ELEC-E7220 Radio Resource and Spectrum Management 5 cr · ELEC-E7220 Radio Resource and Spectrum...

ELEC-E7220 Radio Resource andSpectrum Management 5 cr

Prof. Riku JänttiDepartment of Communications and Networking

E-mail: [email protected]

Spring 2016

2

Radio Resource Management• Radio spectrum is a scarce resource that should be

utilized as efficiently as possible. Consequently, thespectrum must be shard among many users.

• In practice, all sharing (multiple access) techniquescauses interference among the users.

• The objective of Radio Resource Management is tocontrol the division of resources (power, time slots,frequency channels, spreading codes etc.) among theusers such that– The quality of service demands of the users are

satisfied (according to SLA)– The spectral and energy efficiency is maximized– Interference is mitigated– Operator revenue is maximized

Spectrum Management• Spectrum is allocated to different use based on licenses

controlled by national regulators

• When spectrum is shared among multiple systems theirmutual interference need to be controlled.

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Coure overview• Learning Outcomes

The student should understand the principles of sharingthe spectrum among the users and how to mitigate themulti-user interference via resource control such thatquality of service constraints are met while thespectrum efficiency is maximized.

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Spring 2016 OutlineLectures & exercisesPart I: MAC1. Contention based MAC & Random access2. Fairness & scheduling, energy efficiencyPart II: System aspects3. 5G (Ultra dense networks, multiuser MIMO)4. C-RANPart III: Spectrum sharing between systems5. Spectrum sharingGroup workLearning diary

Homework and learning diary are to be done individually,group work is done in a group of two to three persons.

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Grading• Grading rule:

HW Home work grade (rounded average)GW Group work gradeLD Learning diary grade

23.2.2016 6

( )1

2 4roundGrade HW GW LDæ ö

= × ×ç ÷è ø

Lecture 1. Multiple access /Randomaccess

8

Transmitter

ReceiverRadio range

Multiple access control• Wireless transmission is broadcast

in nature. That is more than a singlereceiver can potentially receiveevery transmitted message.

• Transmissions over a broadcastchannel interfere, in the sense thatone transmission coinciding in timewith another may cause none ofthem to be received.

• The success of a transmissionbetween a pair of nodes is notindependent of other transmissions.

• To make a transmission successfulinterference must be avoided or atleast controlled.

• The channel is a shared resourcewhose allocation is critical forproper operation of the network.

• The schemes used for channelaccess are known in literature asMultiple Access Protocols (MAC).

Reuse distance

S-72.3235 NetworkAccess

9

Medium access control protocols• The task of the Medium/Multiple Access Control (MAC)

protocol is to divide the resources between the radiolinks such that– Interference is avoided or kept at controlled level– Utilization of the radio resources is maximized– Quality of service QoS differentiation among the flow

classes is achieved– Fairness inside a QoS class is maintained


10

Medium access control protocols• The operation of the MAC protocol can be

– Centralized such that single entity controls theresource division among the radio links leading toconflict free access

– Decentralized such that each link makestransmission decisions independently leading tocontention based access

• Contention schemes differ in principle from conflict-freeschemes– A transmitting user is not guaranteed to be

successful.– The protocol must prescribe a way to resolve

conflicts once they occur so that all messages areeventually transmitted successfully.


11

Conflict free access• In conflict free protocols, the resource allocation can be

– Static - not dependent on the traffic or channelconditions (TDMA, FDMA, F/TDMA, OFDMA, CDMA,OFCDMA,…)

– Dynamic – based on demand and/or channelconditions• Token passing (Wireless HART)• Channel reservation (satellite systems,

IEEE802.15.4 Beacon mode,…)• Dynamic scheduling (GPRS, WCDMA, WiMAX,

LTE…)• Channel dependent “opportunistic” scheduling

(e.g. CDMA2000 1xEV-DO/DV, HSDPA, LTE)


12

Contention based access• In contention based protocols, the conflicts caused by colliding

packets (interference) must be resolved. Conflict resolutionmethods can be divided into– Static - the actual behavior is not influenced by the

dynamics of the system. The transmission schedule for theinterfering users can be

• Fixed: based on node IDs or priorities• Probabilistic: schedule is chosen from a fixed

distribution (p-persistent CSMA)– Dynamic – the actual behavior of the system depends

system dynamics.• Transmission schedule could be determined by the time

of the arrival• Probabilistic: Transmission schedule depends on the

number of colliding packets (BEB in IEEE802.3 andIEEE802.11)


13

Classification of MAC protocolsMedium Access

Protocols

Contention Conflict free

DynamicResolution

StaticResolution

DynamicResolution

StaticResolution

Time ofArrival

Proba-bilistic

NodeID/

priorities

Proba-bilistic

Reser-Vation

&Schedu-

ling

Tokenpassing

Fixedresource

allocation

Oppor-tunistic


14

Medium access control protocols• The issues affecting the performance of the channel access

– Connectivity• Can all the nodes hear each other or are there hidden terminals?• What is the network topology? Single hop, multi-hop (mesh/ad hoc)

– Channel type• What is the required Signal-to-Interference ratio for correct reception? Is there

possibility for capture in case of collisions?• Do protocol messages get lost due to fading?

– Synchronism• Is the network synchronized, i.e. slotted or can transmissions start and end at

arbitrary time instances.– Feedback information

• Can collisions be detected? Can the colliding nodes be identified?• How much information can be shared among the nodes?• Is correct reception acknowledged by the receiver?

– Traffic• Is the message size fixed or does it vary? Is packets generated randomly or with

steady rate? Can transmission buffers assumed to be saturated (TCP tends tosaturate buffers) or are they likely to be empty at times?

– User population• Is the number of users fixed or random? Can it be known by the system?

– Buffering capability• How many packets can the nodes buffer? Will packets be lost due to buffer

overflow?

Contention based methods• Wireless personal and local area networks

– No centralized control.– MAC protocol is needed to enable co-existence and

sufficient reusedistance– MAC protocols utilize carrier sensing

• Random access channels in cellular systems– A random-access channel (RACH) is a shared

channel used by user equipment to access themobile for call set-up or sometimes also for burstydata transmission.

– Carrier sensing is not possible. Hence, the terminalsattempt transmission when they need to. This leadsto ALOHA

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• Data received was immediatelyre-sent, allowing clients todetermine whether or not theirdata had been receivedproperly.

• Any machine noticing corrupteddata would wait a short timeand then re-send the packet.

ALOHAnet• Aloha was a pioneering

computer networking systemdeveloped at the University ofHawaii in 1970’s.

• The idea was to use radio tocreate a computer networklinking the far-flung campusesof the University.

• The original version of ALOHAused two distinct frequencies ina hub/star configuration, withthe hub machine broadcastingpackets to everyone on thedownlink channel, and thevarious client machines sendingdata to the hub on the uplinkchannel.

Hub

ClientClient

Client

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Infinite user population• Users will generate packets according to Poisson

process• The aggregate packet arrival rate generated by the

users is l

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Pure Aloha

Station A

Station B

0t T+0t 0 3t T+0 2t T+

The packet generated by station B collides ifany station generates a packet on the time intervalt0<t<t0+2T

If station has a packet it will immediately try to transmit it

Packet length is T

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Pure Aloha• Packets arrive to the system with intensity• Packets collide with probability p• In case of collision, the packet will be retransmitted

after random back-off time• Aggregate of new packets and failed packets returning

to the transmitter buffer is still a Poisson process withintensity

ål ( ) 11g p l= -

2l

1 2l l l= +

l

m 2 1pl l=

( ) 11g p l l= - =

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Pure Aloha• Packets arrival process is Poisson with intensity g

packets per second.• Transmission time of the packet is T• Probability that k packets arrive in time window Dt

• Probability that there is no collision is equal to theprobability that no other packets arrive during the timeinterval :

• Normalized load, average number of arrivals in time TG=gT

• Throughput per time T

{ } ( )Pr arrivals in!

kg t g t

k t ek

- D DD =

0 0 2t t t T< £ +{ } { } 2Pr no collision Pr 0 arrivals in 2 g TT e-= =

2GS Ge-=

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Slotted Aloha• Time is slotted. Transmissions in a given slot can collide

only if more than one packet arrived during theprevious slot.

GS Ge-=

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Slotted Aloha

0 0.5 1 1.5 2 2.5 30

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

G

S

Pure ALOHASlotted ALOHA

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Effect of bursty traffic• Bursty arrival in Aloha increases the throughput, since

occasionally the arrival rate is decreased leading to lowcollision probability

10-2 10-1 100 101 102 1030

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Thro

ughp

ut

Offered traffic

MMPP-aMMPP-bPP

lT

a=0.9b=0.1MMPP-a: l2=10l1MMPP-b: l2=0,1l1

Effect of radio channel• Assume that the accessing nodes are uniformly

distributed in the (infinite) area with density r.• Transmission fails if the signal-to-interference ratio

(CIR) is below a given threshold b.• In the Rayleigh fading channel, the probability of

transmission failing (outage) can be calculated usingStochastic Geometry:

where a denotes the pathloss exponent

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{ }2

20

2

Pr SIR 1 exp2sin

p r ap

ab rp bp

a

æ öç ÷ç ÷£ = - -

æ öç ÷ç ÷ç ÷è øè ø

M. Haenggi, ”Outage, Local Throughput, and Capacity of Random WirelessNetworks,” IEEE Transactions on Wireless Communications, Vol. 8, No. 8, August 2009.

Outage probability• Mean number of interferes in a disc of radius r0 around

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20K p rrp=

Effect of radio channel• If the physical layer tolerate low SIR (e.g. use spreading

codes), the ALOHA system performance can beincreased.

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Contention based Random Access in LTE• In LTE Contention based random

Access Procedure is utilized for initialaccess to the network

• The actual Physical Random AccessChannel (PRACH) transmission is anOFDM-based reconstruction of aZadoff-Chu sequence in the timedomain. The OFDM modulator is usedto position the Zadoff-Chu sequence inthe frequency domain (i.e. to place the6RBs of PRACH transmission in the 6consecutive RBs starting from someparticular physical resource.

• If more than one user pick the samesequence, collision will happen.Otherwise, the eNodeB will provideRandom Access Response message.

• The Preamble transmission resemblesmulti-channel ALOHA MAC protocol.

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Contention based Random Access in LTE

• Cellular systems not well suited for short messages– RACH capacity– Large protocol overhead (signaling)

• New protocols are needed to support future massiveMachine Type Communications

23.2.2016

29

Poisson traffic. Packet interarrival time 30 s, 256 and 1024 bit packet sizes

5000 devices / km2

M. Gerasimenko, V. Petrov, O. Galina, S. Andreev and Y. Koucheryavy, ”Impact of machine-type communications onEnergy and delay performance of random access channel in LTE-advanced,” Transactions on emerging telecommunicationTechnologies, April 2013.

New MAC for massive MTC

• MTC traffic– Large number of devices– Low activity– Small message size

• Proposed RACH scheme:– Device register using

normal LTE RACHprocedure.

– Maintain synchronicityand timing advance.

– Certain LTE RB reservedfor MTC contention basedaccess

– Reduced latency: Jointdevice activityrecognition, channelestimation and datareception.

– Multi-user detection

Y. Beyene, C. Boyd, K. Ruttik, C. Bockelmann, O. Tirkkonen and R. Jäntti, “Compressive Sensing for MTC in New LTE Uplink Multi-User Random Access Channel,” In Proc. IEEEAfricon 2015, Addis Ababa, Ethiopia, 14-17 September 2015.


Y. Beyene, N. Malm, J. Kertula, L. Zhou, K. Ruttik, R. Jäntti, O. Tirkkonen and C. Bockelmann, ” Spectrum Sharing for MTC Devices in LTE”, Demo, In Proc.IEEE DySPAN 2015, Stockholm, Sweden, September 29 – October 2, 2015.

CSMA/CA& IEEE802.11 DCF Performance

23.2.2016 37

IEEE802.11 DCF Basic Access Mechanism

• A station with a new packet to transmit monitors the channelactivity.– If the channel is idle for a period of time equal to a

distributed interframe space (DIFS), the station transmits.– If the channel is sensed busy (either immediately or during

the DIFS),• the station persists to monitor the channel until it is

measured idle for a DIFS.• The station generates a random backoff interval before

transmitting to minimize the probability of collision withpackets being transmitted by other stations. (CA = CollisionAvoidance)

• In addition, to avoid channel capture, a station must wait arandom backoff time between two consecutive new packettransmissions, even if the medium is sensed idle in the DIFStime

23.2.2016 38


• For efficiency reasons, DCF employs a discrete-time backoffscale.– The time immediately following an idle DIFS is slotted, and

a station is allowed to transmit only at the beginning ofeach slot time.

– The slot time size, s, is set equal to the time needed atany station to detect the transmission of a packet fromany other station.

• DCF adopts an exponential backoff scheme. At each packettransmission, the backoff time CW is uniformly chosen in therange (CWmin,CWmax).

• The value CW is called contention window, and depends onthe number of transmissions failed for the packet. At the firsttransmission attempt, is set equal to a value CWmin calledminimum contention window. After each unsuccessfultransmission, is doubled, up to a maximum value CWmax.

23.2.2016 39


• The backoff time counter is decremented as long as thechannel is sensed idle, “frozen” when a transmission isdetected on the channel, and reactivated when the channel issensed idle again for more than a DIFS.

• The station transmits when the backoff time reaches zero.• Since the CSMA/CA does not rely on the capability of the

stations to detect a collision by hearing their owntransmission, an ACK is transmitted by the destination stationto signal the successful packet reception.

• The ACK is immediately transmitted at the end of the packet,after a period of time called short interframe space (SIFS).

• The SIFS (plus the propagation delay) is shorter than a DIFS,no other station is able to detect the channel idle for a DIFSuntil the end of the ACK.

23.2.2016 40


• If the transmitting station does not receive the ACK within aspecified ACKTimeout, or it detects the transmission of adifferent packet on the channel, it reschedules the packettransmission according to the given backoff rules.

Station A

Station BACK

DIFSACK

DIFS

0123456

Medium busy

w w-1 w-2

s

Packet A

Packet BPacket B

DIFS

DIFSSIFS

SIFS

Slot time: Slot time:frozen backoff time

23.2.2016 41

IEEE802.11 DCF RTS-CTS Access Mechanism

• A station that wants to transmit a packet, waits until the channel is sensedidle for a DIFS, follows the backoff rules explained above, and then,instead of the packet, preliminarily transmits a special short frame calledrequest to send (RTS).

• When the receiving station detects an RTS frame, it responds, after aSIFS, with a clear to send (CTS) frame.

• The transmitting station is allowed to transmit its packet only if the CTSframe is correctly received.

• The frames RTS and CTS carry the information of the length of the packetto be transmitted. This information can be read by any listening station,which is then able to update a network allocation vector (NAV) containingthe information of the period of time in which the channel will remainbusy.

n A and C want to send to Bn A sends RTS to Bn B sends CTS to An C “overhears” CTS from Bn C waits for duration of A’s

transmissionA B CRTS

CTSCTS

CBA

23.2.2016 42

IEEE802.11 DCF RTS-CTS Access Mechanism

• When a station is hidden from either the transmitting or thereceiving station, by detecting just one frame among the RTSand CTS frames, it can suitably delay further transmission,and thus avoid collision.

• The RTS/CTS mechanism is very effective in terms of systemperformance, especially when large packets are considered, asit reduces the length of the frames involved in the contentionprocess.

• If perfect channel sensing would be possible, then collisioncould only happen among the short RTS packets and thus thetime lost due to collisions would be much shorter compared tothe basic access mechanism.

23.2.2016 43

IEEE802.11 DCF Performance• Consider saturated traffic conditions, in which all the n

users always have packet ready for transmission.• The channel is assumed to be perfect (no packet drop,

no capture)• Let t denote the probability that a given station

transmits a packet in randomly chosen slot time.• A collision happens with probability p.• In order to make the analysis tractable, we make the

approximation that the collision probability is constantand the same for all stations regardless of thetransmissions already suffered.

23.2.2016 44

IEEE802.11 DCF Performance• Let b(t) denote the backoff time counter for a given

station at discrete slot time t. The slot time correspondsto the time instances when the backoff counter value ischanged.

Station A

Station BACK

DIFSACK

DIFS

0123456

Medium busy

w w-1 w-2

s

Packet A

Packet BPacket B

DIFS

DIFSSIFS

SIFS


Station A

Station BACK

DIFSACK

DIFS

0123456

Medium busy

w w-1 w-2

s

Packet A

Packet BPacket B

DIFS

DIFSSIFS

SIFS


t t+1 t+2 t+3 t+4 …

23.2.2016 45

IEEE802.11 DCF Performance• Let us define

– W=CWmin

– CWmax=2m CWmin

– Wi=2iW, i=0,1,2,…m• Let s(t) denote the backoff stage i of the station at slot

time t.• Since the collision probability p is assumed to be

independent of the number of retransmission attempt,we can now model the behavior of the MAC protocol astwo dimensional Markov chain.

23.2.2016 46

IEEE802.11 DCF Performance

G. Bianchi, “Performance of the IEEE802.11 Distributed Coordination Function,”IEEE Journal on Seleceted Areas in Communications, Vol. 18, No. 3, March 2000

i, Wi-2 i, Wi-1

23.2.2016 47

IEEE802.11 DCF Performance• Let P{i,k|j,l}=Pr{s(t+1)=i,b(t+1)=k|s(t)=j,b(t)=l}• The state transition probabilities in the Markov chain

can thus be written as{ }

{ } ( )

{ }

{ }

00

Pr , , 1 1, (0, 2), (0, )

1Pr 0, ,0 1 , (0, 1), (0, )

1Pr , 1,0 , (0, 1), (1, )

1Pr , ,0 , (0, 1)

i

ii

mm

i k i k k W i m

k i p k W i mW

i k i p k W i mW

m k m p k WW

+ = Î - Î

= - Î - Î

- = Î - Î

= Î -

23.2.2016 48

IEEE802.11 DCF Performance• Let

be the stationary state distribution of the Markov chain.• Consider stage 0<i<m

• It follows from (1) that

{ }, lim Pr ( ) , ( ) , (0, 1), (1, )i k t ib s t i b t k k W i m®¥= = = Î - Î

i-1,0

i,1i,0 i,Wi-11

p

1-pp/Wi

Flow out = Flow in

, 1 1,0

, 2 1,0 , 1 1,0

, 1,0

1

1 2

i

i i

i W ii

i W i i W ii i

ii k i

i

b p bW

b p b b p bW W

W kb p bW

- -

- - - -

-

=

= + =

-=

M

,0 1,0 ,0 0,0 , 1,2,..., 1ii i ib pb b p b i m-= Þ = = -

(1)

(2)

23.2.2016 49

IEEE802.11 DCF Performance• For the last stage we have

• It follows from (2) and (3) that

m-1,0

m,1m,0 m,Wm-111-pp/Wm

p/Wm

Flow out = Flow in

( )

( )

( )

, 1 1,0 ,0 1,0 ,0

, 2 1,0 ,0 , 1 1,0 ,0

, 1,0 ,0

1 1 1

1 1 2

m

m m

m W m m m mm m m

m W m m m W m mm m m

mm k m m

m

b p b p b p b bW W W

b p b p b b p b bW W W

W kb p b bW

- - -

- - - -

-

= + = +

= + + = +

-= +

M

( ) 1,0 1,0 ,0 ,0 1,0 0,0 0,01 1 1

mm

m m m m mp p pb p b b b b p b b

p p p-

- -= + Þ = = =- - -

(3)

(4)

23.2.2016 50

IEEE802.11 DCF Performance• For the first stage i=0, we have

• It thus follows from (5) that

0,10,0 0,W0-11

p

1-p

i,0

( )

( ) ( )

( ) ( )

0

0 0

0, 1 ,000

0, 2 0, 1 ,0 ,00 00 0

00, 0, 1 ,0 ,0

0 00 0

11

1 21 1

11 1

m

W ii

m m

W W i ii i

m m

k k i ii i

b p bW

b b p b p bW W

W kb b p b p bW W

-=

- -= =

+= =

= -

= + - = -

-= + - = -

å

å å

å å

M

( ) ( )0,0

0,0 ,0 ,00 0

11

m m

i ii i

bb p b b

p= =

= - Þ =-å å

(5)

(6)

23.2.2016 51

IEEE802.11 DCF Performance• The state probabilities must sum to 1

• The probabilities must sum to 1

, 1,0, ,0 0,0

,0 0,0

ii k i ii i

i i k ii i i

i

W kb p b W k W kW b b p bW Wb p b

-

-ì = - -ï Þ = =íï =î

( ) ( )

( )( )

1 1 1

, ,0 ,0 ,00 0 0 0 0 0 0

10,0

,00 0

0,0

12

22 1 12 12 2 1 1

2(1 2 )(1 )(1 2 )( 1) 1 2

i i iW W Wm m m mi i i

i k i i ii k i k i k ii i

mim mi

ii i

m

W k W k Wb b b bW W

b pWb p Wp p

p pbp W pW p

- - -

= = = = = = =

-

= =

- - += = =

æ öæ ö+= = + + =ç ÷ç ÷ ç ÷- -è ø è ø

- -Þ =

- + + -

åå åå å å å

å å

1

( 1)2

K

k

K Kk=

+=å

1

0

11

NNn

n

xxx

-

=

-=

-å

23.2.2016 52

IEEE802.11 DCF Performance• We can now express the probability t that a station

transmits during a randomly chosen slot time. Thetransmission occurs when the backoff time counter isequal to zero, regardless of the backoff state, hence

• The probability that there is collision is that more thanone user tries transmission at the same slot time:

( )( )0,0

,00

2(1 2 )1 (1 2 )( 1) 1 2

m

i mi

b pbp p W pW p

t=

-= = =

- - + + -å

( ) 11 1 np t -= - -

23.2.2016 53

IEEE802.11 DCF Performance• Probability that at least one user transmits

• Probability of successful transmission conditioned thattransmission takes place

( )1 1 ntrP t= - -

{ }{ }{ }

( )( )

1

Pr exactly one user transmit at least one user transmit

Pr exactly one user transmit 1Pr at least one user transmit 1 1

s

n

n

P

nt t

t

-

=

-= =

- -

23.2.2016 54

IEEE802.11 DCF Performance• The throughput is

where– d is the slot length (propagation delay)– Ps is the probability of successful transmission in a busy

period– Ptr is the probability that there is transmission at given slot– E{P} is the expected length of the packet– Ts is the average time the channel is busy in case of

successful transmission– Tc is the is the time the channel is busy in case of collision

{ }( ) ( )

E1 1

tr s

tr tr s s tr s c

P P PS

P P PT P P Ts=

- + + -

23.2.2016 55

IEEE802.11 DCF Performance• Basic access method:

where H denotes the totallength of PHY and MACheaders

• RTS-CTS handshake

E{ }

E{ }

s

c

T H P SIFSACK DIFS

T H P DIFS

ss

s

= + + ++ + += + + +

E{ }s

c

T RTS SIFS CTS SIFSH P SIFS

ACK DIFST RTS DIFS

ss

ss

= + + + + ++ + +

+ + += + +

G. Bianchi, “Performance of the IEEE802.11 Distributed Coordination Function,”IEEE Journal on Seleceted Areas in Communications, Vol. 18, No. 3, March 2000

23.2.2016 56

IEEE802.11 DCF Performance• The throughput can be written as

Hence, the throughput is maximized if

is maximized.Maximum occurs when

{ }( ) ( )

E1 1tr tr s c

ss

PS

P P P TT

Ps

=- + -

+

( ) ( )( )

( )

11

1 1 1 1

ns

nc c ctr tr s

nPT T TP P P

t t

td d d

--=

æ ö- + - - - -ç ÷è ø

( ) ( )( )( )1 1 1 0n ncT nt t td

- - - - - =

23.2.2016 57

IEEE802.11 DCF Performance• For low load

• The transmit probability that maximizes throughput isgiven by

• Hence, the throughput depends only on the networksize n and on the system parameters m and W.

( )

( )*

2 1 11 1

1 12

c

c c

Tn n

nT Tn n

d

t

d d

æ ö+ - -ç ÷è ø -

= »æ ö- -ç ÷è ø

1t <<

( ) 2( 1)1 12

n n nnt t t-- » - +

23.2.2016 58

IEEE802.11 DCF Performance• The transmission probability depends on the parameters

m and W which should be tuned such that

• For m=0, we have

Hence, the window size should be selected as

That is, the optimal performance is obtained when thewindow size depends on the number of users and theaverage transmission time of the packets.

( )( )2(1 2 ) 1

(1 2 )( 1) 1 22

mc

pTp W pW p n

t

d

-= »

- + + -

2( 1)W

t =+

2 cTW nd

»

Impact of contention window length• A simple simulation of

DCF– 802.11a and fixed CW for

an increasing number ofSTAs,

– Under saturation

– For a given number ofSTAs,too small CW yieldsmany collisionstoo large CW yields manyidle slots

– Cost for a collision is highsince the whole packet istransmitted

– ARQ is needed to detectthe collisions

– Capacity is wasted

23.2.2016 59

0 50 100 1500.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

Number of users

Cha

nnel

Util

izat

ion

CW = 127CW = 255CW = 511CW = 1023

contributed by Thomas Nilsson, Umeå University

23.2.2016 60

IEEE802.11 DCF Performance• In practice, it is very difficult to estimate n.• BEB can be viewed as an attempt to estimate the

optimal window size, but it does not do a very good jobfor large n.

0 50 100 150 200 250 300 350 400 450 5000.4

0.5

0.6

0.7

0.8

0.9

1

number of users

chan

nelu

tiliz

atio

n 802.11 BEB

802.11 CWopt

• A comparison between– 802.11 BEB (normal)– 802.11 CWopt, uses

the optimal CW– BEB used in 802.11 is

far from optimal,– especially for large

number of users– for few stations the

result is satisfactory

IEEE802.11a

contributed by Thomas Nilsson, Umeå University

References• G. Bianchi, “Performance of the IEEE802.11 Distributed

Coordination Function,” IEEE Journal on Selected Areasin Communications, Vol. 18, No. 3, March 2000

More elaborated model with retry limit can be found from

• Y. Xiao, “Performance Analysis of Priority Schemes forIEEE 802.11 and IEEE 802.11e Wireless LANs,” IEEETransactions on Wireless Communications, Vol. 4, No.4, July 2005

23.2.2016 61

IEEE802.11 DCF in Radio Channel• BEP reduces the density of active transmitters even

further compared to basic CSMA

23.2.2016 62

Effectivedensity

Thick blue line = CSMAhttp://arxiv.org/abs/1211.7139

ELEC-E7220 Radio Resource and Spectrum Management 5 cr · ELEC-E7220 Radio Resource and Spectrum...

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