From OFDM to LTE. Fabrizio Tomatis (ST-E) Based on slides from Andrea Ancora (ST-E)

Transcription

1 From OFDM to LTE Fabrizio Tomatis (ST-E) Based on slides from Andrea Ancora (ST-E)

2 Introduction OFDM History Principles of Wireless Communications OFDM principles Serial-to-parallel conversion Cyclic prefix Frequency domain orthogonality Fourier transform OFDM advantages and disadvantages Dimensioning of OFDM system parameters Principles Example of LTE Agenda OFDMA Orthogonal Frequency Domain Multiple Access Combination of OFDMA and TDMA OFDM implementation issues PAPR and non-linearities Frequency offset and time varying channel sensitivity Insufficient cyclic prefix OFDM pilot structure and channel estimation Summary and conclusions CONFIDENTIAL 2

3 Introduction: OFDM History Principles of Wireless Communications CONFIDENTIAL 3

4 Introduction: OFDM History The first OFDM patent was filed in US by Chang at Bell Labs in 1966 and the patent was issued in R.. W. Chang, Synthesis of band-limited orthogonal signals for multichannel data transmission, Bell Systems Technical Journal, vol. 46, pp , December R.W. Chang - US Patent 3,488,445, 1970 A first analysis on this parallel system was done in 1967, at that time only analog design was proposed. B. R. Saltzberg, Performance of an Efficient Parallel Data Transmission System, IEEE Trans. On Communications, vol. 15, no. 6, pp , Dec The employment of the discrete Fourier transform (DFT) to replace the banks of sinusoidal generators and the demodulators was suggested by Weinstein and Ebert in 1971, which made the implementation OFDM cost effective. S. B. Weinstein and P. M. Ebert, Data Transmission by Frequency-Division Multiplexing using the Discrete Fourier Transform, IEEE Trans. on Communications, vol. 19, no. 5, pp , Oct Furthermore, the complexity could drastically be reduced by the application of reduced computational complexity algorithms such as the Winograd Fourier Transform (WFT) or the fast Fourier transform (WFT). A. Peled, A. Ruiz, Frequency domain data transmission using reduced computational complexity algorithms, Proc. IEEE Int. Conf. Acoust., Speech, Signal Processing, pages: , Denver, CO, Since then, the processing power of modern digital signal processors has increased allowing OFDM to find its way into commercial use CONFIDENTIAL 4

5 OFDM has developed into a popular scheme for digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, wireless networking and broadband internet access. Cable ADSL and VDSL broadband access via Plain Old Telephone Service (POTS) copper wiring. Power Line Communication (PLC) for Broadband over Power Lines (BPL). Multimedia over Coax Alliance (MoCA) home networking between televisions, set top boxes and other entertainment devices. Wireless Introduction: OFDM in telecommunication systems The Wireless LAN (WLAN) radio interfaces IEEE a, g, n and HIPERLAN/2. The digital radio systems DAB/EUREKA 147, DAB+, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB. The terrestrial digital TV system DVB-T. The terrestrial mobile TV DVB-H, T-DMB, ISDB-T and MediaFLO downlink. The cellular communication systems Flash-OFDM and the 3GPP Long Term Evolution (LTE) downlink. The wireless MAN/ Fixed broadband wireless access (BWA) standard IEEE (or WiMAX). The Mobile Broadband Wireless Access (MBWA) standards IEEE , IEEE e (Mobile WiMAX) and WiBro. The wireless Personal Area Network (PAN) Ultra wideband (UWB) IEEE a implementation suggested by WiMedia Alliance. CONFIDENTIAL 5

6 Introduction: OFDM History Principles of Wireless Communications CONFIDENTIAL 6

7 Introduction: Wireless communications principles (1) Any wireless telecommunication system can be modeled as an equivalent base-band cascade of: - Low-pass transmitted signal generator, characterized by a transmission bandwidth W; - Time-varying low-pass channel filter, characterized by - Coherence bandwidth B (frequency selectivity) - Doppler spread fd (time variation); - Additive interference source, typically modeled as white (un-limited and frequency flat) or colored (band limited and spectrally shaped) noise. - Adaptive receive filter, whose filter coefficients (or, equivalently, spectral shape) are varied according to some received signal criterion to reconstruct at best the transmitted signal, e.g.: - Match transmitted signal spectrum (MF) - Undo channel response (ZF) - Minimize received signal mean squared error (MMSE) - Minimize symbol error rate (ML) - Minimize bit error rate (MAP) Interference Transmitter Channel + Adaptive filter Receiver CONFIDENTIAL 7

8 Introduction: Wireless communications principles (2) The choice of the transmission scheme in conjunction with the channel characteristics mainly determines: The maximum achievable transmission rate The receiver complexity In single Tx & Rx systems, the maximum achievable transmission rate (i.e. in absence of coding, channel and interference impairment, control overhead) is given by - R=W M b/s - where W is the transmitted symbol bandwidth and M is the modulation order (bits per symbol) In the presence of noise, this rate is practically unfeasible and is instead bounded by the well-known Shannon limit R=W log 2 (1+SINR) b/s - where SINR is the signal to interference+noise power ratio CONFIDENTIAL 8

9 Introduction: Narrow-band vs. broad-band transmissions (1) At receiver side, the received signal is the result of the convolution operation between the time domain transmitted signal waveform and channel impulse response, i.e. low-pass filtering, plus a noise term Frequency selectivity occurs whenever the transmitted signal x(t) occupies an interval bandwidth [-W/2,W/2] greater then the coherence bandwidth B of the channel c(t) (defined as the inverse of the channel delay spread T d ) Narrow-band (e.g. GSM) transmissions are those where the symbol duration (defined as T=1/W) is greater than the channel delay spread T>T d d or, equivalently, W<B Broad-band (e.g. HSDPA/LTE) transmissions are instead those where the symbol duration is small compared to the channel delay spread T<T d or, equivalently, W>B CONFIDENTIAL 9

10 Introduction: Narrow-band vs. broad-band transmissions (2) Signal (Symbol period) T T d <T (Narrow band) convolution Inter-symbol interference Channel T d (Delay spread) T d >T (Broad-band) In general, in case of frequency selectivity, the received signal requires an equalization procedure (filtering) to eliminate the effect of inter-symbol interference. The complexity (i.e. cost) is proportional to the square of the channel memory and could be prohibitively high. C= k (T d /T) 2 or, equivalently, C= k (W T d ) 2 CONFIDENTIAL 10

11 Exploiting the well known time-frequency duality (and then the equivalence of time-domain convolution to spectral multiplication), the frequency selectivity effect can be seen as the frequency components of x(t) with separation exceeding B subject to different frequency channel gains. The receiver aims at compensating independently each channel gain, each compensation requiring consequent complexity. Broad-band Tx signal spectrum Introduction: Narrow-band vs. broad-band transmissions (3) Non-uniform channel gain Equalization necessary to compensate independently each channel gain P P P Channel spectrum P B f W/2 x f W/2 f W/2 f Channel gain constant across bandwidth of transmitted signal Narrow-band Tx signal spectrum P W/2 f x P W/2 f Therefore no equalization necessary (only one channel phase compensation operation) CONFIDENTIAL 11

12 OFDM principles: Serial-to-parallel conversion Cyclic prefix Frequency domain orthogonality Fourier transform OFDM advantages and disadvantages CONFIDENTIAL 12

13 OFDM principles: Serial-to-Parallel Conversion The Orthogonal Frequency Division Multiplexing consists in mixing the advantages of broad-band transmissions, namely the high data rate proportional to the total system bandwidth W, and the low required receiver complexity inherent to narrow-band transmissions. As seen before, in case of broad-band transmissions, the channel delay extends beyond one symbol period. To avoid this, the high rate data symbols can be serially-to-parallel converted and modulated around N different carrier frequencies. The high rate symbols are then converted into N parallel longer duration symbols. By varying N, the modulated symbol duration can be made sufficiently low compared to the channel delay spread and N equivalent narrow-band transmissions are obtained. exp(-j2π t f 1 ) High symbol rate S/P Low symbol rate x exp(-j2π t f n ) Σ x CONFIDENTIAL 13

14 OFDM principles Serial-to-parallel conversion Cyclic prefix Frequency domain orthogonality Fourier transform OFDM advantages and disadvantages CONFIDENTIAL 14

15 OFDM principles: Cyclic prefix Nevertheless, in the time domain, the modulated symbol so obtained would still suffer from inter-symbol interference due to the memory of the channel. symbol inter-symbol interference duration By inserting a guard-interval between the modulated symbols, the inter-symbol interference can be avoided and the memory of the channel emptied. In OFDM, the guard interval is obtained via a cyclic prefix that consists of inserting a replica of last G samples at the beginning of each symbol. While implying some wastage of spectrum, it has the positive effect of reducing the sensitivity to time synchronization. CONFIDENTIAL 15

16 OFDM principles Serial-to-parallel conversion Cyclic prefix Frequency domain orthogonality Fourier transform OFDM advantages and disadvantages CONFIDENTIAL 16

17 OFDM principles: Frequency-domain orthogonality (1) Equivalently, in the frequency domain, modulating each incoming data symbol around N equispaced carriers might generate inter-carrier interference due to spectrum leakage. Again, a guard band could be used to put sufficiently apart the N narrow-band signals. Similarly to the cyclic-prefix, this would imply spectrum wastage. Guard band frequency frequency CONFIDENTIAL 17

18 OFDM principles: Frequency-domain orthogonality (2) Instead, in Orthogonal Frequency Division Multiplexing the inter-carrier interference is avoided more efficiently by exploiting the DFT operation. This is equivalent to modulating N symbols using overlapping orthogonal waveforms (Fourier basis) without cross-talk between the N sub-channels. Hence, no guard bands are required and the spectrum wastage is limited to the use of the cyclic-prefix guard interval in the time domain. CONFIDENTIAL 18

19 OFDM principles Serial-to-parallel conversion Cyclic prefix Frequency domain orthogonality Fourier transform OFDM advantages and disadvantages CONFIDENTIAL 19

20 OFDM principles: Fourier transform (1) Further complexity reduction is achieved by choosing N to be radix-2 and benefiting from the efficiency of the fast Fourier transform (FFT): complexity is proportional to only N log 2 (N). At transmitter side, the IFFT accomplishes the task of modulating around multiples of the sub-carrier frequency f and multiplexing each independent channel at the same time. Cyclic Prefix CONFIDENTIAL 20

21 OFDM principles: Fourier transform (2) At receiver side, the FFT demodulates around multiples of the sub-carrier frequency f and demultiplexes each independent channel at the same time. The main advantage of OFDM transmissions is then to turn the convolutive channel into a multiplicative one in order to simplify the equalization task. Cyclic Prefix removed CONFIDENTIAL 21

22 OFDM principles Serial-to-parallel conversion Cyclic prefix Frequency domain orthogonality Fourier transform OFDM advantages and disadvantages CONFIDENTIAL 22

23 Advantages OFDM principles: Advantages and disadvantages (1) Low complexity equalization, O(N log 2 N), compared to CDMA case, O(N 2 ), with same performance. Transmitter and receiver architecture easily scale with system bandwidth, i.e. by increase of FFT order. Robust against narrow-band co-channel interference, i.e. suppressing only some sub-channels. Robust against inter-symbol interference (ISI) and channel selectivity due by multi-path propagation. High spectral efficiency, as almost the whole available frequency band can be utilized. Efficient implementation using FFT, i.e. numerically stable and supporting digital processing. Low sensitivity to time synchronization errors. Sub-carrier (de)modulation and sub-channel (de)multiplexing happening at same time. Summary of disadvantages Sensitive to Doppler. Sensitive to frequency synchronization problems. High peak-to-average-power ratio (PAPR), requiring high dynamic linear transmitter circuitry suffering from poor power efficiency. CONFIDENTIAL 23

24 OFDM principles: Advantages and disadvantages (2) OFDM requires very accurate frequency synchronization between the receiver and the transmitter; with frequency deviation the sub-carriers will no longer be orthogonal, causing inter-carrier interference (ICI), i.e. cross-talk between the sub-carriers. Frequency offsets are typically caused by mismatched transmitter and receiver oscillators, or by Doppler shift due to movement in LOS case. Whilst Doppler shift alone may be compensated for by the receiver, the situation is worsened when combined with multi-path channels, as reflections will appear at various frequency offsets, engendering a Doppler spread which is much harder to correct. This effect typically worsens as speed increases, and is an important factor limiting the use of OFDM in high-speed vehicles. Several techniques for ICI suppression are suggested, but they may increase the receiver complexity. CONFIDENTIAL 24

25 Dimensioning of OFDM system parameters: Principles Example of LTE CONFIDENTIAL 25

26 Dimensioning of OFDM system parameters: Principles An OFDM system needs proper dimensioning with respect to deployment scenario Given a system bandwidth W, two main parameters must be defined: The sub-carrier spacing, equivalent to define the number N of parallel sub-channels, such that: f=w/n < B min (i.e. the sub-carrier spacing is lower than the minimum channel coherence bandwidth) Or equivalently T =N T > T d,max (i.e. the OFDM symbol duration is greater than the maximum channel delay spread) The OFDM symbol duration must be chosen such that T =N T << 1/f d,max (i.e. the channel must be constant over one OFDM symbol duration) N must be radix-2 for efficient implementation The cyclic prefix length G, must be chosen such that: G> T d,max (i.e. the guard interval is greater than the worst-case channel length) Or equivalently G B min > 1 G must be as small as possible to avoid spectral efficiency wastage. Hence, the channel characteristics, delay spread and maximum Doppler frequency, are uniquely considered in the system dimensioning. In case of wireless channel, worst-case assumptions must be taken. CONFIDENTIAL 26

27 Dimensioning of OFDM system parameters Principles Example of LTE CONFIDENTIAL 27

28 Dimensioning of OFDM system parameters: Example of LTE (1) In the LTE downlink case, propagation scenarios cover: Indoor, urban and sub-urban propagation conditions determining different delay spread Highly mobile user equipments within a cell that can have relative speed of 350 km/h (and more) As MBMS makes use of Macro diversity (combining from multiple cells), a long cyclic prefix is required to cover the full range of delay spread. As a result, 3 OFDM modes are envisaged for efficient use of system resources depending on the deployment. They are as follows: 15kHz sub-carrier spacing (less sensitive to Doppler, so better suited for highmobility applications). Two alternative cyclic prefix lengths: Normal CP (5us): to suit small cell deployments such as indoor and urban. Extended CP (17s): allowing for larger cell deployments such as in suburban / rural. 7.5kHz sub-carrier spacing with 33us cyclic prefix length: intended for Multimedia broadcasting, allowing for longer symbol duration and CP: better for large cell and for multi-cell combining although more prone to Doppler. CONFIDENTIAL 28

29 Dimensioning of OFDM system parameters: Example of LTE (3) CONFIDENTIAL 29

30 Dimensioning of OFDM system parameters: Example of LTE (2) System band-width is imposed by regulatory organizations and cover a wide variety of bandwidths ranging in [1.25 MHz.. 20 MHz] to provide operators deployment flexibility. Backward compatibility against R99 and HSPA favours sampling frequencies being a multiple of 3.84 MHz (the UMTS chip rate). For efficient implementation of multi-mode terminals The maximum sampling frequency is indicated to be f s =30.72 MHz and the FFT order is N=2048 for 20 MHz bandwidth. Given the sampling frequency and the system bandwidth, the ratio between M used subcarriers and the N available at FFT output is lower compared to other systems (DVB, WLAN, etc) slightly reducing FFT efficiency. The current assumption is M=1200 to give M/N 0.7 as ratio between used vs. processed sub-carriers. Lower sampling frequencies (and proportionally lower FFT orders) are possible for narrower bandwidth deployments, to reduce complexity: e.g.: W=5 MHz N=512/M=300/f s =7.68 MHz and so on.. CONFIDENTIAL 30

31 Dimensioning of OFDM system parameters: Example of LTE (4) Channel Bandwidth [MHz] Transmission Bandwidth Configuration [RB] Channel edge Res source block Transmission Bandwidth [RB] Channel edge Active Resource Blocks DC carrier (downlink only) CONFIDENTIAL 31

32 OFDMA: Orthogonal Frequency-Division Multiple Access Combination of OFDMA and TDMA CONFIDENTIAL 32

33 OFDMA Orthogonal Frequency-Division Multiple Access (OFDMA) is a multi-user version of OFDM digital modulation scheme. Multiple Access is achieved in OFDMA by assigning groups of sub-carriers to individual users. Based on feedback information about the channel conditions from each user, adaptive user-to-rb assignment can be performed. If the assignment is done sufficiently fast, this further improves the OFDM robustness to channel selectivity/fast fading of each user and further increases spectral efficiency. Moreover, OFDMA allows the support of differentiated Quality of Service (QoS), i.e. to control the data rate and error probability individually for each user. LTE spectrum allocation granularity is 12 contiguous sub-carriers: Resource Blocks. CONFIDENTIAL 33

34 Combination of OFDMA and TDMA OFDMA can also be used in combination with Time Domain Multiple Access (TDMA), where the resources are partitioned in the time-frequency plane and chunks are assigned along the OFDM symbol index as well as OFDM sub-carrier index. As in LTE: CONFIDENTIAL 34

35 OFDM implementation issues PAPR and non-linearities Frequency offset and Doppler sensitivity Insufficient cyclic prefix OFDM pilot structure and channel estimation CONFIDENTIAL 35

36 OFDM related issues: PAPR and non linearities (1) OFDM can be seen as a linear operation performed over a large block of frequencydomain QAM modulated complex symbols with (almost) constant modulus property. As a result, due to the central limit theorem, the time-domain OFDM symbol can be very well approximated as a Gaussian waveform and therefore losing the interesting constant modulus property. The amplitude of the OFDM modulated signal can then have very large values These amplitude variations are a big disadvantage as they reduce the power efficiency (and increase cost) of the Power Amplifier (PA) in the RF transmitter as the linear operational amplification region is much increased. LTE does not specify any PAPR reduction technique and leaves to enodeb manufacturers the burden of handling the increased required costs. (I)FFT Limited signal dynamic (constant modulus signal) PAPR = 1 High signal dynamic (Gaussian signal) PAPR >> 1 CONFIDENTIAL 36

37 OFDM related issues: PAPR and non linearities (2) OFDM suffers from distortions of time-domain signal as errors spread over all sub-carriers. Hence, non-idealities of real RF transceiver, as limited dynamic amplifiers (i.e. clipping effect) have a direct impact on the quality of received signal and result in an additional source of impairment. Inherent OFDM property such as high PAPR largely contributes to this phenomenon. In case of OFDM, non-linearities can easily be accounted knowing that any non-linear device present along the transmission chain is equivalent to the cascade of a complex gain factor and an additional white noise (distortion) source applied to the received signal. This model is used in setting the performance for LTE and is commonly referred as EVM source. As a result, despite SIR growing as high as possible, OFDM signal would be affected by an irreducible error floor limiting the overall system spectral efficiency. CONFIDENTIAL 37

38 OFDM implementation issues PAPR and non-linearities Frequency offset and time Dopplersensitivity Insufficient cyclic prefix OFDM pilot structure and channel estimation CONFIDENTIAL 38

39 OFDM related issues: Frequency offset and Doppler sensitivity The orthogonality principle of OFDM relies on the fact that the channel stays constant over the transmission of each OFDM symbol corresponding to the DFT block. In wireless environments the multi-path channel is time varying because of the user s mobility (Doppler effect). Rapid channel variations over a symbol period introduce a frequency error. A frequency error may be also the result of a mismatch between the transmitter and receiver local oscillators. The frequency error destroys sub-carriers orthogonality and results in Inter-Carrier Interference (ICI). CONFIDENTIAL 39

40 OFDM implementation issues PAPR and non-linearities Frequency offset and time Dopplersensitivity Insufficient cyclic prefix OFDM pilot structure and channel estimation CONFIDENTIAL 40

41 OFDM implementation issues: Insufficient cyclic prefix OFDM systems are designed to support a cyclic prefix length longer than the channel impulse response in order to benefit from cyclic periodicity of Fourier transform and result in an orthogonal multi-carrier system allowing for extremely low complexity equalization. The condition of a sufficient guard interval is therefore strictly related to the orthogonality property of OFDM. In the unfortunate situation when the channel might be longer than the system designed cyclic prefix length, the orthogonality is destroyed resulting in the introduction of Inter- Carrier Interference (ICI) and Inter Symbol Interference (ISI) from previous OFDM symbol CONFIDENTIAL 41

42 OFDM implementation issues PAPR and non-linearities Frequency offset and time Dopplersensitivity Insufficient cyclic prefix OFDM pilot structure and channel estimation CONFIDENTIAL 42

43 OFDM implementation issues: LTE OFDM pilot structure and channel estimation (1) Transmission schemes can in general be classified as: Non-coherent, where useful signal is modulated using differential schemes (low rate) and requiring energy detection based receiver (e.g. UWB). Coherent, where transmitted signal is modulated using QAM schemes (high rate) and requiring receivers able to recover both amplitude and phase (e.g. LTE downlink). Coherent systems require accurate estimation of the channel Channel estimation can be performed in different ways: by using a parametric model of the channel and exploiting the a-priori knowledge of the model, by exploiting the correlation properties of the channel, by using blind estimation, or by exploiting some known signals. To facilitate and limit the complexity of the channel estimation process, LTE uses known reference signals in the transmitted OFDM symbols allowing for pilot based channel estimation (at the expense of some overhead). CP removal FFT Demux data pilot Channel estimation x Equalized data CONFIDENTIAL 43

44 OFDM implementation issues: LTE OFDM pilot structure and channel estimation (2) LTE reference signal (pilot symbols) are available in the time-frequency plane dispersed on a regular grid. The RS grid is diamond shaped to allow exploiting diversity and for efficient interpolation in the frequency-time 2-dimensional plane. Within one cell, the RS are designed to support multi-antenna transmission: RS are interleaved such that they do not interfere and each antenna avoid transmission on those resource elements where RS of the other antennas are present. Among neighboring cells, the RS resource elements pattern is (mostly) unique to allow for interference randomization. There are 6 available patterns obtained by frequency shifts determined by the Cell ID. Antenna port 0 Antenna port 1 Antenna port 2 Antenna port 3 frequen cy R 0 R 0 R 0 R 0 R 1 R 1 R 1 R 1 R 2 R 2 R 3 R 3 R 0 R 0 R 1 R 1 R 2 R 3 R 0 R 0 time R 1 R 1 R 2 R 3 CONFIDENTIAL 44

45 OFDM implementation issues: LTE OFDM pilot structure and channel estimation (3) The OFDM channel must be estimated over the time-frequency grid and in spatial direction (N Tx x N Rx spatial channels): The channel grid is estimated performing (2D) interpolation and filtering. Each grid for each spatial channel must be estimated separately: no interpolation is possible. The interpolation and filtering technique is critical for channel estimation performance (and complexity). It is based on the exploitation the coherence bandwidth and the time-varying characteristics. In the practical case, the 2-D problem is split into 2 1-D separated interpolation/filtering in frequency and time directions to lower memory complexity as data to be decoded would need to be buffered while the channel estimation is being derived. channel Interpolated channel Channel at RS positions time/frequency frquency 2D interpolation time frquency f-interpolation t-interpolation time CONFIDENTIAL 45

46 Summary and conclusions CONFIDENTIAL 46

47 Summary and conclusions OFDM is a mature technology. It achieves the high transmission rate of broad-band but with the low receiver complexity of narrow-band transmission. Makes use of cyclic-prefix to null Inter-Symbol Interference, enabling block-wise processing. Orthogonal sub-carriers avoid guard-band wastage. Benefits from efficient Fourier transform implementation FFT. System design allows to trade off between Doppler and delay spread depending on the deployment scenarios. Widely deployed: especially suited for broadcast or downlink because of low receiver complexity while requiring high transmitter complexity (expensive PA). Straightforward multi-user extension into OFDMA. CONFIDENTIAL 47

48 CONFIDENTIAL 48