chap-3 media access

8/3/2019 Chap-3 Media Access

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Mobile Communications

Chap -3 Media Access


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Motivation for a specialized MAC

Can we apply media access methods from fixed networks?

So let us considercarrier sense multiple access with collision

detection, (CSMA/CD) which works as follows in wired Network.

A sender senses the medium (a wire or coaxial cable) to see if it is free. If

the medium is busy, the sender waits until it is free. If the medium is free,

the sender starts transmitting data and continues to listen into the medium.

If the sender detects a collision while sending, it stops at once and sends a

jamming signal. Why does this scheme fail in wireless networks?

signal strength decreases proportional to the square of the distance(ISI

Problem)

it might be the case that a sender cannot hear the collision, i.e., CD does

not work furthermore, CS might not work if, e.g., a terminal is hidden


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Motivation:- hidden and exposed terminal

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

Now , B sends something

to A and C wants to send

data to some other mobile

phone outside the range of

A,B and C. C senses the carrier and detects that carrier is busy. Hence C post

pones its transmission. But as A is outside the interference range of C, waitingis not necessary. So in this situation , C is exposed to B.

BA C

A C


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Motivation - 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 As signal

C cannot receive A

If C for example was an arbiter for sending rights, terminal B would drown outterminal A

A B C


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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


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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

CTSCTS

A B C

RTS

CTS

RTS


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Access Methods

To overcome this problems, signals can be divided into Different like

SDMA,FDMA,TDMA, and CDMA

1 ) SDMA :-

Space Division Multiple Access (SDMA) is used for allocating a

separated space to users in wireless networks.

A typical application involves assigning an optimal base station to a

mobile phone user. The mobile phone may receive several basestations with different quality. A MAC algorithm could now decide

which base station is best, taking into account which frequencies

(FDM), time slots (TDM) or code (CDM) are still available (depending

on the technology).

The basis for the SDMA algorithm is formed by cells . use directed antennas

Use cells to reuse frequencies


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Access Methods

2) FDMA : -

Frequency division multiple access (FDMA) comprises all algorithms

allocating frequencies to transmission channels according to the

frequency division multiplexing (FDM) .

Allocation can either be fixed (as for radio stations).

Channels can be assigned to the same frequency at all times, i.e., FDMA,

or change frequencies according to a certain pattern, i.e., FDMA

combined with TDMA.

Furthermore, FDM is often used for simultaneous access to the medium

by base station and mobile station in cellular networks. Here the two

partners typically establish a duplex channel, i.e., a channel that allows

for simultaneous transmission in both directions. The two directions,

mobile station to base station and vice versa are now separated using

different frequencies. This scheme is then called frequency division

duplex (FDD).


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Access Methods

Again, both partners have to know the frequencies in advance; they

cannot just listen into the medium. The two frequencies are also known

as uplink, i.e., from mobile station to base station or from ground controlto satellite, and as downlink, i.e., from base station to mobile station or

from satellite to ground control.

As for example FDM and FDD, Figure shows the situation in a mobile

phone network based on the GSM standard for 900 MHz. The basicfrequency allocation scheme for GSM is fixed and regulated by national

authorities. All uplinks use the band between 890.2 and 915 MHz, all

downlinks use 935.2 to 960 MHz. According to FDMA, the base station,

shown on the right side, allocates a certain frequency for up- and

downlink to establish a duplex channel with a mobile phone.


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FDD/FDMA - general scheme, example GSM

f

t

124

1

124

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz


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Access Methods

3) TDMA : -

Compared to FDMA, time division multiple access (TDMA) offers a

much more flexible scheme, which comprises all technologies that

allocate certain time slots for communication.

Now tuning in to a certain frequency is not necessary, i.e., the receiver

can stay at the same frequency the whole time. Using only one

frequency, and thus very simple receivers and transmitters, many

different algorithms exist to control medium access. As alreadymentioned, listening to different frequencies at the same time is quite

difficult, but listening to many channels separated in time at the same

frequency is simple.

E.g. radio system because it allows several users to share the same

frequency channel by dividing and signal into different timeslots.


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Access method CDMA

4) 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

Advantages:

all terminals can use the same frequency, no planning needed

huge code space compared to frequency space

interferences is not

error correction and encryption can be easily integrated

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


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CDMA in theory

The following (theoretical) example explains the basic function of CDMA

before it is applied to signals:

Two senders, A and B, want to send data. CDMA assigns the following

unique and orthogonal key sequences: key Ak = 010011 for sender A, key

BK = 110101 for sender B. Sender A wants to send the bit Ad = 1, sender B

sends Bd = 0. To illustrate this example, let us assume that we code a

binary 0 as 1, a binary 1 as +1. We can then apply the standard addition

and multiplication rules. Both senders spread their signal using their key as chipping sequence (the

term spreading here refers to the simple multiplication of the data bit with

the whole chipping sequence). In reality, parts of a much longer chipping

sequence are applied to single bits for spreading. Sender A then sends the

signal As = Ad*Ak = +1*(1, +1, 1, 1, +1, +1) = (1, +1, 1, 1, +1, +1).Sender B does the same with its data to spread the signal with the code:

Bs = Bd*Bk = 1*(+1, +1, 1, +1, 1, +1) = (1, 1, +1, 1, +1, 1).


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CDMA in theory

Both signals are then transmitted at the same time using the same frequency,

so, the signals superimpose in space (analog modulation is neglected in this

example). Discounting interference from other senders and environmentalnoise from this simple example, and assuming that the signals have the

same strength at the receiver, the following signal C is received at a

receiver: C = As + Bs = (2, 0, 0, 2, +2, 0).

The receiver now wants to receive data from sender A and, therefore, tunes

in to the code of A, i.e., applies As code for despreading: C*Ak =(2, 0, 0, 2, +2, 0)*(1, +1, 1, 1, +1, +1) = 2 + 0 + 0 + 2 + 2 + 0 = 6. As

the result is much larger than 0, the receiver detects a binary 1. Tuning in to

sender B, i.e., applying Bs code gives C*Bk = (2, 0, 0, 2, +2, 0)* (+1, +1,

1, +1, 1, +1) = 2 + 0 + 0 2 2 + 0 = 6. The result is negative, so a 0

has been detected.


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CDMA on signal level I

data A

key A

signal A

data key

key

sequence A

Real systems use much longer keys resulting in a larger distance

between single code words in code space.

1 0 1

10 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1

01 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0

Ad

Ak

As


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CDMA on signal level II

signal A

data B

key B

keysequence B

signal B

As + Bs

data key

1 0 0

00 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 111 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1

Bd

Bk

Bs

As


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CDMA on signal level III

Ak

(As + Bs)

* Ak

integrator

output

comparator

output

As + Bs

data A

1 0 1

1 0 1 Ad


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CDMA on signal level IV

integrator

output

comparator

output

Bk

(As + Bs)

* Bk

As + Bs

data B

1 0 0

1 0 0 Bd


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comparator

output

CDMA on signal level V

wrong

key K

integrator

output

(As + Bs)

* K

As + Bs

(0) (0) ?

chap-3 media access

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