Orthogonal Frequency Division Multiple Access(OFDMA)Khaja Mohammad Shazzad

OutlineBackgroundMultiple Access (MA) MethodsOrthogonal Frequency Division Multiplexing (OFDM) Based Multiple Access (OFDMA)Orthogonality PrincipleOFDMOFDM-FDMAAdvantages and Disadvantages of OFDMAConclusion

Multiple Access (MA)General wireless cellular systems are multi-users systemsRadio resource are limitedLimited BandwidthLimited number of channelsThe radio resource must be shared among multiple usersMultiple Access Control (MAC) neededContention-basedNon-contention-based

Contention-basedMultiple Access(MA)Contention-basedEach terminal transmits in a decentralized wayNo central controller (Base stations or access points)Example:ALOHA Carrier Sensing Multiple Access (CSMA)Standard:GSM [l] uses the slotted ALOHA in the terminals initial access processIEEE 802.11 uses CSMA/CA based contention access scheme

Non-contention-based Multiple Access (MA)A logic controller (BS or AP) is needed to coordinate the transmissions of all the terminalsThe controller informs each device when and on which channel it can transmitCollisions can be avoided entirelyTwo SubdivisionsNon-channelizationChannelization

Non-channelizationNon-contention-based MATerminals transmit sequentially using the same channelExample:Polling based medium accessStandard:IEEE 802.15(WPAN)IEEE 802.11(WLAN)

ChannelizationNon-contention-based MATerminals transmit simultaneously using different channelsMost commonly used protocols in cellular systemsExample:1. Time Division Multiple Access (TDMA)2. Code Division Multiple Access( CDMA)1. Frequency Division Multiple Access (FDMA)Standard1. GSM (TDMA)2. IS-95 (CDMA)3. American Mobile Phone System, AMPS (FDMA)

Time Division Multiple Access (TDMA)GSMTime slot 0.577 msFrame 4.6 ms8 time slots per frameFrequency band 20 KHz

Code Division Multiple Access (CDMA)IS-95Orthogonal Walsh codes64 codes (channels)One pilot channelSeven paging channels55 traffic channelsEach carrier 1.25 MHz

Frequency Division Multiple Access (FDMA)American Mobile Phone System (AMPS)Total Bandwidth 25 MHzEach Channel 30 KHz

Orthogonal Frequency Division Multiplexing (OFDM) Based Multiple Access (OFDMA)Orthogonality PrincipleOFDMOFDM-FDMA

Orthogonality PrincipleVector spaceA, B and C vectors in space are orthogonal to each otherA.B=B.C=C.A=0(A+B+C).A=(mod A)^2(A+B+C).B=(mod B)^2(A+B+C).C=(mod C)^2

Orthogonality Principle cont..Real Function space

Orthogonality Principle cont..Here mw and nw are called m-th and n-th harmonics of w respectively

Orthogonal Frequency Division Multiplexing(OFDM)It is a special kind of FDMThe spacing between carriers are such that they are orthogonal to one anotherTherefore no need of guard band between carriers.One example makes the thing clear

Example of OFDMLets we have following information bits1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, Just converts the serials bits to parallel bits

C1C2C3C411-1-1111-11-1-1-1-11-1-1-111-1-1-111

Example of OFDM cont..Modulate each column with corresponding sub-carrier using BPSK

Example of OFDM cont..Final OFDM Signal = Sum of all signal

Generated OFDM signal, V(t) V(t)

OFDM-FDMA (OFDMA)Each terminal occupies a subset of sub-carriers Subset is called an OFDMA traffic channel Each traffic channel is assigned exclusively to one user at any time

OFDM-FDMA (OFDMA)The IEEE 802.16e/ WiMax use OFDMA as Multiple access techniqueBandwidth options 1.25, 5, 10, or 20 MHzEntire bandwidth divided into 128, 512, 1024 or 2048 sub carriers20 MHz bandwidth with 2048 sub carriers has 9.8 KHz spacing between sub carriers

OFDM-FDMA (System View)

Advantages of OFDMAMulti-user Diversitybroadband signals experience frequency selective fadingOFDMA allows different users to transmit over different portions of the broadband spectrum (traffic channel)Different users perceive different channel qualities, a deep faded channel for one user may still be favorable to others

Advantages of OFDMA cont..Multi-user Diversity

Advantages of OFDMA cont..Efficient use of Spectrum

4/3 Hz per symbol6/5 Hz per symbol

Advantages of OFDMA cont..Receiver SimplicityIt eliminates the intra-cell interference avoiding CDMA type of multi-user detectionOrthogonality of code destroyed by selective fading Only FFT processor is requiredBit Error Rate performance is better only in Fading environment

Disadvantages of OFDMAPeak to average power ratio (PAPR)

The large amplitude variation increases in-band noise and increases the BER when the signal has to go through amplifier nonlinearities.

Disadvantages of OFDMA cont..SynchronizationTight Synchronization between users are required for FFT in receiverPilot signals are used for synchronizationsCo-channel interference Dealing with this is more complex in OFDM than in CDMA Dynamic channel allocation with advanced coordination among adjacent base stations

Research issues and ConclusionFuture worksPeak-to-average power reduction in OFDM Timing and Frequency SynchronizationEfficient digital signal processing Implementation of OFDM Multiple input/Multiple output (MIMO) OFDM ConclusionDifferent variations of OFDMA are proposed and have different pros and cons

Thank YouQuestions or Comments?