AN ANALYSIS OF OFDMA, PRECODED OFDMA AND SC-FDMA FOR THE UPLINK IN CELLULAR SYSTEMS

Transcription

1 AN ANALYSIS OF OFDMA, PRECODED OFDMA AND SC-FDMA FOR THE UPLINK IN CELLULAR SYSTEMS Cristina Ciochina 1,2, David Mottier 1 and Hikmet Sari 2 1 Mitsubishi Electric ITE-TCL, 1 Allée de Beaulieu, Rennes Cedex 7, France 2 Supélec, Plateau de Moulon, 1-3 Rue Joliot Curie, Gif sur Yvette, France Abstract: Key words: Orthogonal Frequency Division Multiple Access (OFDMA) appears today as a strong candidate in on-going standardization. Despite its attractive features, it has several drawbacks when employed in the uplink: The high peak-to-average power ratio (PAPR) inherited from Orthogonal Frequency Division Multiplexing (OFDM) and the inherent frequency diversity loss. The loss of frequency diversity can be combated by precoding. Variants of Precoded OFDMA include Spread Spectrum Multi-Carrier Multiple Access (SS-MC- MA) and the frequency-domain implementation of single-carrier (SC) FDMA called DFT-Spread OFDM. The latter transforms OFDMA into a SC transmission system, avoiding the PAPR problem. SC-FDMA with uniformly spaced carriers can be also generated by time-domain processing, a technique known as Interleaved Frequency Division Multiple Access (IFDMA). This paper analyzes OFDMA, several variants of Precoded OFDMA as well as SC- FDMA in its time- and frequency-domain implementations and compares them for uplink transmission. The spectrum and performance analysis confirm the benefits of SC-FDMA in terms of both high-power amplifier (HPA) output back-off and bit error rate (BER) performance on selective channels. OFDMA; SC-FDMA; DFT-Spread OFDM; IFDMA; SS-MC-MA; Peak to Average Power Ratio (PAPR); HPA Back-off. 1. INTRODUCTION Future broadband cellular systems should meet stringent requirements such as high data rates over dispersive channels, coexistence of different services, good coverage, robustness to interference, high flexibility and high

2 2 Cristina Ciochina, David Mottier and Hikmet Sari performance. These requirements turn the design of such a system into a real challenge, especially for the uplink, where low-cost and low-complexity mobile terminals are demanded. Although the principle of multi-carrier (MC) systems is not new, it is only in the past decade that this technique gained recognition and became a key component of many standards. Coded OFDM schemes are today used for terrestrial digital video broadcasting (DVB-T), digital audio broadcasting (DAB), wireless local area networks (IEEE a, ETSI Hiperlan2) and wireless metropolitan area networks (IEEE ). Almost all current proposals for the air interface of Beyond Third Generation (B3G) and Fourth Generation (4G) cellular systems involve OFDM, OFDMA or one of its derivatives (e.g., MC code division multiple access MC-CDMA, SS-MC- MA). Nevertheless, MC systems suffer from one major problem: The high PAPR. This counterbalances the well-known advantages of MC techniques, particularly on the uplink of cellular systems, since the output power of user terminals is strictly limited and must be efficiently utilized in order to increase coverage. At this point, the debate on the choice between SC and MC systems is not closed. The SC transmission alleviates the PAPR problem. On the other hand, MC transmission opens the way to OFDMA 1, which significantly increases the cell range compared to a SC system or an OFDM system that uses time division multiple access. Indeed, FDMA concentrates the available transmit power in a fraction of the channel bandwidth, which improves the signal-to-noise ratio (SNR). The considerations above lead to the conclusion that the multiple access technique best suited for the uplink is SC-FDMA, as it combines the low- PAPR characteristics of SC transmission with the advantages of FDMA. This can be precisely achieved by the IFDMA technique 2-4. Its basic principle is the following: The input data stream is split into symbol blocks, each block is repeated a predetermined number of times and multiplied with a user specific phase ramp. This results in interleaving different users signals in the frequency domain without having to make any transformations between the frequency and the time domains. The 3GPP (Third Generation Partnership Project), which focuses on the Long Term Evolution (LTE) of UMTS (Universal Mobile Terrestrial Systems) radio access, has favored a frequency-domain implementation of SC-FDMA, which is actually a Precoded OFDMA scheme, where precoding is carried out by means of a DFT matrix. The major argument in favor of this implementation, also called DFT-Spread OFDM, is its flexibility in terms of sharing the spectrum between different users. This paper provides an analysis of these different multiple access schemes. The paper is organized as follows: Section 2 gives the fundamentals of the multiple access techniques reviewed above. Section 3

3 AN ANALYSIS OF OFDMA, PRECODED OFDMA AND IFDMA FOR THE UPLINK IN CELLULAR SYSTEMS 3 presents the system model used in our simulations. Performance is evaluated and the results are discussed and compared in section 4. Finally, the conclusions are given in section MULTIPLE ACCESS TECHNIQUES FOR THE UPLINK Fig. 1 presents the baseband structure of a general MC transmitter, which applies to all types of SC or MC modulation signals transmitted in blocks 5. Data blocks of size M are precoded with the [M M] matrix P. The M-sized output vector is then mapped on M out of N inputs of the inverse DFT according to the subcarrier mapping [N M] matrix Q. To combat the effect of the frequency selective channel, a cyclic prefix of length CP is inserted in front of each N-sized block delivered by the inverse DFT. Transmission with different rates among users is available according to each user s requirement, as a different number of subcarriers and a different modulation and coding scheme can be assigned to each user. Let us denote by s(n) the information symbols which are parsed into data blocks of size M. The i-th data block s i can thus be written as: [ sim sim M ] i s = ( ),..., ( + 1) T. (1) The index i will be omitted in the sequel. Let us denote by the Kronecker product, by 0 M N the all-zero matrix of size [M N] and by I M the [M M] identity matrix. For clarity, we assume that the size N of the inverse DFT is a multiple of the block size, i.e., N = MK. Figure 1. General MC transmitter.

4 4 Cristina Ciochina, David Mottier and Hikmet Sari 2.1 OFDMA The trivial case when P is the identity matrix, P= I M, leads to OFDMA. The user-specific data block s is directly mapped onto a subset of M subcarriers, conveniently chosen by the user-specific subcarrier mapping matrix Q. The vector Qs is fed to the entries of the inverse DFT. The form of the matrix Q might lead to either a localized (Eq. 2) or a distributed (Eq. 3) subcarrier mapping: Q N M q M = IM 0 0 ( N q M) M, (2) Q N M 0n 1 = IM 1. (3) 0( K n 1) 1 By assigning different groups of subcarriers to different users, each user s transmit power can be concentrated in a restricted part of the channel bandwidth, resulting in significant coverage increase. Different user signals remain orthogonal only if carrier synchronization is maintained and an appropriate cyclic prefix is appended to compensate for timing misalignment at the reception. In order to keep good performance on frequency-selective channels, efficient forward error correction must necessarily be employed. 2.2 Precoded OFDMA Precoded OFDMA consists of using a precoding matrix P that spreads the energy of symbols over the subcarriers allocated to the user. Uniform energy distribution is favored in practice. One of the most well known precoding matrices is the Walsh-Hadamard (WH) matrix: [ ] T 0 1 M 1 P= p,p,,p, (4) where the row vectors p i, i = 0... M 1, are orthogonal WH sequences of length M. This type of Precoded OFDMA was coined SS-MC-MA 6,7. The precoding operation Ps consists in spreading the data symbols by multiplication with orthogonal WH sequences and superimposing them on the same set of subcarriers according to matrix Q. Another precoding matrix that spreads the symbol energy uniformly is the DFT matrix. We will discuss

5 AN ANALYSIS OF OFDMA, PRECODED OFDMA AND IFDMA FOR THE UPLINK IN CELLULAR SYSTEMS 5 this precoding in the following section. Precoded OFDMA conserves the advantages of OFDMA in terms of cell range extension and spectrum spreading, which is expected to provide some robustness against cellular interference. With respect to MC-CDMA, it loses some frequency diversity, but this loss can be compensated by frequency interleaving or frequency hopping techniques 8. The well-known advantages of MC systems are sometimes counterbalanced by their high PAPR. If we want to avoid nonlinear effects, the input signal must lie in the linear region of the HPA. In order to avoid the use of extremely high back-offs and costly amplifiers, occasional clipping and/or soft thresholding must be allowed. This leads to in-band distortion (which degrades the bit error rate) and to spectral widening that increases adjacent channel interference. Many PAPR reduction algorithms have been developed in order to alleviate this problem. Unfortunately, they do not always yield significant performance gains in practical applications SC-FDMA As an alternative to MC-FDMA, SC-FDMA schemes have been envisioned, since a single-carrier system with an OFDMA-like multiple access would combine the advantages of the two techniques: low PAPR and high coverage. The first SC-FDMA concept 2 was IFDMA, which is based on compression and block repetition in the time domain of the modulated signal. As theoretically proven 4, this manipulation has a direct interpretation in the frequency domain. The spectrum of the compressed and K-times repeated signal has the same shape as the original signal, with the difference that it presents exactly K-1 zeros between two data subcarriers. This feature enables us to easily interleave different users in the frequency domain by simply applying to each user a specific frequency shift, or equivalently, by multiplying the time-domain sequence by a specific phase ramp. Besides, as for OFDMA, robustness to cellular interference can be achieved by coordinating resource allocation between adjacent cells. The same waveform can be obtained in the frequency domain. Indeed, by using in Fig. 1 the discrete Fourier matrix: kn j2 p,, M kn pkn, e π P = = (5) as precoding matrix, we obtain a DFT-Precoded OFDMA, which is mathematically identical to IFDMA in a distributed scenario. The precoding operation Ps is equivalent to an M-point DFT transform. With a mapping

6 6 Cristina Ciochina, David Mottier and Hikmet Sari matrix Q as given in Eq. (3), the spectrum of the distributed DFT-Precoded OFDMA signal is identical to the IFDMA signal spectrum, and thus it corresponds to the same waveform. This is also called DFT-Spread OFDM. The two techniques are just different implementations of SC-FDMA. The advantage of DFT-Precoded OFDMA is its more flexible structure: While IFDMA imposes a distributed signal structure, DFT-Precoded OFDMA allows us to choose the mapping matrix Q as desired. Localized versions of implementation or channel-dependent mappings are possible. Also, pulse shape filtering can eventually be performed in the frequency-domain, with a lower complexity than the time-domain filtering. Note that in case of a frequency selective channel, interference may occur within the M elements of each data block. This degradation, which is more important in a distributed subcarrier mapping, impacts WH-Precoded OFDMA as well. 3. SYSTEM MODEL In what follows, WH-Precoded OFDMA will simply be referred to as Precoded OFDMA. The simulated system model employs OFDMA, Precoded OFDMA and SC-FDMA transmission. We use a signal with N = 512 subcarriers, among which 300 are data carriers, split into 25 resource units of 12 subcarriers. 1 DC is added in the case of OFDMA transmission, and the remaining 211 (OFDMA) or 212 (Precoded OFDMA, SC-FDMA) are guard carriers. With these parameters, the sampling frequency corresponding to a 5 MHz channel is 7.68 MHz. The signal constellation is 16QAM with Gray mapping. We employ a (753,531) 8 convolutional code with rate 1/2, 3/4 or 5/6. The codes of rate 3/4 and 5/6 are obtained by puncturing the 1/2 code. The data is scrambled before coding and interleaved prior to QAM mapping. Groups of 7 OFDMA-type symbols are encoded together in order to take advantage of the channel diversity. We used soft Viterbi decoding. Frequency-domain equalization is performed when necessary. The HPA is Rapp s solid state power amplifier model 10 : v OUT = v IN ( ( ) ) 1 2 p vin / vsat p, (6) where v IN, v OUT are respectively the complex input and complex output signals (baseband equivalent, normalized) and v SAT corresponds to the 2 output saturation level normalized to unity, P SAT = v SAT. We consider a Rapp model HPA with knee factor p = 2, since it is reported 11 to be a good

7 AN ANALYSIS OF OFDMA, PRECODED OFDMA AND IFDMA FOR THE UPLINK IN CELLULAR SYSTEMS 7 representation of typical HPAs in the sub-10ghz frequency range. We also define the input back-off (IBO) and output back-off (OBO) with respect to the saturation values: IBO OBO db db { 2 IN () } E v t = 10log10, (7) P SAT, IN { 2 OUT () } E v t = 10log10. (8) P SAT, OUT 4. IMPACT OF NONLINEARITIES AND PERFORMANCE CONSIDERATIONS In this section, we compare the performance of OFDMA, Precoded OFDMA and SC-FDMA. We separately evaluate the impact of nonlinearities on these multiple access techniques and study their behavior on fading channels. 4.1 Signal Envelope Variations, Output back-off (OBO) and Total Degradation In order to illustrate the PAPR performance, we evaluate the Complementary Cumulative Distribution Function (CCDF) of the instantaneous normalized power (INP), which is defined as: 2 y i 2 CCDF( INP( vin )) = Pr > γ, (9) Pavg, IN where v IN is the signal present at the input of the HPA, y i denotes its samples and P { 2 avg, IN = E vin () t } is its average power. The CCDF of INP 9 is a more relevant performance criterion than the widely used CCDF of PAPR: It takes into account all of the signal samples that are susceptible of causing degradation when passing through the HPA, and not only the highest peak of each OFDM symbol. The signal is oversampled by a factor of 4. Fig. 2 presents the CCDF of INP for the three transmission schemes with M = 24 distributed subcarriers, 16QAM signal mapping and 3/4 coding rate. The SC properties of SC-FDMA results in better PAPR performance: 2 db better

8 8 Cristina Ciochina, David Mottier and Hikmet Sari 4 than OFDMA at a clipping probability per sample of 10. OFDMA and Precoded OFDMA have similar PAPR. It has to be noted that the performance gain predicted by the CCDF of INP can be directly interpreted in terms of (input) back-off difference only in the case of an ideal (clippertype) HPA 9. With a realistic HPA, a back-off difference given by the CCDF curves should only be perceived as an upper bound of performance gain. For practical system design, the main relevant evaluation criterion is the necessary amount of OBO that is needed to reach some performance, e.g., 4 BER= 10, while complying with the spectrum mask requirements and the out-of-band (OOB) radiation limits. In order to evaluate OOB radiation, the adjacent channel leakage ratio (ACLR) is defined as the ratio between the inband signal power and the power radiated in the adjacent band. Fig. 3 presents the spectrum of a SC-FDMA signal complying with the UMTS uplink 5 MHz spectrum mask 12 and an ACLR restriction of at least 33 db. In order to comply with the spectrum mask, current regulations require the verification of the case when all resource units are allocated to the same user which is a rather unlikely scenario in practice. A verification of typical allocation scenarios would be preferable, but no such scenarios were defined Figure 2. CCDF of INP for SC-FDMA, OFDMA and Precoded OFDMA with 16QAM 3/4, 24 distributed subcarriers, oversampling by a factor of 4. so far. Here, actual specifications were treated (300 occupied subcarriers); an OBO of 5.76 db is necessary. From Fig. 2, the back-off advantage of SC-

9 AN ANALYSIS OF OFDMA, PRECODED OFDMA AND IFDMA FOR THE UPLINK IN CELLULAR SYSTEMS 9 FDMA over OFDMA is at most 1.6 db (in order to maintain the clipping probability of corresponding to 5.3 db of IBO in the SC-FDMA case). Indeed, by reproducing the results in Fig. 3 for OFDMA, we find that 7.17 db of OBO (6.75 db of IBO) are necessary in order to maintain the same constraints. This leads to a relative gain of 1.4 db. The results obtained for Precoded OFDMA are similar to those of OFDMA. Nonlinear effects also cause BER degradation. In order to approximate the real performance of a practical system, an estimation of the total degradation ( OBO + Eb / N0 loss) must be performed. Fig. 4 shows the total 4 degradation at the BER target of 10. BER simulations take into account an AWGN channel in order to evaluate the impact of nonlinearities separately from the channel fading effects. We can notice that OFDMA and Precoded OFDMA have similar behavior. SC-FDMA can achieve a minimum total degradation that is 1.2 db lower than that of OFDMA. However, this result cannot be directly interpreted, as the operating point of the HPA is imposed by three conditions: Spectral requirements (mask, ACLR), BER degradation and amplifier type. For SC-FDMA, the minimum total degradation is attained when working with an OBO of 4 db, which corresponds to an E b /N 0 Figure 3. SC-FDMA spectrum, 300 occupied subcarriers, complying with UMTS spectrum mask and ACLR constraints 13

10 10 Cristina Ciochina, David Mottier and Hikmet Sari Figure 4. Total degradation vs. OBO for SC-FDMA, OFDMA and Precoded OFDMA, 16QAM 3/4, 24 distributed subcarriers, AWGN channel, target BER = loss of 1.6 db. But in practice (see Fig. 3,) an OBO of 5.76 db is required in order to comply with the spectral constraints. Therefore, the performances of the three schemes have to be compared by considering their respective operating points (marked on Fig. 4,) given by the necessary OBO values to fit with the spectrum constraints. This corresponds to a total degradation of 6.25 db for SC-FDMA and of 7.65 db for its two counterparts, thus a gain of 1.4 db in the favor of SC-FDMA. 4.2 BER Performance on Frequency Selective Channels Fig. 5 presents the BER performance in the case of a transmission over a frequency selective COST 259 channel 13 in the absence of HPA. SC-FDMA and Precoded OFDMA have similar BER performance because they both recover the diversity of the system thanks to the spreading component. OFDMA performance is very dependent on the coding rate. When low coding rate (5/6) is employed, OFDMA performs poorly because coding does not manage to compensate the influence of carriers with a low SNR. When stronger coding is present (1/2), OFDMA benefits from the coding diversity and thus it recovers the difference and even outperforms SC- FDMA / Precoded OFDMA transmissions, whose performances are limited by inter-symbol interference.

11 AN ANALYSIS OF OFDMA, PRECODED OFDMA AND IFDMA FOR THE UPLINK IN CELLULAR SYSTEMS CONCLUSIONS In this paper, we have reviewed the fundamentals of three multiple access techniques which are potential candidates for the uplink of future cellular communications systems: OFDMA, WH-Precoded OFDMA and SC-FDMA. Precoded OFDMA combines the advantages of spectrum spreading with OFDMA-type multiple access. SC-FDMA is based on SC transmission and FDMA-type multiple access. The second technique can be implemented in the time domain (IFDMA) or in the frequency domain (DFT-Precoded OFDMA, also called DFT-Spread OFDM). The two implementations are completely equivalent in the distributed case. DFT-Precoded OFDMA has the advantage of the flexible subcarrier allocation. A pertinent analysis of realistic HPA s impact on performance was conducted taking into account the constraints of practical systems (spectrum mask, ACLR). BER performance on frequency selective channels was also Figure 5. BER performance of OFDMA, Precoded OFDMA and SC-FDMA with 16QAM and 24 distributed subcarriers. investigated. With 16QAM over distributed sub-carriers, OFDMA was found to have good BER performance only in the presence of robust channel coding. It is outperformed by both Precoded OFDMA and SC-FDMA when less powerful coding is employed. Precoded OFDMA has similar

12 12 Cristina Ciochina, David Mottier and Hikmet Sari performance to SC-FDMA on frequency selective channels, since their spreading properties recover frequency diversity. On the other hand, it inherits the high PAPR disadvantages of OFDMA. Thanks to its singlecarrier signal structure, SC-FDMA has the advantage of lower PAPR and thus lower back-off requirements than OFDMA and Precoded OFDMA. ACKNOWLEDGEMENT The present work was carried out in the framework of the CELTIC project WISQUAS 14 (CP2-035). REFERENCES [1] H. Sari and G. Karam, Orthogonal frequency-division multiple access and its application to CATV networks, European Transactions on Telecommunications (ETT), vol. 9, no. 6, pp , Nov. Dec [2] U. Sorger, I. De Broeck and M. Schnell, Interleaved FDMA A new spread-spectrum multipleaccess scheme, ICC 98, Atlanta, Georgia, USA, June [3] M. Schnell and I. De Broeck, Application of IFDMA to Mobile Radio Transmission, ICUPC 98, Florence, Italy, Oct [4] T. Frank, A. Klein, E. Costa and E. Schultz, Robustness of IDFMA as Air Interface Candidate for Future High Rate Mobile Radio Systems, Advances in Radio Science, vol. 3, pp , [5] Wireless World Research Forum, Technologies for the Wireless Future, vol. 2, John Wiley & Sons, Ltd, [6] S. Kaiser and K. Fazel, A flexible spread-spectrum multi-carrier multiple-access system for multimedia applications, PIMRC 97, Helsinki, Finland, Sept [7] S. Kaiser and W.A. Krzymien, An asynchronous spread spectrum multi-carrier multiple access system, GLOBECOM 99, vol. 1, pp , Dec [8] N. Chapalain, D. Mottier and D. Castelain, Performance of Uplink SS-MC-MA Systems with Frequency Hopping and Channel Estimation Based on Spread Pilots, PIMRC 05, Berlin, Germany, Sept [9] C. Ciochina, F. Buda and H. Sari, An Analysis of OFDM Peak Power Reduction Techniques for WiMAX Systems, ICC 06, Istanbul, Turkey, June [10] C. Rapp, Effects of the HPA-nonlinearity on a 4-DPSK/OFDM signal for a digital sound broadcasting system, Tech. Conf. ECSC 91, Luettich, Oct [11] T. Kaitz, Channel and interference model for b Physical Layer, contribution to the IEEE b standard, [12] 3GPP TS V6.11.0: User Equipment (UE) radio transmission and reception (FDD), Mar [13] Luis Correia, "Wireless Flexible Personalized Communications: Cost 259: European Co-Operation in Mobile Radio Research," John Wiley & Sons Jan [14] CELTIC project WISQUAS website: