THE CODED ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (COFDM) TECHNIQUE, AND ITS APPLICATION TO DIGITAL RADIO BROADCASTING 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. multipath propagation 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 radio broadcasting 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 modulation, 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 (Digital Audio 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 information 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 spectral efficiency 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 fading, 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 modulations 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.