Department of Telecommunications. The Norwegian Institute of Technology. N-7034 Trondheim, Norway. and the same power.

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1 OFDM for Digital TV Terrestrial Broadcasting Anders Vahlin and Nils Holte Department of Telecommunications The Norwegian Institute of Technology N-734 Trondheim, Norway ABSTRACT This paper treats the problem of digital TV terrestrial broadcasting in presence of co-channel interference from analogue TV services. The channel capacity is calculated for the optimum distribution of the transmitted power. An OFDM scheme is presented which is designed to be robust to the analogue TV interference. Power and number of bits per symbol are assigned to each OFDM channel to give the maximum data rate for a given signal to interference ratio. Simulations of the proposed scheme, for 8 MHz bandwidth and interference from a system G PAL signal, show that acceptable error rate can be achieved for e.g. 27 Mbit/s at a signal to interference ratio of -6 db. The proposed scheme is suitable for the next generation terrestrial TV networks which are going to coexist with the current analogue systems. 1. INTRODUCTION There are currently activities, in both Europe and North America, on developing the next generation terrestrial TV broadcasting systems. The development goes in the direction of all-digital systems for both continents [1]. In the design of the new digital systems, one important consideration is that they are going to coexist with the current analogue TV systems. The frequency bands allocated to TV broadcasting are in many areas severely congested and ecient use of available bands is of great importance. The existing analogue systems are relatively sensitive to interference and reuse of frequencies has therefore been limited. The frequencies that can not be used for analogue systems in a geographical area, can be used by the new digital systems provided that they transmit with suciently low power. However, reducing the power of the digital signal makes it more susceptible to interference from the analogue signal. The capacity of the digital network is therefore likely to be limited by interference from the analogue co-channels. OFDM has been proposed as modulation method in systems designed to be robust to the analogue TV signal [2], [3]. These systems avoid interference by not transmitting in the OFDM channels at frequencies where there are peaks in the spectrum of the analogue signal. All other channels use the same number of bits per symbol and the same power. In this paper, the water lling theorem of information theory [4] is used to nd the optimum distribution of power and the theoretical capacity of a channel with interference from an analogue TV signal. An OFDM system which was designed to be robust to this interference is then presented. The robustness is achieved by using pulse shaping with low spectral sidelobes and an elaborate scheme for allocation of power and information among the OFDM channels. 2. INTERFERENCE FROM ANALOGUE TV SIGNAL The signals used for broadcasting of analogue TV consist of a frequency multiplex of luminance, colour and sound signals. The luminance and sound carries form large peaks in the spectrum of the signal. When designing a digital system it is crucial to know the details about this interfering signal to achieve good performance. Fig. 1 shows the power spectral density of an analogue TV signal. The signal is a system G PAL signal [5, 6] with NICAM digital sound. The luminance to sound carrier spacing is 5.5 MHz. The colour subcarrier is located at 4.43 MHz and the NICAM sound at 5.85 MHz. The signal shown in Fig. 1 is a colour-bar test signal [7]. However, the signal is representative for a real signal as the dominating parts are independent of the transmitted picture [7]. The PAL signal presented in this section was used for the calculations and design in this paper.

2 PSD [db] Power distribution according to the water lling theorem is illustrated in Fig. 2. S n (f) represents the bottom of a water container. The shaded regions are water and the depth of the water is S x (f). The water level is the constant. S n (f) Frequency [MHz] Figure 1. Power spectral density of a system G PAL signal with digital NICAM sound. Other colour television standards, e.g. NTSC, will in principle give the same results, but the numerical values will be dierent. 3. CHANNEL CAPACITY This section presents the theoretical channel capacity for a system with co-channel interference from an analogue TV service. The capacity is the upper bound on achievable information rate. The capacity was calculated for a total available channel bandwidth of 8 MHz (-1.25 { 6.75 MHz in Fig. 1) and with the analogue TV signal specied in the previous section as the only interference. To calculate the channel capacity, the analogue TV signal was modelled as a stationary, Gaussian, random process. This model is not accurate but it gives a reasonable approximation in this case. With these assumptions, the maximum capacity is achieved if the power spectral density of the transmitted signal, S x (f), is shaped according to the water lling theorem of information theory [4]. The theorem states that S x (f) should be chosen as S x (f) = (? S n (f)) + (1) where S n (f) is the power spectral density of the interference and () + is dened as (x) + = x x > otherwise (2) The constant is chosen such that the transmitted signal has power P, i.e. Z S x (f) df = P (3) Figure 2. Illustration of the water lling theorem. With S x (f) distributed according to the water lling theorem, the channel capacity, in bits per second, is given by [4] C = 1 2 Z log 2 (1 + S x(f) ) df (4) S n (f) This integral was solved numerically with the interference given in Section 2. The resulting capacity is shown in Fig. 3 as a function of the Signal to Interference Ratio (SIR). The SIR is dened as the ratio of received power of the desired signal to received power of the interference. C [Mbit/sec] SIR [db] Figure 3. Capacity of channel with interference from analogue TV signal. f

3 4. INTERFERENCE ROBUST OFDM SYSTEM This section presents the design and the performance of the robust OFDM system. In OFDM the modulated signal consists of several carriers equally spaced in frequency. Each carrier is modulated by an ordinary linear modulation method such as PSK or QAM. In transmission environments where the interference has sharp peaks, as in TV broadcasting, OFDM is a well suited modulation method. This property arises as the spectrum of the transmitted signal can easily be shaped by using dierent power in dierent channels. This technique has successfully been used in systems for broadcasting [2, 3]. These systems use around 5 channels and leave some of them unused, forming a gap in the spectrum of the transmitted signal where there are peaks in the interference spectrum. All used channels transmit the same constellation with the same power. The systems use rectangular pulse shape with guard interval to get simple equalisation of multipath propagation. To evaluate the proposed system, a reference system based on the principles of [3] was used. The reference system has 53 channels in bandwidth of 7.5 MHz, where the bandwidth is dened as number of channels times the channel spacing. The symbol interval is 75.3 s, of which 4.4 s is guard interval. All active channels use 16-QAM symbols and equal power. To reduce the eect of interference from analogue co-channels, the 18 channels with the strongest interference are left unused. The data rate is then 27 Mbit/s OFDM with Pulse Shaping One of the main dierences between the proposed system and the reference system is the use of pulse shaping lters. The OFDM system that was used is described in detail in [8, 9]. The pulse shaping of this system is optimised to have minimum out-of-band energy for a given length. The system is thus designed to have low spectral sidelobes and low complexity. In the proposed system, 64 channels in a 7.7 MHz bandwidth was used. The pulse length was 4 symbol intervals and the symbol interval was 8.3 s. Compared to the reference system, this system has the following advantages: It uses matched lters in the receiver and minimum channel spacing. The loss in bandwidth and signal to noise ratio associated with use of guard interval [1] is therefore avoided. The low spectral sidelobes make it possible to use relatively few channels. With 64 channel, the sidelobes outside the 8 MHz band is nearly 5 db below the main lobe. This is more than 2 db lower than for the reference system. The complexity of modulator and demodulator is about half of that of the reference system. However, the proposed system requires a slightly more complex equaliser which reduces the dierence. The low spectral sidelobes also make it possible to allocate channels close to the peaks of the interference spectrum and still avoid degradation. Fig. 4 demonstrates the performance of the OFDM system with pulse shaping compared to the reference system with rectangular pulses. The gure shows simulated BER as a function of SIR. The system with pulse shaping transmits 16-QAM symbols with same power in all used channels. The 57 channels with the lowest interference power are used which gives the same data rate as the reference system. The simulations show that approximately 5 db is gained by using the pulse shaping in this example. Fig. 4 also shows the BER of a system designed for uniform interference. The system is equal to the reference system but transmits in all OFDM channels. Figure 4. Bit error rate as a function of signal to interference ratio for the system with pulse shaping (dash-dot line) and the reference system with rectangular pulses (dashed line). The dotted line is for a system transmitting in all OFDM channels Allocation of Power and Information Using the same power in all active channels and no power where there are peaks in the interference,

4 as in the reference system, can be seen as a coarse approximation to water lling. The system presented in this paper gives a ner approximation by using dierent power and symbol constellation in dierent channels. The scheme used for allocation of power and symbol constellations to the individual channels is described next. The allocation strategy was to design the system for a Bit Error Rate (BER) equal to at an SIR equal to and transmit as many bits per second as possible. Rectangular QAM symbol constellations [11] with 2, 4, 6 or 8 bits per symbol were used. The allocation was done according to the following steps: 1. Calculate the power of the interference in each channel. The interference power in channel n is given by Z 2 = jh(f? f n n )j 2 S n (f) df (5) where H(f) is the transfer function of the receiver lters and f n is the centre frequency of channel n. 2. Set initial values of power and bits per symbol to zero in all channels. For all n set P n = and M n = 3. For all channels, calculate the power P n needed to transmit 2 bits per symbol with BER =. To calculate the error rate, it is assumed that the interference at decision has Gaussian distribution. The BER for 4-QAM (M=2) in channel n is then [11] BER n = Q s! T P n 2 2 n (6) where T is the symbol interval and Q(x) is the integral from x to innity of the unit variance Gaussian distribution. P n is thus the solution to BER n =. 4. Find the channel n with the smallest P n. As the constellation size was limited to 8 bits per symbol, only channels which have less than that allocated are consider. 5. Allocate the power and increase the constellation by 2 bits per symbol. Let P n = P n + P n and M n = M n + 2. The information is thus allocated to the channel where it costs least, in terms of power, to get the prescribed BER. 6. Update P n. The power needed to transmit M n + 2 bits per symbol in channel n is calculated similar to the calculation for M = 2. P n is the power needed in addition to the power already allocated. 7. Repeat from 4 until all power is allocated. The available power P is given by the target SIR as P = 2 (7) where 2 is the total power of the interference. The iteration stops when the dierence between the available and the allocated power is smaller than the smallest P n. As the number of bits per symbol is limited to 8, there is a possibility that the iterative procedure does not allocate all available power. However, this is not likely at SIR for which the interference is the dominating source of errors. The system designed according to this procedure can be seen as a discrete approximation to the water lling theorem. The available frequency band is made discrete as it is divided into a nite number of channels. The SIR in the individual channels are quantised by the combination of prescribing the BER and discrete number of bits per symbol Performance of the Robust System To provide acceptable TV quality, a BER in the order of 1?9 is required [12]. With suitable error correction coding this can be achieved for an uncoded BER of 1?3 [13]. The system used for evaluation of the proposed OFDM scheme was therefore designed for a target BER of 1?3. A systems was designed according to the procedure described above, for an SIR of -6 db. The resulting data rate is 27 Mbit/s. The number of bits per symbol for each channel of this system is shown in Fig. 5. Comparing to the power spectral density of the interference in Fig. 1, there is a match in that large constellations are used where the interference is weak. The inuence of changing the number of channels in the OFDM system is illustrated by Table 1. The bit rates achieved with the proposed allocation scheme for OFDM systems with 32, 64, 128 and 256 channels are shown. The systems were designed for a BER of 1?3 at SIR of -6 db and db, respectively. There is a small increase in achieved data rate when the number channels is increased. The complexity increases as log 2 (N), where N is the number of channels. Hence, neither achieved

5 8 7 6 Bits per symbol Channel no. Figure 5. Number of bits per symbol for each of the OFDM channels in the proposed system with 27 Mbit/s. data rate nor complexity is dramatically aected by increasing the number of channels. Data rate [Mbit/s] No. of channels -6 db db Table 1. Achieved data rate with the proposed allocation scheme for OFDM systems with dierent number of channels. The performance of the proposed OFDM scheme was evaluated by computer simulations. Samples of the analogue TV signal, specied in Section 2, was used as interference. Fig. 6 shows the BER as a function of SIR for the proposed system with 27 Mbit/s and for the reference system. The proposed system has approximately 16 db better resistance to the interference than the reference system. 5 db was due to the pulse shaping and the rest comes from the allocation scheme. 5. THE EFFECT OF MULTIPATH PROPAGATION Apart from co-channel interference, multipath fading is probably the largest transmission problem in broadcasting of TV. In the reference system, a guard interval is used to get simple equalisation of a multipath channel [3]. If the delay spread is smaller than the guard interval, there is neither in- Figure 6. Bit error rate as a function of signal to interference ratio for the proposed system (solid line), the reference system (dashed line), the system with pulse shaping (dash-dot line) and the system that transmits in all channels (dotted line). All systems transmit 27 Mbit/s in an 8 MHz channel. tersymbol nor interchannel interference and equalisation is done by only a multiplication of each symbol. The proposed system requires a linear lter equaliser [11] to perform on its best. This equaliser gives close to optimum performance [14] and can be implemented in a relatively simple structure [15]. It is, however, more complex than the equaliser used with guard interval. The same equaliser as with guard interval could be used but that would give degradation due to intersymbol and interchannel interference. 6. CONCLUSIONS An OFDM scheme for terrestrial broadcasting of TV has been presented. The proposed scheme is designed to be robust to co-channel interference from analogue TV services. The robustness is achieved by an elaborate allocation of power and information among the OFDM channels together with use of pulse shaping with low spectral sidelobes. The performance was evaluated by computer simulations for an 8 MHz channel with a system G PAL signal as interference. The results show that an acceptable error rate can be achieved for e.g. 27 Mbit/s at an SIR of -6 db. The robustness to co-channel interference is important to being able to utilise the congested frequency bands allocated to terrestrial broadcasting of TV. There is available capacity for transmission of digital TV in these frequency bands, provided that the new systems use low transmitted power.

6 Use of low power means that the new systems must be able to operate with a relatively high level of interference from an analogue co-channel. ACKNOWLEDGEMENT The authors would like to thank Vidar Ringset for fruitful discussions and for providing background material. REFERENCES [1] M. Sablatash, \Transmission of all-digital advanced television: State of the art and future directions", IEEE Trans. Broadcasting, vol. 4, no. 2, pp. 12{121, June [2] N. K. Lodge and A. G. Mason, \A rugged and exible digital modulation scheme for terrestrial high denition television", EBU Technical Review,, no. 253, pp. 34{39, Autumn [3] P. Appelquist and O. Franceschi, \Development of a terrestrial digital HDTV system { The HD-Divine project", EBU Technical Review,, no. 253, pp. 4{47, Autumn [4] T. M. Cover and J. A. Thomas, Elements of Information Theory, John Wiley & Sons, Inc., New York, USA, [5] D. H. Pritchard and J. J. Gibson, \Worldwide color television standards { similarities and dierences", SMPTE Journal, vol. 89, Feb [6] M. Robin, \Addendum to \Worldwide color television standards { similarities and dierences"", SMPTE Journal, vol. 89, Dec [7] G. H. Hutson, Colour Television Theory, McGraw-Hill, London, UK, [8] A. Vahlin and N. Holte, \Optimal nite duration pulses for OFDM", in Proc. IEEE GLOBECOM, San Francisco, CA, USA, Nov. 1994, vol. 1, pp. 258{262. [9] A. Vahlin and N. Holte, \Optimal nite duration pulses for OFDM", IEEE Trans. Commun., 1995, Accepted for publication. [1] A. Vahlin and N. Holte, \Use of a guard interval in OFDM on multipath channels", IEE Electron. Lett., vol. 3, no. 24, pp. 215{216, 24th Nov [11] J. G. Proakis, Digital Communications, McGraw-Hill, New York, USA, [12] Y. Wu and B. Caron, \Digital television terrestrial broadcasting", IEEE Communications Magazine, pp. 46{52, May [13] E. Stare, \Development of a prototype system for digital terrestrial HDTV", TELE,, no. 2, pp. 1{6, [14] A. Vahlin and N. Holte, \Maximumlikelihood sequence estimation for OFDM", in Proc. 7th Tyrrhenian International Workshop on Digital Communications, Viareggio, Italy, Sept. 1995, To be published. [15] B. Hirosaki, \An analysis of automatic equalizers for orthogonally multiplexed QAM systems", IEEE Trans. Commun., vol. COM{28, no. 1, pp. 73{83, Jan. 198.