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THE CODED ORTHOGONAL FREQUENCY DIVISION (COFDM) TECHNIQUE, AND ITS APPLICATION TO DIGITAL TOWARDS MOBILE RECEIVERS

J.C. RAULT, D. CASTELAIN, B. LE FLOCH

CCETT (Centre Commun d'Etudes de Telediffusion et Telecommunications) 3551 2 CESSON SEVIGNE - FRANCE

Abstract

The broadcasting channel towards mobile receivers, especiallv in a dense urban area, is particularly hostile,which makes th'e transmission of high bit rate very challenging. 2. Characteristics the urban radio channel The conjunction of an Orthogonal Frequency Division of Multiplexing (OFDM technique and a convolutional coding scheme (associated to a Viterbi decoding al orithm) is ,a As previously said, the two main characteristics of the promising solution (COFDM) studied at CCkTT, that is radio channel towards mobile receivers are the presence of suitable to cope with such a channel. and the continual changing of the channel. In the first part of the pa er, the theoretical principles of the system are detailed. f he second part concerns the In fact, studies led to a channel model in two parts: realization of a com lete COFDM s stem, designed within - the first one gives the average received ener y in an the framework of tt?e DAB (Digitay Audio Broadcasting) area of small dimensions ( a few hundreds of wavefength ). EUREKA 147 project, which is able to broadcast 5.6 Mbit/s in Experimental studies have shown that in an urban area, the a bandwidth of 7 MHz. For the time being, this rate received energy follows a log-normal distribution, of which corresponds to 16 high quality stereophonic programs. the mean value is a simple function of the received power, network aspects are pointed out as far as the deduced from the free space propagation. Fz%ction of a new service is concerned. - the second one takes into account the combination of several waves, arising from specular reflections and received after scattering by material structures near the vehicle, that cannot be considered as simple reflectors. 1. Introduction A mathematical modellin of this second art leads to the block diagram of figure 1, wkch represents $e channel [2]. The broadcasting channel towards mobile receivers, especially in a dense urban area, is particular1 hostile [l]. In The terms Aj(t) represent the Rayleigh process associated fact, the presence of multipath propagation &e to multiple with the path j, delayed by T. and produced by scattering on reflections by buildings and other scattering structures material structure near the vkhicle. Aj(t) describes a process around the vehicle, together with the electrical interferences of which the spectrum is limited to the band [fov/c,+fov/c].The arising from domestic and industrial sources makes the least favourable model that we have to consider in our transmission of high bit data rate very challen ing. Another application corresponds to the absence of a constant difficulty to face to is the continual variation o? the channel amplitude path (Twl). characteristics as a result of the changing environment of Furthermore, depending on the relation between the the vehicle. delay spread (range of values of T, over which Aj(t) is In the first section of the paper, we recall the essentially non-zero) and the bandwidth considered for the characteristics of the urban radio channel and introduce the transmission, frequency selectivity will or will not affect the problems which have to be solved in order to ensure the received signal. In urban reception, this delay usually transmission of high bit rates. extends over several microseconds. Therefore, the non- The second section deals with the eneral principles of the selectivity concerns only low bit rates which cannot be Coded Orthogonal Frequency 8ivision Multiplexing assumed for high quality sound broadcasting. (COFDM) technique that we propose in order to cope with Taking into acount the above channel modelling, it is the multipath propagation. possible to represent the effects of the transmission by The demodulation process and the decoding rules are combining the channel frequency response and time developed in section 3, while section 4 ives the variation (figure 2). This two-dimension function performances of the COFDM , particukrly in the characterizes the "selective Rayleigh channel" and admits a case of the selective Rayleigh channel. decomposition in surfaces of different sizes : In section 5, we present a hardware realization of a - the small surfaces represent the frequency-time areas com lete COFDM system, designed within the framework of where the channel can be considered as locally invariant. the gAB ( Broadcasting) EUREKA 147 roject, - the large surfaces indicate the minimum separation for which is able to broadcast 5.6 MbiVs in a bandwi&h of 7 which two small surfaces are statistically independent. MHz. For the time being, this rate corresponds to 16 high quality stereophonic programs Finally, network aspects are This decomposition constitutes the basis of the channel pointed out as far as the introduction of a new radio modulation and coding method described in the following broadcasting service is concerned. sections.

12.3.1. 0428 CH2682-318910000-0428$1 .OO 0 1989 IEEE General principles of the COFDM system 1 / f,,, . The minimum value of the symbol duration being 3. fixed b the delay spread, and the speed v of the vehicle being {xed by the service (about 200 km/h), fo must, be chosen low enough to keep the Doppler distortion negligible 3.1. The OFDM solution to cope with the frequency ( fo < 2 GHz, see section 6 ). selectivity The first idea of the system is to suppress the intersymbol 3.3. The channel coding interference due to the frequency selectivity of the channel. For this purpose, the to be transmitted is s lit The second main principle of the system is to use channel into a large number of modulated carriers. The effect of tiis coding. In fact, we have just demonstrated that the OFDM process is to decrease the frequency selectivity of the technique wipes out the intersymbol interference in the channel on each carrier of which the bit rate is reduced. This multipath channel. technique, called OFDM, is equivalent to split the time However, the OFDM technique does not suppress frequency domain into small surfaces with a dimension t, fadings. As a matter of fact, the amplitude of each carrier (symbol duration) on the time axis and l/t, on the frequency generally follows a Rayleigh law. In such a channel, the axis [3]. decrease of the error rate as a function of EJN, is extremely A simple Frequency Division Multiplexing (FDM) slow. This is why an effective channel coding system is technique, in which the spectra of the N carriers are essential. separate, has two main drawbacks, the first one being a low Let us notice that the evolution of the Rayleigh law as a and the second one bein a technolo ical frequency-time function is relatively slow when compared to difficulty in implementing a large number 07 matched ffters the density of transmitted samples on the t.ime-frequency (one for each carrier). domain. It means that the value of one received sample is As a consequence, we propose to use another solution correlated to the values of its neighbours. As a which consists in tolerating an overlapping in the spectra of consequence, in the case of a , all the received the emitted signals (figure 3), provided that certain samples taken into account by the decoder (which takes its orthogonality conditions are satisfied, which guarantee the decisions by observin a finite number of samples), can be absence of interference between the different carriers.[4]. considered as erase8 by the channel. Of course, this will lead to decision errors, whatever the coding efficiency may We define a base of N elementary orthogonal signals be. gk(t), for k=O to N-1: Nevertheless, an efficient interleaving s stem (working on both time and frequency dimensions) ahws the received for 0 It< t, g,(t) = ezMf,+k/?,t amplitudes to be independent from one sample to another, which will feed the decoder with a set of independent otherwise q(t) = 0 Rayleigh samples. In such conditions the probability of The OFDM transmitted signal can be written : receiving a group of "erased" samples at the input of the decoder decreases considerably. +m N-i The interleavin depth is relative to the dimensions of the large surfaces os the channel representation of figure 2 Such a coding system is designed to take benefit from the Cj ,represents the emitted information, having complex wideband transmission, and it can be pointed out that values taken from a finite alphabet depending on the chosen multipaths as a source of frequency diversity can be modulation. For the time being, we have concentrated our considered as an advantage. efforts on 4-PSK modulated carriers because of the sim licity and efficiency of this modulation. Nevertheless, the OFh system allows the use of more sophisticated 4. Demodulation and decoding processes if the application requires it. The s ectrum of the signal tends asymptotically towards The signal transmitted during the time interval T, = t, + A an idear rectangular spectrum, which corresponds to a can be written: spectral efficiency of 2 bitsMHz for a 4-PSK modulation. Nevertheless, the conditions of, ortho onality are no longer maintained at the receiver input, %ecause of the intersymbol interference in the time domain, resulting from where f,= fo+Wts , t, is the duration of the useful symbol the multiple paths of the channel. The implementation of a safe uard interval before each useful symbol solves this and A is the duration of the safeguard interval. probfem. The OFDM technique, by increasing the symbol ck takes its complex value in the alphabet {1+i, 1-i, -1+i, duration proportionally to the number of carriers, permits to -1 -i} for a 4-PSK modulation. choose a safeguard interval A longer than the delay spread, with an acceptable loss in the spectral efficiency. Therefore, Assumjng that the safeguard interval duration A is longer the useful period of the signal remains free of interference than the impulse response of the channel, we can say that and the orthogonality remains perfect. the received si nal will not be affected by the intersymbol interference anithus can be written Finally, the OFDM solution does not lead to implementation problems since the modulation and demodulation processings can be carried out by fast Fourier t E [O , Ts1 transform algorithms, which can easily be performed with the avalaible digital technology [4]. where H, = pk ei% stands for the channel frequency response at the frequency f,. 3.2. The temporal coherence The received signal Y(t) is translated in baseband by the The channel variations in time are essentially due to the mean of projection on two quadrature carriers of frequency Doppler effect, which is characterized b its maximum fo + N/2ts and then sampled at the frequency N I ts = 1 IT frequency f,,x = fov/C . The temporal coKerence of the channel implies that the symbol must be much shorter than

12.3.2. 0429 The obtained complex samples are The differential encoding is written : N. 1 y(nT)=(-1)" HK CK~ a1.k + i b.1.k = (l+i)Ci,k/ Ci-l,k k-0 where ai, and bik equal to +/- 1 are the outputs of the Let us write y, = (-i)'Y(nT) IN convolutional code;. The weightings of ai,k and bi,k in the N-1 branch metric of the decoder are thus thus we have Yn = 1 HKCKV Vj.k ,k and Im K,k ,k (1-i) k-0 Re Y.j-1 / (1-i) $k) / 4k)

{yJ appears as the inverse discrete Fourier transform of 5. Performances of the COFDM system {HkCk}' Figure 4 represents the evolution of the Bit Error Rate as a {H,C,} can thus be calculated by using a FFT algorithm. function of the EJN, ratio. The modulation scheme is 4-PSK We can see that, in the absence of noise, the emitted (either coherent or differential). We have used a symbols can be recontructed without error, if the frequency convolutional code with a constraint length of 7 and a rate of response H, of the channel can be estimated. 1/2. Figure 4 also oints out the results obtained when using a concatened CO& (convolutional code + Cyclotomatically As far as the choice of the coding scheme is concerned, Shortened Reed Solomon (CSRS) code). Instead of a convolutional codes can be considered as very interesting if progressive degradation, we can observe a "virtually error the condition of independence can be ensured at the input of free" channel. Especially if the system is used for data the decoder. Such a code famil can take into account the broadcasting, this approach is all the more interesting Rayleigh law, with an acceptabre increase in the decoding because the CSRS decoder is able to indicate a decoding complexity. failure. Let us remark that the CSRS codes are Maximum Distance Separable ( dmi, = N - K + ,1 ), but that they only In fact, the convolutional code is associated with a require processing on the Galois Field GF(2) instead of maximum likelihood decoding (Viterbi soft decision algorithm). The required conditions of independence are GF(2*) in the case of a Reed Solomon code, which reduces carried out by an interleaving arranjement, in the time and considerably their complexity. The chosen parameters in our frequency domains, the frequency omain being necessary application are N = 336 and N - K = 48, with 12 bit words. for a fixed reception. Let us detail the decoding rules. In fact, the received signal 6. A hardware realization Y. at the instant j is disturbed by a complex gaussian noise (which is not necessary white) and can be expressed as : A hardware realization was implemented in the UHF band, (L (L in order to validate the COFDM principles [5]. The system is 'j,k = 'j,k + Nj.k @le to,process 16, stereo programs,of 288 kBiVs (4.6 MbiVs The Viterbi decoder implements the criterion of a including additional data), in a bandwidth of posteriori maximum likelihood, which consists in maximizing 7 MHz[4]. with respect to {Ci,k} and under the code constraint the The information is transmitted on 448 carriers s aced by probability density : 15625 Hz, each carriers being 4-PSK modulated. The total duration of the symbol is short enough to ensure the n P ( ffih 11 {Hj.k 1 . {Cj.k 1 tem ral coherence of the channel, even at a speed of 200 ik kxfora carrier frequency of 1.5 GHz. This leads to minimize C 11 Yj,k - Hj,k Cj,k 112 I2 cfj,k In each frame of 24 ms (300 symbols),, one s mbol is ik forced to zero, which allows the noise analysis and tie frame synchronization, and one symbol corresponds to a fixed being the variance of the real and imaginary parts of sine-sweep, which constitutes a phase reference for the the noise. differential demodulation of the 448 carriers. Moreover, this In the case of a 4-PSK modulation, when Ci ,= Ai + iBj,, sine-sweep signal allows the computation of the impulse response of the channel, which is very useful for improving and B. being equal to +I- l), the decoder 'has to I , the accuracy of the receiver synchronization. maximize wiih respect of all the values (Ai,k, Bi,,) of the code: The binary information to be transmitted was previously processed by a convolutional code of constraint length 7 and rate 1/2, associated with a frequency interleaving over 448 carriers together with a time interleaving over 384 ms, and then differentially encoded. The modulation of the 448 carriers is achieved by means of a FFT-l algorithm. The receiver architecture is described in figure 5. The Yi.kH'i,k/&i,k appears to be the weighting of Ai,k and Bi,k in analog RF art is conventional, the channel si nal being the computation of the branch metrics in the Viterbi decoder. filtered in ltby a SAW filter (bandwidth of 7.5 dHz). After The estimation of the channel frequency response Hi,, could demodulation, the "I" and "Q" si nals are sampled and be carried out by a coherent demodulation scheme, but we converted to a diftal, form.. Tf!e FFT algorithm are have developed another solutjon based on differential performed by a ZOR N rocessor which is able demodulation. The implementation of this solution is very to compute a 51 2 complex oints FF? in about 1.1 ms. The simple. The performance degradation is in reality slight if ZORAN DSP also deals wit1the differential demodulation of account is taken of the practical limitation of coherent the 448 carriers. After the desinterleaving arrangements, the demodulation in such a hostile channel. Viterbi and the CSRS decodin s are carried out by two ASICs, developed b the SORfP French company under The differential demodulation consists in estima!ing the CCETT contracts. {he number of operations per useful frequency response Hi,, by using the values of the signal at transmitted bit remains relatively low when compared to the instant j-1, which means : other techniques such as equalization.

Hj.k = 'j-1 .k I 'j-1 .k

12.3.3. 0430 7. Network aspects Additive goussion Flexibility of a system is an important advantage when noise frequency planning must be considered. The COFDM system can easily be adapted to different configurations and several possibilities are investigated like UHF local TJ broadcasting (bandwidth of 4 to 7 MHz) by using TV Emission Reception channels in adjacent regions or broadcasting in a frequency range of 0.5 to 2 GHz, with a national L coverage. Another configuration which is very promising is a single iI frequency network (national or re ional coverage) in the 60- i 200 MHz band, using only one 4 bHz channel. It consists in a network of synchronized working on the same signal, each being considered as an active echo by any receiver. The delay s read of the equivalent channel is related to the distances getween the transmitters. It is therefore necessary to implement very long symbols (about 1 ms), with a safeguard interval able to do away with echoes from 100 km distant transmitters. The number of carriers in a given bandwidth is increased but the complexity is hardly Wove of constont omplitude greater because of the efficiency of the FFT algorithms. (may not exist)

Figure 1 : Modelling of the transmission channel 8. Conclusion

In this paper, we have presented an original system, which is able to broadcast high data rates in a selective Rayleigh channel. This technique, called COFDM, implements sophisticated processes such as Orthogonal Frequency Division Multiplexing and Viterbi decodin .The system is thus. able to take benefit from the widebanjtransmission by turning to account the information contained in all the echoes of the multipath channel while having a very good spectral efficiency and a low computation complexity. The flexibility of the system is also a very promissing point, as far as frequency planning is concerned. A field demonstration of digital sound broadcasting at the WARC ORB 88 conference in Geneva has stron ly proved the complete feasibility of the COFDM technique[67.

9. References

11 Lee W.C.Y : Mobile communications engineering. Published by McGraw-Hill 1982 [2] Pommier D., Wu Yi : Interleaving or s ectrum spreading in digital radio intended for vehicgs. EBU time Y review N0217, June 1986 Figure 2 : Channel time-frequency response [3] Weinstein S.B., Ebert D.M. : Data transmission by Frequency Division Multiplexing using the discrete Fourier Transform. IEEE trans. on comm. technology, vol. COM 19, N015, October 1971 k=O1234 [4] Alard M., Halbert R. : Principles of modulation and channel coding for for mobile receivers. EBU review ~0224,August 1987 [5] Alard M., Halbert R., Le Floch B,.Pommier D. : A new system of sound broadcasting to mobile receivers. Eurocon conference 1988 [6] Sound broadcasting lobby proves a point on a bus. Financial Times, October 7, 1988

Figure 3 : Spectrum of gk Signals

12.3.4. 0431 Performances of 4P S K-COFD M with coherent demodulation

0 : No coding A : Convolutional and algebraic coding B : Convolutional coding only G : Gaussian channel R : Rayleigh channel

Performances of 4PSK-COFDM with differential demodulation

Figure 4 : Performances of COFDM-4PSK system 1

SELECTION

UP CHANHeL SELECTION

1 Figure 5 : Synoptic diagram of the receiver I

12.3.5. 0432