ofdm-and-sc-fdma-for-lte

1、OFDM and SC-FDMA for LTE,,1,OFDM introduction History of OFDM OFDM Applications OFDM Basic Principles OFDM advantages and disadvantages OFDM for LTE OFDM Multi Access for LTE SC-FDMA for LTE,Course Contents,3,OFDM

2、 introduction History of OFDM OFDM Applications OFDM Basic Principles OFDM advantages and disadvantages OFDM for LTE OFDM Multi Access for LTE SC-FDMA for LTE,Course Contents,,History of OFDM,5,The basic principle

3、s of OFDM was proposedin several publications in the 1960’s.Since 1966 FDM systems with overlappingspectra were proposedThe next step is a proposal to realize anFDM system with DFTFinally, in 1971 Weinstein and

4、Ebert proposed a complete OFDM system, which included generating the signal with an FFT and adding a guard interval in the case of multipath channels,OFDM introduction History of OFDM OFDM Applications OFDM Basic Prin

5、ciples OFDM advantages and disadvantages OFDM for LTE OFDM Multi Access for LTE SC-FDMA for LTE,Course Contents,,OFDM Applications,BroadcastingDAB (Digital Audio Broadcasting)DVB (Digital Video Broadcasting)WLAN (

6、Wireless local area network)IEEE 802.11aHiperLan/2WMAN (Wireless metropolitan area network)IEEE 802.16 (WiMax)LTEDL: OFDMAUL: SC-FDMA,OFDM introduction History of OFDM OFDM Applications OFDM Basic Principles O

7、FDM advantages and disadvantages OFDM for LTE OFDM Multi Access for LTE SC-FDMA for LTE,Course Contents,Modulation and Demodulation in an OFDM System,Orthogonal Sub-carriers,Time-Frequency View,OFDM VS Traditional FDM

8、,OFDM spectrum efficiency,Without consideration of GI： M-ary modulation,Spectrum efficiency of the traditional FDM,,Bandwidth ：,Bit Rate：,Spectrum Efficiency：,Single Carrier,N Carriers,OFDM System：,Spectrum efficiency

9、 of OFDM,M-ary modulation，N sub-carriers， The symbol duration of OFDM is NT Sub-carriers inter-spacing： 1/NT Bandwidth： W=（N+1）/NT Bit rate：,Spectrum Efficiency：,,,OFDM Transmitter and Receive

10、r in Base-band,DFT,,,,,,,,DFT&IDFT,OFDM signal in the time domain：,OFDM Core Technologies,N—# of sub-carriers；T—OFDM symbol duration,,K=0,OFDM signal in based-band：,OFDM Modulation-IDFT,Discrete signal representation

11、：,Ts is the sampling interval or the period of the QAM (QPSK) symbols,IDFT,Demodulation-DFT,Demodulation in base- band correspinding the kth sub-carrier,DFT (Notice the factor：1//N in different books),OFDM symbol,Orthog

12、onal Property,,19,Multi Path Propagation,The cancellation of inter-symbol interference makes more complex the hardware design of the receiversIn WCDMA for instance the RAKE receiver requires a huge amount of DSP capacit

13、y.One of the goals of future radio systems is to simplify receiver design and thus the rectangular pulse is the first choice.Inter-symbol interference originating from the pulse form itself is simply avoided by startin

14、g the next pulse only after the previous one finished completely, therefore introducing a Guard Period (Tg) after the Pulse.There is no inter-symbol interference between symbols as long as the multi-path delay spread (e

15、.g. delay difference between first and last detectable path) is less than the guard period duration Tg.,Multi-Path Propagation and Inter-Symbol Interference,Multi-Path Propagation and the Guard Period,,Obviously when the

16、 delay spread of the multi-path environment is greater than the guard period duration (Tg), then we encounter inter-symbol interference (ISI),Propagation delay exceeding the Guard Period,Most systems use only the cyclic

17、prefix which then occupies the whole guard period. A receiver typically uses the high correlation between the cyclic prefix and the last part of the following symbol to locate the start of the symbol and begin then with

18、 decoding. In multi-path propagation environments the delayed versions of the signal arrive with a time offset, so that the start of the symbol of the earliest path falls in the cyclic prefixes of the delayed symbols.

19、As the CP is simply a repetition of the end of the symbol this is not a inter-symbol interference and can be easily compensated by the following decoding based on discrete Fourier transform.,Cyclic Prefix,OFDM introducti

20、on History of OFDM OFDM Applications OFDM Basic Principles OFDM advantages and disadvantages OFDM for LTE OFDM Multi Access for LTE SC-FDMA for LTE,Course Contents,Advantages:Easy to extend carrier bandwidth and

21、 support larger bandwidthHigh Spectrum efficiencyMultipath propagation fadingAdaptive Frequency SchedulingEasy to be implemented in UE Disadvantages:Sensitive to frequency shift and system synchronization Errors (T

22、iming offset)High Peak to Average Power Ratio Transmit power amplifier has a large backoff High resolution A/D in the receiver (Complexity and Power Burden),OFDM Advantages and Disadvantages,OFDM introduction History

23、 of OFDM OFDM Applications OFDM Basic Principles OFDM advantages and disadvantages OFDM for LTE OFDM Multi Access for LTE SC-FDMA for LTE,Course Contents,,The Beauties of LTE,Channel only changes amplitude and phas

24、e of subcarriers,Advanced Scheduling Time & Freq. (Frequency Selective Scheduling),,TX,,RX,,,Tx,Rx,,,,,MIMOChannel,DL: OFDMAUL: SC-FDMA,,scalable,HARQ: Hybrid Automatic Repeat Request,,,,,,,,,,,1,2,NACK,ACK,Rx Buf

25、fer,Combined decoding,,,,Downlink Air Interface TechnologyOFDMA,Constant symbol length for all bandwidths15 kHz subcarrier spacing20 MHz = 1200 subcarriers10 MHz = 600 subcarriers etc.Subcarriers assigned quality d

26、ependent,In LTE, the DL air interface is based in OFDMA LTE uses a 15 kHz subcarrier spacing (fs). Therefore the Symbol duration (Ts) is 66.67 μs. EUTRAN uses OFDM with 2048 subcarriers. Not all 2048 subcarriers are

27、for use, the subcarriers at the edges must be left free as guard bands against interference from other systems. A single cell of EUTRAN uses at least 72 subcarriers and maximum 1320 subcarriers. This corresponds to ban

28、dwidths from 1.4 MHz, 3 MHz, 5 MHz,10 MHz, 15MHZ and up to 20 MHz. Its an operator’s choice how many subcarriers (bandwidth) a cell should get.,LTE/EUTRAN Air Interface,,(1320 Subcarriers),Subcarrier Spacing ?f:15

29、kHz (7.5 kHz)Symbol durationTs66.67 µsNo. of usable subcarriers per cell:72 … 1320Total no. of subcarriers:2048,* NOTE:For MBMS (Multimedia Broadcast Multicast Service) EUTRAN defines also a subcarrie

30、r spacing of 7.5 kHz.,EUTRAN Subcarriers,*,Bandwidth Scalability,Scalable bandwidth 1.4 – 20 MHz using different number of subcarriers Large bandwidth provides high data rates Small bandwidth allows simpler spectrum re

31、farming, e.g. 450 MHz and 900 MHz,,,,,,,,,,,,,,,,1.4 MHz,3.0 MHz,5 MHz,10 MHz,15 MHz,Bandwidth,,Narrow Spectrum Refarming,High Data Rates,20 MHz,,,OFDM resource Grid for LTE/EUTRAN,,,frequency,,time,,,,,,,,,,…,,,,,,,…,,,

32、,,,,…,,,,,,,…,,,,,,,…,,,,,,,…,,,,,,,…,,,,,,,…,,,,,,,…,,,,,,,…,…,…,…,…,…,…,,,,,,,…,,,,,,,…,,,,,,,…,,,,,,,…,,,,,,,…,,,,,,,…,,,,,,,…,,,,,,,…,,,,,,,…,,,,,,,…,…,…,…,…,…,…,…,…,…,…,…,…,…,…,…,…,…,…,,,,Slot = 0.5 ms,,,,12 subcarr

33、iers,,,,,,6 or 7 Symbols/slot,OFDM symbols are arranged in a 2 dimensional matrix called the resource grid: One axis of the grid is the subcarrier indexThe other axis is the time. Each OFDM symbol has its place in th

34、e resource grid.,,Subframe = 1 ms,,,LowPass,,,cos(2πfct),,,-sin(2πfct),,,,I,Q,ModulationMapper,IFFT,,s0,ModulationMapper,,s1,ModulationMapper,,sN-1,,b10 ,b11,…,Serial toParallel Converter(BitDistrib.),,b20 ,b21,…

35、,,bN-1 0 …,BinaryCodedData,...,D A,,x0, x1, …, xN-1,IQSplit,LowPass,D A,,,,,,RF,s0,s1,sN-1,s2,,Frequency Domain,,,time,t1,t2,t0,tN-1,,,,,…,x0,x1,xN-1,x2,,TimeDomain,CP/GuardGeneration,I,Q,OFDM T

36、ransmitter,Time Domain Signal,,Applies different Modulation Schemes:OOK,BPSK,QPSK, 1&QAM, 64QAM,,Frequency Domain Signal:Collection of Sinusoids),Channel Correction,,Demodulator,Bit Mapping,,,j,,,,I,Q,A D,

37、,ChannelEstimation,,,RF,Low Noise Amp.+ Bandpass,,,,A D,,AGCAutomaticGain Control,,,De-rotator,,,signal strength,LNA gain,,,Frequency And Timing Sync,,signal autocorreation,phase correction,,,timeeadjust,,

38、,,,,,,,,...,s’0,s’1,s’N-1,,,reference(pilot),,,channelresponse,,,s0,Bit Mapping,,,s1,Bit Mapping,,,sN-1,...,...,,,,,,,...,B10 ,B11,…,B20 ,B21,…,BN-1 0 …,Bit Distribution,Soft BitCodedData,s0,s1,sN-1,s2,Freque

39、ncy Domain,,Time Domain,,,time,t1,t2,t0,tN-1,,,,,…,y0,y1,yN-1,x2,OFDM Receiver,,,,Windowing +FFT,,Frequency Domain,,The price for the optimum subcarrier spacing is the sensitivity of OFDM to frequency errors.If the rec

40、eiver’s frequency slips some fractions from the subcarrier spacing (subcarrier bandwidth) then we encounter not only interference between adjacent carriers, but in principle between all carriers. This is known as Inter-

41、Carrier Interference (ICI) and sometimes also referred to as Leakage Effect in the theory of discrete Fourier transform. If we have a little frequency drift between transmitter and receiver, then we decode the symbol of

42、 subcarrier 2, for instance, a little bit offset from its true center frequency.,Inter-Carrier Interference (ICI) in OFDM,Leakage effect due to Frequency Drift: ICI,,Two effects begin to work:1.-Subcarrier 2 has no lon

43、ger its power density maximum here - so we loose some signal energy. 2.-The rest of subcarriers (0, 1, 3 and 4) have no longer a null point here. So we get some noise from each other subcarrier.,,,The result is a lower

44、 signal to noise ratio by a decreased signal level and an increased noise level. This is the Inter-Carrier Interference effect for OFDM. To limit the influence of the ICI on OFDM systems completely by hardware we would

45、have to have receivers and transmitters with under 0.1 ppm frequency stability. This would drastically increase the cost and complexity of hardware. Thus quite a big part of the OFDM software in the receiver deals with

46、frequency correction using the cyclic prefix, but also reference or pilot signals sent with the signal.,Inter-Carrier Interference (ICI) in OFDM,OFDM introduction History of OFDM OFDM Applications OFDM Basic Principle

47、s OFDM advantages and disadvantages OFDM for LTE OFDM Multi Access for LTE SC-FDMA for LTE,Course Contents,Up to here we have only discussed simple point-to-point or broadcast OFDM.Now we have to analyze how to hand

48、le access of multiple users simultaneously to the system. OFDM can be combined with several different methods to handle multi-user systems:-Plain OFDM-Time Division Multiple Access via OFDM-Orthogonal Frequency D

49、ivision Multiple Access OFDMA®,OFDM and Multiple Access,Plain OFDM: Normal OFDM has no built-in multiple-access mechanism.This is suitable for broadcast systems like DVB-T/H which transmit only broadcast and multic

50、ast signals and do not really need an uplink feedback channel (although such systems exist too).,1.- Plain OFDM,Time Division Multiple Access via OFDM: The simplest model to implement multiple access handling is by putti

51、ng a time multiplexing on top of OFDM.Except some system specific information, all subcarriers go to user 1 in the first symbol period, then come user 2, user 3 and so on; this allocation scheme will be repeated periodi

52、cally.The disadvantage of this simple mechanism is, that every user gets the same amount of capacity (subcarriers) and it is thus rather difficult to implement flexible (high and low) bit rate services.Furthermore it i

53、s nearly impossible to handle highly variable traffic (e.g. web traffic) efficiently without too much higher layer signaling and the resulting delay and signaling overhead.,2.- Time Division Multiple Access via OFDM,Orth

54、ogonal Frequency Division Multiple Access OFDMA®: The term OFDMA is a registered trademark by Runcom Ltd. and was introduced with 802.16 (WiMAX) WirelessMAN-OFDMA for the downlink.The basic idea is to assign subcar

55、riers to users based on their bit rate services. With this approach it is quite easy to handle high and low bit rate users simultaneously in a single system.But still it is difficult to run highly variable traffic effic

56、iently.The solution to this problem is to assign to a single users so called resource blocks or scheduling blocks.Such block is simply a set of some subcarriers over some time. A single user can then use one or more b

57、locks. 802.16d uses such a mechanism with variable block sizes. The first OFDM symbols in each frame are used to indicate which user gets which blocks with which size. EUTRAN will use a similar system, but with fixed b

58、lock sizes and the assignment mechanism is not specified yet (2007-08).,3.- Orthogonal Frequency Division Multiple Access OFDMA@,OFDMA® is registered trademark of Runcom Technologies Ltd.,Different Methods for OFDM

59、Multiple Access,OFDM introduction History of OFDM OFDM Applications OFDM Basic Principles OFDM advantages and disadvantages OFDM for LTE OFDM Multi Access for LTE SC-FDMA for LTE,Course Contents,One of the major p

60、roblems with OFDMA systems is, that the transformation of a complex symbol mapped sequence (e.g. BPSK, QPSK, etc.) onto a small set of subcarriers produces time sequences that have a quite big ratio between the maximum

61、power and the averaged power (PAPR = Peak-to-Average Power Ratio). This results in requirements for expensive transmission amplifiers especially on mobile side. It is thus a major design goal to limit this effect.Anoth

62、er variant of OFDMA is used to reduce the PAPR for lower RF hardware requirements. It is called SC-FDMA (Single Carrier Frequency Division Multiple Access). This mechanism can reduce the PAPR of 6..9 dB compared to norm

63、al OFDMA. SC-FDMA is one option in 802.16d and it is the method selected for EUTRAN in the uplink direction.,SC-FDMA,1.- High Peak-to-Average Power Ratio (PAPR) of the transmitted signal: this results in requirements fo

64、r expensive non-linear power amplifiers. (especially on UE side).,OFDM Challenges,2.- Sensitivity to frequency error: frequency errors between the TX and the RX cause the sub-carriers to start to interfere each others (

65、Refer to ICI in this module),Another variant of OFDMA is used to reduce the PAPR for lower RF hardware requirements. It is called SC-FDMA (Single Carrier Frequency Division Multiple Access). SC-FDMA is a new hybrid m

66、odulation scheme that cleverly combines the low PAR of single-carrier systems with the multipath resistance and flexible subcarrier frequency allocation offered by OFDM.This mechanism can reduce the PAPR of 6..9 dB

67、compared to normal OFDMA. SC-FDMA is one option in WiMAX (802.16d) and it is the method selected for EUTRAN in the uplink direction.For a formal definition of SC-FDMA, please refer to TS 36.201 &36.211, which giv

68、es the mathematical descriptionof the time-domain representation of an SC-FDMAsymbol.,SC-FDMA,User multiplexing in frequency domain One user always continuous in frequency Low PAPR compared to OFDMA,Uplink Air Interf