THE CODED ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING. (COFDM) TECHNIQUE, AND ITS APPLICATION TO DIGITAL RADIO. BROADCASTING ...
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) 35512 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. The conjunction of an Orthogonal Frequency Division Multiplexing (OFDM technique and a convolutional coding scheme (associated to a Viterbi decoding al orithm) is ,a promising solution (COFDM) studied at CCkTT, that is suitable to cope with such a channel. In the first part of the pa er, the theoretical principles of the system are detailed. he second part concerns the realization of a com lete COFDM s stem, designed within the framework of tt?e DAB (Digitay Audio Broadcasting) EUREKA 147 project, which is able to broadcast 5.6 Mbit/s in a bandwidth of 7 MHz. For the time being, this rate corresponds to 16 high quality stereophonic programs. network aspects are pointed out as far as the F z % c t i o n of a new radio broadcasting service is concerned.
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1. Introduction The broadcasting channel towards mobile receivers, especially in a dense urban area, is particular1 hostile [l]. In fact, the presence of multipath propagation &e to multiple reflections by buildings and other scattering structures around the vehicle, together with the electrical interferences arising from domestic and industrial sources makes the transmission of high bit data rate very challen ing. Another difficulty to face to is the continual variation o? the channel characteristics as a result of the changing environment of the vehicle. In the first section of the paper, we recall the characteristics of the urban radio channel and introduce the problems which have to be solved in order to ensure the transmission of high bit rates. The second section deals with the eneral principles of the Coded Orthogonal Frequency 8ivision Multiplexing (COFDM) technique that we propose in order to cope with the multipath propagation. The demodulation process and the decoding rules are developed in section 3, while section 4 ives the performances of the COFDM modulation, particukrly in the case of the selective Rayleigh channel. In section 5, we present a hardware realization of a com lete COFDM system, designed within the framework of the gAB (Digital Audio Broadcasting) EUREKA 147 roject, which is able to broadcast 5.6 MbiVs in a bandwi&h of 7 MHz. For the time being, this rate corresponds to 16 high quality stereophonic programs Finally, network aspects are pointed out as far as the introduction of a new radio broadcasting service is concerned.
2. Characteristics of the urban radio channel
As previously said, the two main characteristics of the radio channel towards mobile receivers are the presence of multipath propagation and the continual changing of the channel. In fact, studies led to a channel model in two parts: - the first one gives the average received ener y in an area of small dimensions ( a few hundreds of wavefength ). Experimental studies have shown that in an urban area, the received energy follows a log-normal distribution, of which the mean value is a simple function of the received power, deduced from the free space propagation. - 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. A mathematical modellin of this second art leads to the block diagram of figure 1, wkch represents $e channel [2]. The terms Aj(t) represent the Rayleigh process associated with the path j, delayed by T. and produced by scattering on material structure near the vkhicle. Aj(t) describes a process of which the spectrum is limited to the band [fov/c,+fov/c].The least favourable model that we have to consider in our application corresponds to the absence of a constant amplitude path (Twl). Furthermore, depending on the relation between the delay spread (range of values of T, over which Aj(t) is essentially non-zero) and the bandwidth considered for the transmission, frequency selectivity will or will not affect the received signal. In urban reception, this delay usually extends over several microseconds. Therefore, the nonselectivity concerns only low bit rates which cannot be assumed for high quality sound broadcasting. Taking into acount the above channel modelling, it is possible to represent the effects of the transmission by combining the channel frequency response and time variation (figure 2). This two-dimension function characterizes the "selective Rayleigh channel" and admits a decomposition in surfaces of different sizes : - the small surfaces represent the frequency-time areas where the channel can be considered as locally invariant. - the large surfaces indicate the minimum separation for which two small surfaces are statistically independent. This decomposition constitutes the basis of the channel modulation and coding method described in the following sections.
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3. General principles of the COFDM system 3.1.
The OFDM solution to cope with the frequency selectivity
The first idea of the system is to suppress the intersymbol interference due to the frequency selectivity of the channel. For this purpose, the information to be transmitted is s lit into a large number of modulated carriers. The effect of tiis process is to decrease the frequency selectivity of the channel on each carrier of which the bit rate is reduced. This technique, called OFDM, is equivalent to split the time frequency domain into small surfaces with a dimension t, (symbol duration) on the time axis and l/t, on the frequency axis [3]. A simple Frequency Division Multiplexing (FDM) technique, in which the spectra of the N carriers are separate, has two main drawbacks, the first one being a low spectral efficiency and the second one bein a technolo ical matched ffters difficulty in implementing a large number (one for each carrier). As a consequence, we propose to use another solution which consists in tolerating an overlapping in the spectra of the emitted signals (figure 3), provided that certain orthogonality conditions are satisfied, which guarantee the absence of interference between the different carriers.[4]. We define a base of N elementary orthogonal signals gk(t), for k=O to N-1:
07
for 0 I t < t,
g,(t) = ezMf,+k/?,t
otherwise
q(t) = 0
The OFDM transmitted signal can be written : +m
N-i
,
Cj represents the emitted information, having complex values taken from a finite alphabet depending on the chosen modulation. For the time being, we have concentrated our efforts on 4-PSK modulated carriers because of the sim licity and efficiency of this modulation. Nevertheless,the O F h system allows the use of more sophisticated modulations if the application requires it. The s ectrum of the signal tends asymptotically towards an idear rectangular spectrum, which corresponds to a 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 the multiple paths of the channel. The implementation of a safe uard interval before each useful symbol solves this probfem. The OFDM technique, by increasing the symbol duration proportionally to the number of carriers, permits to choose a safeguard interval A longer than the delay spread, with an acceptable loss in the spectral efficiency. Therefore, the useful period of the signal remains free of interference and the orthogonality remains perfect. Finally, the OFDM solution does not lead to implementation problems since the modulation and demodulation processings can be carried out by fast Fourier transform algorithms, which can easily be performed with the avalaible digital technology [4].
3.2. The temporal coherence The channel variations in time are essentially due to the Doppler effect, which is characterized b its maximum frequencyx, , f = fov/C . The temporal coKerence of the channel implies that the symbol must be much shorter than
1, ,/, f . The minimum value of the symbol duration being 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 ( fo < 2 GHz, see section 6 ).
3.3. The channel coding The second main principle of the system is to use channel coding. In fact, we have just demonstrated that the OFDM technique wipes out the intersymbol interference in the multipath channel. However, the OFDM technique does not suppress fadings. As a matter of fact, the amplitude of each carrier generally follows a Rayleigh law. In such a channel, the decrease of the error rate as a function of EJN, is extremely slow. This is why an effective channel coding system is essential. Let us notice that the evolution of the Rayleigh law as a frequency-time function is relatively slow when compared to the density of transmitted samples on the t.ime-frequency domain. It means that the value of one received sample is correlated to the values of its neighbours. As a consequence, in the case of a fading, all the received samples taken into account by the decoder (which takes its decisions by observin a finite number of samples), can be considered as erase8 by the channel. Of course, this will lead to decision errors, whatever the coding efficiency may be. Nevertheless, an efficient interleaving s stem (working on both time and frequency dimensions) a h w s the received amplitudes to be independent from one sample to another, which will feed the decoder with a set of independent Rayleigh samples. In such conditions the probability of receiving a group of "erased" samples at the input of the decoder decreases considerably. 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 wideband transmission, and it can be pointed out that multipaths as a source of frequency diversity can be considered as an advantage. 4. Demodulation and decoding processes
The signal transmitted during the time interval T, = t, can be written:
+A
where f,= fo+Wts , t, is the duration of the useful symbol and A is the duration of the safeguard interval. c k takes its complex value in the alphabet {1+i, 1-i, -1+i, -1-i}for a 4-PSK modulation. Assumjng that the safeguard interval duration A is longer than the impulse response of the channel, we can say that the received si nal will not be affected by the intersymbol interference a n i thus can be written t
E [O
, Ts1
where H, = pk ei% stands for the channel frequency response at the frequency .,f The received signal Y(t) is translated in baseband by the mean of projection on two quadrature carriers of frequency fo + N/2ts and then sampled at the frequency N I ts = 1 I T
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The obtained complex samples are
The differential encoding is written : a1.k+ i b.1.k = (l+i)Ci,k/ Ci-l,k
N. 1
y(nT)=(-1)"
HK C
K
~
where ai, and bik equal to +/- 1 are the outputs of the convolutional code;. The weightings of ai,k and bi,k in the branch metric of the decoder are thus
k-0
Let us write
y, = (-i)'Y(nT) IN N-1
thus we have
Yn =
1
H K C K V
Re
k-0
{yJ appears as the inverse discrete Fourier transform of {HkCk}'
{H,C,}
can thus be calculated by using a FFT algorithm.
We can see that, in the absence of noise, the emitted symbols can be recontructed without error, if the frequency response H, of the channel can be estimated. As far as the choice of the coding scheme is concerned, convolutional codes can be considered as very interesting if the condition of independence can be ensured at the input of the decoder. Such a code famil can take into account the Rayleigh law, with an acceptabre increase in the decoding complexity. In fact, the convolutional code is associated with a maximum likelihood decoding (Viterbi soft decision algorithm). The required conditions of independence are carried out by an interleaving arranjement, in the time and frequency domains, the frequency omain being necessary for a fixed reception. Let us detail the decoding rules. In fact, the received signal Y. at the instant j is disturbed by a complex gaussian noise (which is not necessary white) and can be expressed as :
L( ,j'k
=
j',k
+
Nj.k
The Viterbi decoder implements the criterion of a
posteriori maximum likelihood, which consists in maximizing with respect to {Ci,k} and under the code constraint the probability density :
n i
.
P ( ffih 11 {Hj.k 1 {Cj.k 1 k
This leads to minimize
C i
11 Yj,k
- Hj,k Cj,k 112 I2 c f j , k
k
being the variance of the real and imaginary parts of the noise. In the case of a 4-PSK modulation, when Ci ,= Ai + iBj,, and B. being equal to +I- l ) , the decoder 'has to I maximize wiih respect of all the values (Ai,k, Bi,) of the code:
,
Yi.kH'i,k/&i,k appears to be the weighting of Ai,k and Bi,k in the computation of the branch metrics in the Viterbi decoder. The estimation of the channel frequency response Hi,, could be carried out by a coherent demodulation scheme, but we have developed another solutjon based on differential demodulation. The implementation of this solution is very simple. The performance degradation is in reality slight if account is taken of the practical limitation of coherent demodulation in such a hostile channel. The differential demodulation consists in estima!ing the frequency response Hi,, by using the values of the signal at the instant j-1, which means : Hj.k
= 'j-1
.k
I 'j-1
5.
Y.j-1 ,k / (1-i) $k)
and Im K , k
,k
/ (1-i) 4k)
Performances of the COFDM system
Figure 4 represents the evolution of the Bit Error Rate as a function of the EJN, ratio. The modulation scheme is 4-PSK (either coherent or differential). We have used a convolutional code with a constraint length of 7 and a rate of 1/2. Figure 4 also oints out the results obtained when using a concatened CO& (convolutional code + Cyclotomatically Shortened Reed Solomon (CSRS) code). Instead of a progressive degradation, we can observe a "virtually error free" channel. Especially if the system is used for data broadcasting, this approach is all the more interesting because the CSRS decoder is able to indicate a decoding failure. Let us remark that the CSRS codes are Maximum Distance Separable ( dmi, = N - K + ,1 ), but that they only require processing on the Galois Field GF(2) instead of GF(2*) in the case of a Reed Solomon code, which reduces considerably their complexity. The chosen parameters in our application are N = 336 and N - K = 48, with 12 bit words.
6. A hardware realization A hardware realization was implementedin the UHF band, in order to validate the COFDM principles [5]. The system is @le to,process 16,stereo programs,of 288 kBiVs (4.6 MbiVs including additional data), in a bandwidth of 7 MHz[4]. The information is transmitted on 448 carriers s aced by 15625 Hz, each carriers being 4-PSK modulated. The total duration of the symbol is short enough to ensure the tem ral coherence of the channel, even at a speed of 200 k x f o r a carrier frequency of 1.5 GHz. In each frame of 24 ms (300 symbols),, one s mbol is forced to zero, which allows the noise analysis and t i e frame synchronization, and one symbol corresponds to a fixed sine-sweep, which constitutes a phase reference for the differential demodulation of the 448 carriers. Moreover, this sine-sweep signal allows the computation of the impulse response of the channel, which is very useful for improving the accuracy of the receiver synchronization. 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 analog RF art is conventional, the channel si nal being filtered in l t b y a SAW filter (bandwidth of 7.5 dHz). After demodulation, the "I" and "Q" si nals are sampled and converted to a diftal, form.. Tf!e FFT algorithm are performed by a ZOR N digital signal rocessor which is able to compute a 512 complex oints FF? in about 1.1 ms. The ZORAN DSP also deals wit1 the differential demodulation of the 448 carriers. After the desinterleaving arrangements, the Viterbi and the CSRS decodin s are carried out by two ASICs, developed b the SORfP French company under CCETT contracts. {he number of operations per useful transmitted bit remains relatively low when compared to other techniques such as equalization.
.k
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7. Network aspects
A d d i t i v e goussion noise
Flexibility of a system is an important advantage when frequency planning must be considered. The COFDM system can easily be adapted to different configurations and several possibilities are investigated like UHF local broadcasting (bandwidth of 4 to 7 MHz) by using TV channels in adjacent regions or satellite radio broadcasting in a frequency range of 0.5 to 2 GHz, with a national coverage. Another configuration which is very promising is a single frequency network (national or re ional coverage) in the 60200 MHz band, using only one 4 bHz channel. It consists in a network of synchronized transmitters working on the same signal, each transmitter 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 greater because of the efficiency of the FFT algorithms.
L
TJ
Reception
Emission
i
i I
W o v e o f constont omplitude
(may n o t e x i s t )
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 review N0217, June 1986 [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 [4] Alard M., Halbert R. : Principles of modulation and channel coding for digital broadcasting 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
time
Y
Figure 2 : Channel time-frequency response
k
=
O
1
2
3
4
Figure 3 : Spectrum of gk Signals
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Performances of 4PSK-COFDM 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
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