Motivation for media access control

Multiuser Access

MOTIVATION

Motivation for media access control

S

₁

T

₁

S

₂

T

₂

collision domain

Infrastructure based

with a coordinating base station or ad-hoc without infrastructure

Base station

( )

Motivation

Can media access procedures be taken over from fixed networks?

Example CSMA / CD

 Carrier sense multiple access with collision detection

 Send as soon as the medium is free, hear whether a collision took place (original procedure in Ethernet IEEE802.3)

Wireless network problems

 Signal strength decreases quadratically or more with distance

 CS and CD would be used at the transmitter, but collision happens at the receiver

 This means that the collision may no longer be audible at the transmitter, i.e. CD fails

 Furthermore, CS can also give incorrect results, e.g. if a device is "hidden“

or “exposed”

Or for example CDMA systems and the near far problem

End user devices A and B are sending, C should receive

 the signal strength decreases quadratically or more with the distance

 therefore the signal from device B "drowns out" that from device A

 C cannot hear A



If, for example, C gave transmission rights, B could physically overrule station A.

Also a big problem for CDMA networks - exact power control is necessary!

Near/Far Problem

A B C

Dimensions to support multi user access

SDMA (Space Division Multiple Access)

 Division of space into sectors, directional antennas

 see. cell structure

FDMA (Frequency Division Multiple Access)

 Time-controlled assignment of a transmission channel to a frequency

 permanent (e.g. radio), slow hopping (e.g. GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum)

TDMA (Time Division Multiple Access)

 Time-controlled access right of a transmission channel to a fixed frequency

CDMA (Code Division Multiple Access)

 Use of orthogonal codes in spread spectrum to allow several transmissions in the same frequency spectrum at the same time

The basis are the multiplex methods SDM, FDM, TDM and CDM

r₂

r₃ r₁

Multiplexing

Multiplexing in 4 dimensions:

 space (r_i)

 time (t)

 frequency (f)

 code (c)

Goal: multiuser of a

shared mediums

^f

t c

k₂ k₃ k₄ k₅ k₆ k₁

f t

c

f t

c channels k_i

Recap Antennas: directional and with sectors

side view (xy plane) x y

side view (yz plane) z y

top view (xz plane) x z

top view, 3 sectors x z

top view, 6 sectors x z

Frequently used antenna types for direct microwave connections and base stations for mobile radio networks (e.g. coverage of valleys and street canyons)

directed antenna

sector antennas

Frequency multiplex

The entire available bandwidth is divided into individual frequency sections

Transmission channel occupies frequency section over the entire period

Benefits:

- no dynamic coordination necessary

- also for analog signals Disadvantage:

- Bandwidth waste with uneven load

- inflexible

k₂ k₃ k₄ k₅ k₆ k₁

f

t

c

f

t

c

k₂ k₃ k₄ k₅ k₆ k₁

Time multiplex

Channel occupies the entire frequency space for a certain period of time

Benefits:

- only one carrier on the medium in a period of time

- Throughput remains high even with a large number of

participants Disadvantage:

exact synchronization

necessary

Code multiplex

Transmission is assigned by personal code All participants can at the same time

transmit in the same frequency segment Benefits:

- Bandwidth efficiency

- no coordination and synchronization necessary

- Protection against interference Disadvantage:

- complex due to signal regeneration Realization: spread spectrum technology

k₂ k₃ k₄ k₅ k₆ k₁

f

t

c

f

Time- und frequency multiplex

Combination of the above procedures

Transmissions occupy a frequency segment for a period of time Example: GSM

Benefits:

- relatively safe against evesdropping - Protection against interference

but: precise coordination required

t

c

k₂ k₃ k₄ k₅ k₆ k₁

Cognitive Radio

Typically in the form of a spectrum sensing CR

 Detect unused spectrum and share with others avoiding interference

 Choose automatically best available spectrum (intelligent form of time/frequency/space multiplexing)

Distinguish

 Primary Users (PU): users assigned to a specific spectrum by e.g. regulation

 Secondary Users (SU): users with a CR to use unused spectrum

Examples

 Reuse of (regionally) unused analog TV spectrum (aka white space)

 Temporary reuse of unused spectrum e.g. of pagers, amateur radio etc.

PU PU

PU PU

SU

SU SU

f

PU

PU

PU PU PU PU

SU PU

SU

SU

SU SU SU

SPREAD SPECTRUM

Spread spectrum technique

Problem of wireless communication: frequency selective fading occasionally erases narrow band signal

Solution: spreading the signal based on a code sequence to a broader spectrum

Protection against narrowband signal cancellation and errors

Protection against narrow band disturbance

side effect:

 coexistence of more than one communication without the need for dynamic coordination

 protection against eavesdropping

Two general solutions: direct sequence, frequency hopping

detection at receiver disturbing

signal

spreaded used signal

used signal

spreaded disturbing signal

Spreading and frequency selective fading

frequency channel

quality

1 2

3

4

5 6

narrowband signals

protection

22 22 2

frequency channel

quality

1

spreaded signals

narrowband channels

spreaded channels

DSSS (Direct Sequence Spread Spectrum) I

XOR of the signal with a pseudo random number (chipping sequence)

 many chips per Bit (e.g. 128) imply a higher bandwidth required for the signal

Advantages

 reduces frequency selective fading

 in cellular networks

 base stations can use the same frequency range

 several base stations can detect and reconstruct a signal

 soft handover

Disadvantage

 precise power control required

used data

chipping sequence

resulting signal

0 1

0 1 1 0 1 0 1 0 1 1 0 1 0 1

XOR

0 1 1 0 1 0 1 1 0 0 1 0 1 0

= t_b

t_c

t_b: bit duration t_c: chip duration

DSSS (Direct Sequence Spread Spectrum) II

X data

chipping sequence

modulator carrier

frequency spreaded

signal transmitted

signal

sender

demodulator received

signal

carrier frequency

X chipping sequence

lowpass filterted signal

receiver

integrator product

decision

data sum

correlator

FHSS (Frequency Hopping Spread Spectrum) I

discrete jumps between different carrier frequencies

 sequence determined by a sequence of pseudo random numbers

Two versions

 fast changes (fast hopping)

several frequencies for one data bit

 slow changes (slow hopping)

several data bits per current carrier frequency

Advantage

 frequency selective fading and interference limited to short periods

 simple implementation

 uses only a narrow range of the spectrum at each time instant

Disadvantages

 not as robust as DSSS

 eavesdropping is easier

FHSS (Frequency Hopping Spread Spectrum) II

daten

slow hopping (3 bit/hop)

fast hopping (3 hops/bit)

0 1

t_b

0 1 1 t

f

f₁ f₂ f₃

t t_d

f

f₁ f₂ f₃

t t_d

t : bit period t : dwell time

FHSS (Frequency Hopping Spread Spectrum) III

modulator data

jump sequence modulator

narrowband signal

spreaded signal

sender

received signal

receiver demodulator

data frequency

synthesizer

jump sequency

demodulator

frequency synthesizer

narrowband signal

Code Division Multiple Access

General Idea

Chip Sequence

11

01

s₁ and s₂ Frequency

Time Bit Sequence

11

01

r₁ s₁

Media access procedure CDMA

CDMA (Code Division Multiple Access)

 all stations operate on the same frequency and thus use the entire bandwidth of the transmission channel at the same time

 Signal is combined on the transmitter side with a pseudo random number that is unique to the transmitter (XOR)

 Receiver can restore the original signal using the known transmitter pseudo random sequence and a correlation function

CDMA in der theory

Sender A

 transmits A_d = 1, chipping sequence A_k = 010011 (use: „0“= -1, „1“= +1)

 transmit signal A_s = A_d * A_k = (-1, +1, -1, -1, +1, +1)

Sender B

 transmits B_d = 0, chipping sequence B_k = 110101 (use: „0“= -1, „1“= +1)

 transmit signal B_s = B_d * B_k = (-1, -1, +1, -1, +1, -1)

Both signals are superimposed in the air

 other disruptions neglected here (noise etc.)

 A_s + B_s = (-2, 0, 0, -2, +2, 0)

Receiver wants to receive transmission of A

 apply chipping sequence A_k bitwise (inner product)

 A_e = (-2, 0, 0, -2, +2, 0)  A_k= 2 + 0 + 0 + 2 + 2 + 0 = 6

 Result is greater than 0, thus the transmitted bit was „1“

 analog B

 B_e = (-2, 0, 0, -2, +2, 0)  B_k = -2 + 0 + 0 - 2 - 2 + 0 = -6, thus „0“

CDMA – at signal level I

In practice, longer keys are used in order to achieve the greatest possible distance in the code space.

1 0 1

1

0 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1

0

1 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0

data A code A

signal A daten  code code-data A

A_d

A_k

A_s

CDMA - at signal level II

1 0 0

0

0 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1

1

1 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1

signal A data B code B code-data B

signal B

A_s + B_s data  code

B_d

B_k

B_s A_s

CDMA - at signal level III

1 0 1

A_k

(A_s + B_s) * A_k integrator output comparator output A_s + B_s

1 0 1

data A A_d

CDMA - at signal level IV

1 0 0

integrator output comparator output B_k

(A_s + B_s) * B_k A_s + B_s

1 0 0

daten B B_d

CDMA - at signal level V

(0) (0) ?

comparator output wrong code K

integrator output (A_s + B_s) * K A_s + B_s

Benefits and disadvantages of CDMA benefits

 Robustness due to frequency diversity

 everyone can transmit on the same frequency, no frequency planning

 very large code space (e.g. 2³²) compared to the frequency space

 Interference (white noise) not coded

 Multipath resistance

 Privacy (encryption can be easily integrated)

 Forward correction can be easily integrated

 Graceful degradation

disadvantage

 Self-jamming

 Near-far problem (all signals must be of the same strength at the receiver)

 More complex handoff and higher implementation complexity due to signal regeneration in general

CDMA: RAKE Receiver





Orthogonal‐Frequency‐Division‐Multiplexing

Orthogonal Frequency Division Multiplexing

used bandwidth?

bit rate per subcarrier?

main advantage:

• Frequency selective 

disturbance (fading) affects  only a few bits (error 

correction)

• Inter‐symbol interference 

significantly reduced. What is 

the bit time per channel?

What means orthogonality in OFDM?

Orthogonal Frequency Division Multiple Access

Contention based media access

Motivation

• Nodes in a network might dynamically appear and  disappear; examples:

– Ad‐hoc scenario 

– Mobile nodes in a cellular network

• Nodes have to contend for the medium

– without coordinator (base station): always – with coordinator:

• at least at the beginning but

• for short packages overhead of coordinated media access 

might not payoff

ALOHA and slotted ALOHA

ALOHA

Slotted‐ALOHA improvement

Start transmission whenever there is a data packet

However, start the transmission only at the beginning of fixed time slot

time time slot

packet arrival packet transmission

Comparison between ALOHA and Slotted‐

ALOHA

1 G

S 1

the ideal

Carrier Sense Multiple Access

Possible Solution: CSMA

S

₁

T

₁

S

₂

T

₂

Wants to send Wait

CSMA Problem: Collision Still Possible

S

₁

T

₁

S

₂

T

₂

Wants to send Collision

t

₁

t

₂

t

₃

Solution: Collision Detection

S

₁

T

₁

S

₂

T

₂

Detect Collision Detect Collision

Cancel Transm. Cancel Transmission

Problem: sending and listening for other transmissions on the same channel

is not applicable in wireless communication

Solution: Collision Avoidance (1/2)

S

₁

T

₁

S

₂

T

₂

Signal first Wait

t

₁

t

₃

Send if no signal

received

t

₂

Wants to send

Wait

t

₄

Solution: Collision Avoidance (2/2)

S

₁

T

₁

S

₂

T

₂

Signal first Wait

t

₁

t

₃

Don’t send

but wait Don’t send

but wait Signal first

Wait

t

₂

The Hidden Terminal Problem

S

₁

T

₁

S

₂

T

₂

CSMA does not prevent S

₂

from sending

Collision

The Exposed Terminal Problem

S

₁

T

₁

S

₂

T

₂

CSMA prevents S

₂

from sending

Busy Tones

Data Frequency

Busy Tone Frequency

S

₁

T

₁

S

₂

T

₂

Busy tone

while reception

t

₁

t

₂

Other nodes are blocked while receiving busy tone Data

transm.

BT and the Hidden Terminal Problem

S

₁

T

₁

S

₂

T

₂

Busy tone prevents S from sending

Busy Tone

BT and the Exposed Terminal Problem

S

₁

T

₁

S

₂

T

₂

Busy tone does not prevent S

₂

from sending

Busy Tone

The Problem with Busy Tones (1/2)

S

₁

T

₁

S

₂

T

₂

Data and busy tone frequency are subject to different fading and attenuation characteristics. Busy tone may possibly be

Busy Tone

Collision

The Problem with busy Tones (2/2)

S

₁

T

₁

S

₂

T

₂

Busy tone may reach a node S which

Busy Tone

MACA – preventing collisions with RTS and CTS

MACA (Multiple Access with Collision Avoidance) uses short signaling packages to avoid collisions

 RTS (request to send): Request from a sender to a receiving node before a packet can be sent

 CTS (clear to send): Confirmation of the receiving node as soon as it is ready to receive

Signaling packages include:

 sender address

 receiver address

 packet size

Variants of this procedure are used in IEEE802.11 as DFWMAC

(Distributed Foundation Wireless MAC)

Example:CSMA & RTS/CTS

S

₁

T

₁

RTS

CTS

Data

NAV indicates busy medium Respect CTS

response time

MACA variant: DFWMAC in IEEE802.11

silence

wait for send permission

wait for ack

sender receiver

paket ready to send; RTS time-out;

RTS

CTS; data ACK

RxBusy

silence

wait for data

RTS; RxBusy RTS;

CTS data;

ACK

time-out  data;

NAK

ACK: positive acknowledgment RxBusy: receiver busy time-out 

NAK;

RTS

RTS/CTS and the HT Problem

S

₁

T

₁

S

₂

T

₂

CTS prevents S

₂

from sending

RTS

CTS CTS

RTS/CTS and the ET Problem

S

₁

T

₁

S

₂

T

₂

S does not hear CTS and is not blocked by NAV

RTS CTS

HT Problem Always Resolved?

S

₁

T

₁

T

₂

S

₂

RTS

CTS

Data

RTS

CTS

Data

S

₁

T

₁

S

₂

T

₂

Side note: How to Assess a Clear Channel?

occupied free

Ideally The reality

Side note: Use Threshold dBm Value

How to reduce many false negatives on free channel?

free occupied free

Side note: Removing Outliers

Observation: Valid packet will not have an outlier significantly below noise floor

Sample n times (e.g. 5 times)

 No outlier found: channel is busy

 Else: channel is free

t₁ t₂ t₃ t₄ t₅

Noise floor

s₁

s₂

s₃

s₄

s₅

Side note: Example

Problem: Sync of Deterministic RTS Retry

RTS RTS

RTS RTS

CTS CTS

Expected CTS Retry

First Round

s

₁

t s

₂

Collision

Collision

Solution: Add Random Component

RTS RTS

RTS

RTS

CTS CTS

Expected CTS Random Backoff First Round

s

₁

t s

₂

Collision

Random Backoff

CTS

Waiting how long?

RTSRTS RTSCTSCTS RTS CTS CTS CTSCTS CTSCTSRTS RTS RTS

RTS RTS RTS CTS

Waiting too short

Data

CTS

RTSRTS CTSCTS RTS

RTS CTS Data

CTS Data

t s₁

s t s₁

s₂

2 ¢ data waste

RTS CTS Data

Binary Exponential Backoff

success?

if (b ꞏ b_max ) { b = b ¢ 2 }

b = b₀

RTS/CTS within [0,b]

no

yes

Data transmission

CELLULAR NETWORKS

Cell structure

Realization of space multiplex: Base stations each cover a certain spatial area (cell)

Mobile stations only communicate via base stations Advantages of the cell structure:

•

more capacity, more participants reachable

•

less transmission power necessary

•

more robust against failures

•

more manageable propagation conditions problems:

•

Network for connecting the base stations

•

Handover (transition between two cells) necessary

•

Interference with in other cells

•

Concentration in certain areas

Cell size from e.g. 100 m (city) to 35 km (rural area) with GSM (even

Macro cells and micro cells

Cells with particularly small coverage are also known as micro‐

cells. Otherwise one speaks of macro cells.

Typical parameters [1]

(Delay spread = time between the first and last reception of a  signal in the case of multipath propagation)

macro cells micro cells

cell radius 1 – 20 km 0,1 – 1 km

transmit power 1 – 10 W 0,1 – 1 W

average delay spread 0.1 – 10 s 10 – 100 ns

maximum bit rate 0.3 Mbps 1 Mbps

Using the available spectrum in cellular networks

• Frequency multiplex

– e.g. used in GSM

– e.g. OFDM used in LTE

• Time multiplex

– e.g. GSM uses time and frequency

• Code multiplex

– e.g. UMTS uses 

• Space multiplex

– basically using cells is already space multiplex

– in addition cell splitting 

Ideal cell geometry

•

First consider three transmitters s₁, s₂, s₃, which should completely cover  the triangle D enclosed by them.

•

Assume every transmitter has the same maximum transmission range r.

•

How should the transmitters by positioned so that the area of D is  maximized?

Ideal cell geometry

• Now continue this infinitely for the subsequent regions.

• What does the region R of the points around a transmitter s that are closest to s look like with this transmitter 

positioning?

Ideal cell geometry

Conclusion: hexagonal cell geometry satisfies

• maximizes the covered area with a fixed number of  transmitters or

• minimizes the number of transmitters required to cover a  given area

comment

• Each base station costs money. The hexagonal cell geometry  is therefore useful for cell planning.

• Attention, idealized cell geometry: the same maximum 

transmission range for each base station is an idealized 

FREQUENCY PLANNING AND 

FREQUENCY REUSE

Frequency planning I

Frequencies can only be reused if there is a sufficient distance between the cells or the base stations

Model with 7 frequency ranges:

Fixed channel assignment:

•

certain amount of channels assigned to a certain cell

•

Problem: change in load on the cells Dynamic channel allocation:

•

Channels of a cell are selected depending on the already assigned channels of the neighboring cells

•

more capacity in areas with higher demand

•

assignment based on interference measurements also possible

k4 k5 k1 k3

k2 k6 k7 k3

k2 k4

k5 k1

Frequency planning II

f₁ f₂ f₃ f₂

f₁ f₁ f₂ f₃

f₂ f₃

f₁ f₂ f₁ f₃ f₃

f₃ f₃

f₃

f₄ f₅ f₁ f₃

f₂ f₆ f₇ f₃

f₂ f₄

f₅ f₁ f₃ f₅ f₆

f₇ f₂

f₂

f₁

f₁ f₂ f₁

f₃ f₂

f₃

f₂ f₃ h₁h₂

h₃ g₁g₂

g₃ h₁h₂

h₃ g₁ g₂

g₃ g₁g₂

g₃

3 cells/cluster

7 cells/cluster

3 cells/cluster plus 3 sectors/cell

HANDOFF IN CELLULAR

NETWORKS

Handoff (aka. handover)

Handoff – process to pass a mobile device from one cell to an  adjacent one

• Network‐initiated – only based on measurements of the  received signals from the mobile station

• Mobile unit supported – signal strength measurements on 

the mobile station are fed back to the base station

Handoff (aka. handover)

General parameter for handoff decisions ‐ signal strength (averaged) Handoff Strategies

• Relative signal strength

• Relative signal strength with threshold

• Relative signal strength with hysteresis

• Relative signal strength with hysteresis and threshold

• Prediction techniques

Remark: Handoff is even more complicated due to transmission power control

Hard- und Soft-Handoff



Handoff procedures in TDMA and FDMA always such that a device is connected to a base station (hard handover).



The above-described idea for RAKE-Receiver can also be transferred to handoff in the CDMA case (for soft handover)



When a mobile device can receive several base stations well

• Signals sent by the mobile device are received by all of these base stations and forwarded to the mobile switching station; The mobile switching station combines the signals (e.g. selection combining)

• The same goes in the opposite direction. All base stations transmit with the code of the mobile station. The mobile station can also combine the signals

(Compare with RAKE receiver on the previous slide)

Handover types



Inter-system (e.g. WCDMA and GSM)



Inter-frequency (needed at different cell layers or at hot spots)



Intra-frequency (what we look at here)

• Soft handover

• Softer handover

GSM GSM GSM GSM

WCDMA WCDMA WCDMA

GSM GSM

capacity extension coverage extension

F1 F1 F1 F1

F2 F2

handover at hot spot

F1 F1 F1 F1

F2 F2 F2 F2 F2 F2 F2

handover to support macro and micro layers

The idea of soft handover

 Exploiting multi path/antenna diversity (Macro diversity)

 Uplink

• No additional signal is transmitted

• In principal, always increases performance

 Downlink

• Each link causes interference at other users

• Trade-off

NodeB1

NodeB2

Soft handover: the downlink perspective

 Maximal ratio combining (MRC) in the rake receiver

 Recall: MRC used to exploit multi path diversity

NodeB1

NodeB2

Soft handover: the uplink perspective



Selection combining (SC) in the RNC



Target SIR decided after SC

NodeB1

NodeB2

NodeB1 NodeB2 SC

frame with CRC

frame with CRC RNC

Softer handover

 Sectored antenna

 Downlink: similar to soft handover

 Uplink: the more effective MRC NodeB

Ingredients of the soft handover procedure

cell 1

cell 2

cell 3

CPICH E_c/I₀  Measurement quantity, e.g.

CPICH E_c/I₀

 Active set: soft handover connection of UE

 Neighbor/monitored set: set of cells that UE can measure

 In the following example the active set size is 2

Adding a cell to the active set

cell 1

cell 2

cell 3



_add _add = reporting_range –

hysteresis_event1A

= window_add Active set is

not full Best

pilot

Replacing a cell in the active set

cell 1

cell 2

cell 3

 

Worst pilot in full active set Best candidate pilot

_replace

Removing a cell from the active set

cell 1

cell 2

cell 3

  

_remove

Best pilot

_remove = reporting_range + hysteresis_event1B

= window_drop

POWER CONTROL

The near-far problem of CDMA



Large area may become blocked



Need to balance emitted power



Assume for now a target SIR for each UE



Goal: minimum TX power to keep the SIR

NodeB

Fast fading spoils our plans

The solution: fast close loop power control

NodeB

execute in NodeB at rate 1.5kHz:

foreach UE i assigned to NodeB

estimate SIR_est after rake combining if SIR_est > SIR_target then

generate TPC “DOWN” command for i if SIR_est ≤ SIR_target then

generate TPC “UP” command for i

Compensates a fading channel

Further remarks



And the downlink? basically the same…



A short reflection: closed loop power control

• Tight interaction between sender and receiver

• Useful for an interaction period



What if sender and receiver are not connected so far?



Example random access on RACH for

• Initial access

• Short packages



Open loop power control…

Open loop power control

 Transmit power needs to be known to UE

 Inaccurate! Fast fading between uplink and downlink is uncorrelated in WCDMA FDD

 Does not consider interference at receiver

 (Use power ramping to avoid excessive interference)

NodeB

• estimate path loss

• adapt power

• estimate path loss

• adapt power

How to choose the right target SIR?



Adjust target SIR to meet the link quality



Consider quality as BER or BLER



SIR for quality depends on

• Mobiles speed

• Multipath profile



Adjust SIR to the worst case?

• Unnecessary high SIR wastes capacity

• Desirable: minimal SIR which fulfils the quality requirement



How to find such SIR?

Finding the target SIR: outer loop power control



Similar method for the downlink



Downlink method resides in UE



Why is uplink handled in RNC?



Soft handover combining! …

NodeB

execute in RNC at rate of max 100Hz:

foreach UE i assigned to a NodeB

determine the quality from CRC attachment if quality better than required then

decrease SIR_target = SIR_target – Δ_down else

increase SIR_target = SIR_target + Δ_up

Radio Network

target SIR adjustment frame reliability

information

Cell breathing

Example GSM (no cell breathing)

 The terminal receives full power from the base station

 The number of registered devices has no influence on the cell size

Example UMTS (cell breathing)

 Cell size is closely correlated with the capacity of the cell

 Capacity is determined by the signal-to- noise ratio

 Noise is caused by interference

 other cells

 other participants

 Interference increases noise

 Terminal devices at the cell boundary cannot further amplify the signal (due to the transmission power limitation)  no communication possible

 Limitation of the number of participants necessary

 Cellular breathing makes network planning

Cell breathing: example