function of time and location

(1)

Wireless Transmission & Media Access

Signals and Signal Propagation

Multiplexing

Modulation

Media Access

(2)

Signals

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 ϕ

sine wave as special periodic signal for a carrier:

s(t) = A_t sin(2 π f_tt + ϕ_t)

(3)

Signal propagation ranges

Transmission range

communication possible

low error rate

Detection range

detection of the signal possible

no communication possible

Interference range

signal may not be detected

signal adds to the background noise

distance sender

transmission

detection interference

(4)

Signal propagation

Propagation in free space always like light (straight line) Receiving power proportional to 1/d²

(d = distance between sender and receiver) Receiving power additionally influenced by

fading (frequency dependent)

shadowing

reflection at large obstacles

refraction depending on the density of a medium

scattering at small obstacles

diffraction at edges

reflection refraction scattering

shadowing diffraction

(5)

Real World Examples

(6)

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

Î

quick changes in the power received (short term fading) Additional changes in

distance to sender

obstacles further away

Î

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

long term fading power

short term fading t

(7)

Time Division Multiplexing

Statistical (temporal) alignment of multiply sourced packet streams

Shared use of medium with one carrier at any time

High throughput, efficient use of medium

Synchronisation necessary

Typical use: wired LANs

(8)

Frequency Division Multiplexing

Frequency spectrum divided in smaller bands

Any user (channel) gets a certain band for the whole time

No coordination/synchronisation

Works for analog signals as well

Guarantied throughput, inefficient use of bandwidth

Typical use: telephone system (standard 12 x 4.000 Hz between 12 and 60 kHz)

Frequency

User

Time

(9)

Optical: Wavelength Division Multiplexing

Colours of different sources are added to different carrier frequencies and combined

Subtype of frequency division multiplexing

Allows combined use of fibers

Processing in pure optical components

Fiber 1 Fiber 2 Fiber 4

Fiber 3 Combiner Splitter

4 3

2

: λ

1

+ λ + λ + λ

Spectrum

(10)

Code multiplex

Each channel has a unique code

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

more complex signal regeneration

k₁ k₂ k₃ k₄ k₅ k₆

t

c

f

(11)

Multiplexing in Wirless Networks

Additional dimension:

Space division multiplexing

Attempts to combine multiplexing technologies Goal: multiple use of a (limited) shared medium Example: Time and frequency multiplexing in GSM

Caveat: precise coordination/guarding of ‘spaces’ required

(12)

Modulation and demodulation

synchronization decision

digital analog data

demodulation radio carrier

analog baseband signal

101101001 radio receiver digital

modulation digital

data analog

modulation analog

baseband signal

radio carrier

radio transmitter 101101001

Sinus baseband signal: s(t) = A sin(2 π f t + ϕ)

(13)

Spread spectrum technology

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 at receiver interference spread

signal

spread interference

f f

power power

(14)

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

z base stations can use the same frequency range

z several base stations can detect and recover the signal

z soft handover

Disadvantages

precise power control necessary

t_b

user data XOR

0 1

t_c

chipping sequence 0 1 1 0 1 0 1 0 1 1 0 1 0 1 =

resulting signal 0 1 1 0 1 0 1 1 0 0 1 0 1 0

t_b: bit period t_c: chip period

(15)

DSSS (Direct Sequence Spread Spectrum) II

X user data

modulator spread

spectrum signal

transmit signal

chipping sequence

radio carrier transmitter

demodulator received

signal

radio carrier

X chipping sequence

lowpass filtered signal

integrator products

decision sampled

sums correlator

data

(16)

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

(17)

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

(18)

Cell breathing

CDM systems: cell size depends on current load

Additional traffic appears as noise to other users

If the noise level is too high users drop out of cells

(19)

Media Access

How does the shared media air differ?

Example: Think of CSMA/CD

But: Signal propagation is different

Signal strength decreases proportional to the square of distance

Senders may not hear or drown each other

Collisions occur at receiver, CS & CD at the sender

Different access methods needed (taken from multiplexing techs):

SDMA (Space Division Multiple Access)

FDMA (Frequency Division Multiple Access)

TDMA (Time Division Multiple Access)

(20)

Hidden and exposed terminals

Hidden terminals

A sends to B, C cannot receive A

C wants to send to B, C senses a “free” medium (CS fails)

collision at B, A cannot receive the collision (CD fails)

A is “hidden” for C

Exposed terminals

B sends to A, C wants to send to another terminal (not A or B)

C has to wait, CS signals a medium in use

but A is outside the radio range of C, therefore waiting is not necessary

C is “exposed” to B

B

A C

(21)

Near and far terminals

Terminals A and B send, C receives

signal strength decreases proportional to the square of the distance

the signal of terminal B therefore drowns out A’s signal

C cannot receive A

If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer

Also severe problem for CDMA-networks - precise power control

A B C

(22)

MACA - collision avoidance

MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance

RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet

CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive

Signaling packets contain

sender address

receiver address

packet size

Variants of this method can be found in IEEE802.11 as DFWMAC

(Distributed Foundation Wireless MAC)

(23)

MACA examples

MACA avoids the problem of hidden terminals

A and C want to send to B

A sends RTS first

C waits after receiving CTS from B

MACA avoids the problem of exposed terminals

B wants to send to A, C to another terminal

now C does not have to wait for it cannot receive CTS from A

A B C

RTS

CTS CTS

A B C

RTS CTS

RTS

(24)

MACA variant: DFWMAC in IEEE802.11

sender receiver

idle

wait for data idle

wait for the right to send

wait for ACK

packet ready to send; RTS time-out;

RTS

CTS; data RxBusy

data;

ACK

RTS;

time-out ∨ CTS data;

NAK ACK time-out ∨

NAK;

RTS

RTS; RxBusy ACK: positive acknowledgement

NAK: negative acknowledgement

RxBusy: receiver busy

(25)

Access method CDMA

CDMA (Code Division Multiple Access)

all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel

each sender has a unique random number, the sender XORs the signal with this random number

the receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function

Disadvantages:

higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal)

all signals should have the same strength at a receiver

Advantages:

all terminals can use the same frequency, no planning needed

huge code space (e.g. 2³²) compared to frequency space

interferences (e.g. white noise) is not coded

(26)

Comparison SDMA/TDMA/FDMA/CDMA

Approach SDMA TDMA FDMA CDMA

Idea segment space into cells/sectors

segment sending time into disjoint time-slots, demand driven or fixed patterns

segment the

frequency band into disjoint sub-bands

spread the spectrum using orthogonal codes

Terminals only one terminal can be active in one cell/one sector

all terminals are active for short periods of time on the same frequency

every terminal has its own frequency,

uninterrupted

all terminals can be active at the same place at the same moment,

uninterrupted

Signal separation

cell structure, directed antennas

synchronization in the time domain

filtering in the frequency domain

code plus special receivers

Advantages very simple, increases capacity per km²

established, fully digital, flexible

simple, established, robust

flexible, less frequency planning needed, soft handover

Dis-

advantages

inflexible, antennas typically fixed

guard space needed (multipath propagation), synchronization difficult

inflexible,

frequencies are a scarce resource

complex receivers, needs more complicated power control for senders

Comment only in combination with TDMA, FDMA or CDMA useful

standard in fixed networks, together with FDMA/SDMA used in many mobile networks

typically combined with TDMA

(frequency hopping patterns) and SDMA (frequency reuse)

still faces some problems, higher complexity,

lowered expectations; will be integrated with

TDMA/FDMA