

f1

f2

f1

f3f3

f3f3

f3

f1f1 f1

f2

f3

f2

f3

f2

f3h1

h2

h3g1

g2

g3

h1

h2

h3g1

g2

g3g1

g2

g3

3 cell cluster 3 cell cluster with 3 sectors


Sectorization II

What does that mean? We can now assign frequency sets to sectors and decrease

the re-use distance or improve S/I ratio (i.e. signal quality). Question: By how much? Depends on number of sectors

(i.e., 60% or 120%).

First Tier(all use samefrequencies in

center cell)A”

A”

A”A’

A’A’

A

A

“A”: set of frequencies in a sector

A:Do not interfere with “A”sector of center cell

A”:Cause Mobile to cell site interference

A’:Cause Cell site tomobile interference

sectors as


Other Capacity or Signal Improvement Tech.

Dynamic channel allocation (DCA): allows cells to borrow frequencies from other cells within the cluster if not used by them. Can be used to alleviate hotspots. Another implementation basically has all channels available to all cells, they get allocated based upon demand.

Power control: by reducing the transmitted power, the battery life of a mobile can be extended. It also helps in reducing -channel and adjacent channel interference.

Wireless & Mobile Communications Chapter 2: Wireless Transmission

Documents

Transcript of Wireless & Mobile Communications Chapter 2: Wireless Transmission