Resource Allocation Mechanisms - INSA Lyon

research & development

Resource Allocation MechanismsOverview & Fundamentals

Dr Mischa DohlerSenior ExpertFT R&D

5 December 2006

MastersPHY – Mischa Dohler – 2/45 research & development France Telecom Group

� Review and Overview

� Resource Allocation Dimensions

� Ideal and Existing Systems

� Capacity and Closing Remarks

1

2

3

Lecture's Outlook

4


1Review and Overview


Semester Review/Overview

� Introduction to Wireless Communication Systems� overview of the topic [JMG]� validity of assumptions [JMG]

� Point-to-Point Communication Modelling� wireless channel characteristics [MD]� error rates and PHY-layer modelling [JMG]

� System Modelling and Behaviour� resource sharing and MAC-layer [MD]� interference modelling [JMG]

� Practical System Studies� wireless local area network [JMG]� wireless sensor network [MD]

� Chosen Topics� scientific paper presentations [students]


Review of Last Lecture

� The sum in dB (i.e. product in linear scale!) of pathloss (blue), shadowing (red), fading (green) is our total wireless channel (black).

1000 2000 3000 4000 5000 6000 7000 8000 900010000-140

-120

-100

-80

-60

-40

-20

0

20

Distance [log of meter]

Rec

eive

d P

ower

[dB

W]

fading +

shadowing +

pathloss =

total channel


Aim of Today's Lecture [1/2]

� Modern communication systems are built in a cellular and hierarchical fashion:

Global

Suburban

Macro-Cell

Urban

Micro-Cell In- Building

Pico-Cell

Home-Cell


Aim of Today's Lecture [2/2]

� Conflicts are managed by an entity called 'Medium Access Control' or simply MAC.

� MAC is situated at the Data Link Layer (Layer 2) in the OSI Model.

� A MAC protocol manages many things at the same time:� resource allocation (i.e. which resources a user may use)

� division between up and downlink (i.e. whether the same frequency or different)� coordination of access (i.e. how each user can use the wireless medium)

� Today, we would like to understand how the MAC works and how allof us can communicate wirelessly without creating conflicts.


2Resource Allocation Dimensions


Allocation in Time – TDMA [1/3]

� We assign each link a non-overlapping time-slot.



� It is known as Time Division Multiple Access (TDMA).

� It has the following basic properties:� entire system needs to be time synchronised� no overlap of time-slots among users (no intra cell interference)

� adjacent cells cannot use same slot at same frequency (no inter cell interference)

� It has the following advanced properties:� modern TDMA systems have many MPCs (need of equalizer)

� channels between users are independent (multi-user diversity)� different propagation delays between users (need of guard-periods)

� Main problem of TDMA is cell roll-out pattern to minimise interference.



� Frequency re-use pattern of 7 with hexagonal cells:

. F

. G

A .

. E

. D

. B

. C

. G

. F

. E

. D

. B

. A

. G

. B

Cell 8

Cell 1

MEAN RE-USE DISTANCE


Allocation in Frequency – FDMA [1/2]

� We assign each link a non-overlapping frequency-band.


Allocation in Frequency – FDMA [2/2]

� It is known as Frequency Division Multiple Access (FDMA).

� It has the following basic properties:� no time synchronisation is needed� no overlap of frequency-bands among users (no intra cell interference)

� adjacent cells cannot use same frequency at same time (no inter cell interference)

� It has the following advanced properties:� channels between users are independent (multi-user diversity)

� band filters are imperfect (need of guard-bands)

� FDMA systems are used with other access methods (e.g. TDMA).


Allocation per Subcarrier – OFDMA [1/3]

� We assign each link a set of orthogonal subcarriers.



� The block diagram and spectral behaviour are as follows:



� Known as Orthogonal Frequency Division Multiple Access (OFDMA).

� It has the following basic properties:� subcarriers can be allocated in dependency of users, QoS, etc.� bases on OFDM, hence loss of spectral efficiency due to cyclic prefix

� adjacent cells cannot use same frequency at same time (no inter cell interference)

� It has the following advanced properties:� there is no intersymbol interference per subcarrier (simple receivers)

� it is very susceptible to timing and frequency offsets

� subcarriers between users are independent (multi-user diversity)

� OFDMA requires highly linear amplifiers (problem of PAPR).


Allocation per Code – CDMA [1/5]

� We assign each link an orthogonal code sequence.



� We multiply ('spread') user signal by code sequence:

User Signal: '011'

User Code: '1001'

Coded Signal:



CodeSync.

CodeGenerator

CodeGenerator

tcωcostcωcos2 ( )tc

bb Tf /1=

cc Tf /1=Chip rate

)()( tctb

1)( ±=tc

∫bT

dt0

(.)1)( ±=tb

� transmitted signal: s(t) = b(t)�c(t)�cos(ωcT)� received before filter: r(t) = b(t)�c(t) + b(t)�c(t)�cos(2ωcT)

� received after filter: r(t) = b(t)�c(t)

� after code multiplier: r(t) = b(t)�c(t)�c(t)� after summation: r(t) = b(t)�Σ c2(t) = b(t)�SPF (spreading factor)



� The spectral behaviour of a CDMA system is as follows:



� It is known as Code Division Multiple Access (CDMA).

� It has the following basic properties:� system does not need to be time synchronised, but it helps if it is� same and adjacent cells can use the same frequency at the same time

� it can reject narrowband interferers

� It has the following advanced properties:� modern CDMA systems have many MPCs (need of equalizer, Rake receiver, etc.)

� swapping codes does not give another channel (no multi-user diversity)� suffers near-far-problem (need of good power control mechanisms)

� Main problem of CDMA is power control!


MC-CDMA = OFDMA + CDMA [1/2]

� We assign each link an orthogonal code sequence over some chosen subcarriers.


MC-CDMA = OFDMA + CDMA [2/2]

� Known as Multi Carrier Code Division Multiple Access (MC-CDMA).

� It has the following basic properties:� it is very flexible because resources can be assigned in t, f & code� same and adjacent cells can use the same frequency at the same time

� It has the following advanced properties:� it operates closest to capacity limits with proper schedulers

� it can operate asynchronously (proposed for 4G uplink)

� Main problem of MC-CDMA is scheduling complexity, as well as power control and need of linear amplifiers!


3Ideal and Existing Systems


Ideal Communication Systems [1/2]

� An ideal system would use all possible resource dimensions for uplink (reverse) and downlink (forward).


Ideal Communication Systems [2/2]

� Advantages:� very flexible (resources are allocated according to needs)� high diversity order can be achieved

� close to system capacity

� therefore highest revenue

� Disadvantages:� very complex to manage with NP-complete scheduler complexity� large overhead to inform scheduler about state of the channel & user needs

� difficult to realise technologically (up & downlink bands cannot be very close)

� impossible to realise technologically (cannot talk at the same time & frequency)

� Real systems therefore need to use pragmatic approaches.


Real Communication Systems [1/2]

� Today's systems allow flexibility in terms of:� frequency band assigned to user � number of subcarriers assigned to user

� number & length of codes assigned to user

� Today's systems keep fixed:� reference/minimal slot-length (note that user can have several slots)

� total number of subcarriers� bandwidth (determining sampling frequency)

� guard times and guard bands

� Future systems (cognitive radio, 4G, etc.) will also allow for:� flexible choice of carrier frequency and bandwidth

� flexibility concerning used slot-lengths, synchronisation, etc.


Real Communication Systems [2/2]

� Practical systems, also suffer from the following 2 problems:� one cannot transmit and receive at the same time in the same frequency band;� resolution of access conflicts in the absence of centralised control entity.

� A terminal cannot transmit and receive at the same time in the same frequency band; however, a communication system always has:

� an uplink (reverse link) from a terminal to a base station, and

� a downlink (forward link) from a base station to a terminal.

� Many users wanting to use the channel always means conflict, which can be resolved by:

� centralised scheduler (yielding contention-free MACs)� distributed approach (yielding contention-based or random MACs)


Problem of Up/Downlink [1/2]

� One can use Frequency Division Duplex (FDD):� up & downlink use different bands at the same time� advantage: simultaneous communication in both directions

� disadvantages: channels in both directions are different

� One can use Time Division Duplex (TDD):� up & downlink use the same frequency band at different times

� advantages: channel in both directions is the same & allocation is more flexible� disadvantages: communication distance is limited due to guard times

� For instance, we can estimate the guard times:� minimum guard times with 35 km cell is: (2 x 35*1000m)/c = 230µs

� this is a lot compared to the symbol duration; hence TDD is not good for cellular


Problem of Up/Downlink [2/2]

� With TDMA & FDMA as examples, the possible combinations with TDD & FDD are:

frequency

downlink #1

time

TDD & TDMA

downlink #2 uplink #1 uplink #2

frequency

downlink #1

time

TDD & FDMA

downlink #2

uplink #1

uplink #2

frequency

downlink #1

time

FDD & TDMA

downlink #2

uplink #1 uplink #2

frequency

downlink #1

time

FDD & FDMA

downlink #2

uplink #1

uplink #2


Problem of Conflict Resolution [1/2]

� Conflicts always occur if many users want to use the same medium.

� Centralised entity can ensure contention-free access:� in the downlink, by statically or dynamically allocating resources (e.g. time slot )� in the uplink, by asking terminals to allocate resources (e.g. code sequence)

� Distributed approach requires contention-based access:� in cellular system, a non-registered terminal asking for resources in uplink

� in ad hoc type networks, a terminal trying to transmit a packet

� Although other combinations are possible, traditionally:� contention-free MACs are used in circuit-switched systems (regular traffic)

� contention-based MACs are used in packet-switched systems (irregular traffic)


Problem of Conflict Resolution [2/2]

� Fundamental contention-based MACs are:� ALOHA (invented in, guess where, Hawaii)� Carrier Sensing Multiple Access (CSMA)

� Principle of ALOHA:� packet arrives in buffer and is transmitted immediately

� if no ACK, packet collided and hence retransmitted after random delay

� un-slotted and slotted ALOHA exist, with the latter giving better throughput

� Principle of CSMA:� in principle like ALOHA, but user senses channel prior to transmission

� throughput & delay are better than for ALOHA

� different variants exist, e.g. non-persistent, 1-persistent, p-persistent


Existing Communication Systems [1/4]

� Requirements for given system trigger choice of below approaches:

� Resources allocation:� FDMA, TDMA, CDMA� OFDMA, MC-CDMA

� Up/downlink duplex:� FDD

� TDD

� Conflict management:� centralised

� distributed (ALOHA, CSMA, etc)



� Global System for Mobile Communication (GSM) – 2G:

� Resources allocation:� FDMA: 25 MHz divided into 124 carrier frequency channels of 200 kHz� TDMA: 8 radio timeslots per frequency channel yielding a TDMA frame

� Up/downlink duplex:� FDD: 890-915 MHz (uplink) and 935-960 MHz (downlink)

� Conflict management:� ALOHA: new user asking for resources in uplink

� central: registered users in up/downlink and new user in downlink



� Universal Mobile Telecommunication System (UMTS/FDD) – 3G:

� Resources allocation:� FDMA: up to 3 x 5 MHz channels are available per operator� CDMA: codes of different length can be assigned to users

� Up/downlink duplex:� FDD: 1885-2025 MHz (uplink) and 2110-2200 MHz (downlink)

� Conflict management:� ALOHA: new user asking for resources in uplink

� central: registered users in up/downlink and new user in downlink



� Wireless Local Area Network – IEEE 802.11b:

� Resources allocation:� FDMA: up to 4 x 22 MHz channels are available per ISM band� TDMA: time slots are allocated for up & downlink data

� CDMA: different codes are assigned to different access points

� Up/downlink duplex:� TDD: ISM Band, i.e. 83.5 MHz around 2.4 GHz carrier frequency

� Conflict management:� CSMA: carrier sense variant for uplink and ad hoc communication

� polling: downlink from access point towards terminal


4Capacity and Closing Remarks


Capacity of TDMA/FDMA Systems

� Capacity is measured in N = Number of Channels / Cell.

� It can be derived as:

� where� Bt total bandwidth

� Bc channel bandwidth� CIR minimum carrier-to-interference ratio for good service

� K number of cells per cluster

CIR

BB

K

BBN ctct

⋅≈=

32

//


Capacity of CDMA Systems

� Capacity is measured in N = Number of Channels / Cell.

� It can be derived as:

� where� Bt total bandwidth

� fb bit rate� CIR minimum carrier-to-interference ratio for good service

� λ interference increase factor (=2/3)

+⋅= 1CIR

fBN btλ


Some Thoughts

� The only variables given by nature are time and space.

� Engineers, using some tricks, have managed to allocate resources not only within time and space, but also frequency, subcarrier, code, etc.

� Although seemingly attractive, lots of design degrees of freedom are difficult to optimise and hence implement.

� Resource allocation and scheduling schemes are vital for your subsequent studies!


Acronyms [1/2]

� A list of some important acronyms used in the lecture:� ACK Acknowledgment� BS Base Station

� CDMA Code Division Multiple Access

� CIR Carrier-to-Interference Ratio� CP Cyclic Prefix

� CSMA Carrier Sensing Multiple Access

� FDD Frequency Division Duplex� FDMA Frequency Division Multiple Access

� GSM Global System for Mobile Communication

� (I)FFT (Inverse) Fast Fourier Transform

� ISM Industrial, Scientific, Medical (frequency band)� JMG Jean-Marie Gorce

� MAC Medium Access Control

� MC-CDMA Multi Carrier CDMA


Acronyms [2/2]

� … continued …� MD Mischa Dohler� MPC Multi-Path Component

� MS Mobile Station

� NP Non Polynomial (problem complexity)� OFDMA Orthogonal Frequency Division Multiple Access

� P/S, S/P Parallel-to-Serial, Serial-to-Parallel

� PAPR Peak-to-Average Power Ratio� QoS Quality of Service

� Rx, Tx Receiver, Transmitter

� SPF Spreading Factor

� TDD Time Division Duplex� TDMA Time Division Multiple Access

� WLAN Wireless Local Area Network

� UMTS Universal Mobile Telecommunications Systems


Advanced Topics

� If you really want to get into resource allocation mechanisms, here some important topics which I didn't have time to deal with:

� throughput analysis

� delay analysis

� stability issues of contention-based MACs� game-theoretic resource allocation

� distributed power control

� MACs for sensor networks


My Recommendations

� Some good books related to this lecture are:� Hamid Aghvami, et al. “Multiple Access Protocols for Mobile Communications”� Andreas Molisch “Wireless Communications”

� Mischa Dohler, et al., “Understanding UMTS Radio Network …”

� Some good articles related to this lecture are:� Sunil Kumar, Vineet S. Raghavan, Jing Deng, "Medium Access Control protocols

for ad hoc wireless networks: A survey," Elsevier Ad Hoc Networks, 2006.� I.F. Akyildiz, J. McNair, L.C. Martorell, R. Puigjaner, Y. Yesha, "Medium access

control protocols for multimedia traffic in wireless networks," IEEE Network 13 (July/August) (1999) 39–47.

� A.C.V. Gummalla, J.O. Limb, "Wireless Medium Access Control Protocols," IEEE Communication Tutorials & Surveys, 2000.

� Some good online articles related to this lecture are:� http://www.cs.ucdavis.edu/research/tech-reports/2006/CSE-2006-4.pdf� and then … there is always http://en.wikipedia.org/


Credits

� Some graphs/figures in this lecture have been reproduced from:

� Hamid Aghvami, King's College London, UK

� http://de.wikipedia.org/wiki/

� http://www.iec.org/online/tutorials/ofdm/topic04.html� http://www.ecl.sys.hiroshima-u.ac.jp/~ohno/details-j.html

� http://umtslink.at/cgi-bin/reframer.cgi?../UMTS/wcdma.html

Resource Allocation Mechanisms - INSA Lyon

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