HARDWARE ARCHITECTURE AND LABORATORY TEST ON AN ADVANCED CD3-DVB-T RECEIVER

Transcription

1 HARDWARE ARCHITECTURE AND LABORATORY TEST ON AN ADVANCED CD3-DVB-T RECEIVER A. Morello, B. Sacco Rai Research Centre, Italy ABSTRACT CD3-OFDM is a channel estimation and equalisation algorithm developed by RAI, to improve SFN reception probability in DVB-T (Digital Video Broadcasting - Terrestrial) compliant [1] receivers. CD3 allows direct estimation of the channel transfer function in every OFDM carrier frequency position through a loop exploiting the error correction capability of a forward error correction () decoder and a frequency and time domain ing to mitigate the effects of noise and residual errors. A receiver based on CD3-OFDM is not affected by frequency-domain sub-sampling in the channel estimation, therefore the receiver can equalise also longer echoes, up to the limit of twice the guard interval duration, giving a gradual performance degradation in the interval [Tg, 2Tg]. This significantly improves reception probability in wide single-frequency networks [3]. The CD3-OFDM channel estimation and equalisation algorithm has been implemented in hardware, as an experimental prototype, based on FPGAs and DSP blocks; the CD3 board has been fitted into the European dttb Demonstrator (the first DVB-T compliant receiver), replacing the original channel correction subsystem. In this article the overall CD3 prototype architecture is highlighted. The laboratory tests are reported, showing that the CD3 prototype offers gradual performance degradation for echoes outside the guard interval, in accordance with the computer simulations. INTRODUCTION During the development of the DVB-T specification an advanced demodulation method for OFDM signals, named CD3, has been invented and proposed by the RAI Research Centre [2]. Although the final DVB-T specification was not designed to allow the full exploitation of the CD3 capabilities (i.e. a bit-rate gain of about 8%, by removing the scattered pilots), nevertheless other significant advantages of this technique were soon discovered. In fact a document (TM-1706), presented by RAI to DVB in July 1996, indicated that a DVB-T compliant CD3 receiver could significantly improve the coverage in large SFNs, for transmitter distances exceeding Km and high bit-rates (15-20 Mbit/s). This improvement derives from the gradual performance degradation offered by CD3 for echoes falling outside the guard interval [3], allowing an improved rejection of the network self-interference produced by distant (not adjacent) transmitters. All these problems have been widely discussed within the CEPT FM/PT 24 group in its preparatory meetings to the Chester 97 Agreement. Figure 1, derived by computer simulations for a regular hexagonal lattice SFN, shows the potential coverage gains offered by a CD3 receiver at 20 Mbit/s, as a function of the SFN transmitter distance (coverage limited by network self-interference, without noise) Minimum coverage probability [%] Conventional CD3-OFDM Portable Reception Fixed Reception Distance [Km] Figure 1: Minimum coverage probability versus transmitter distance, for 64-2/3 (R u = 20 Mbit/s). The results show that CD3 allows to increase the transmitters spacing in the network of about % for the same percentage of coverage. For example a good fixed coverage is

2 achieved with transmitter distances of 50 and 72 Km for conventional and CD3 receivers, respectively. Simulation results apply also to the case of portable reception. Since D 2 is inversely proportional to the number of transmitters in the network (assuming a very large coverage area), this implies that regarding the network self-interference the CD3 system potentially supports a large SFN with a number of transmitters halved or divided by three. The above results, initially based only on theory and computer simulations, are now supported by laboratory tests on the hardware prototype. THE CD3-OFDM RECEIVER ARCHITECTURE The conventional DVB-T receiver is described by the block diagram of fig.2. The first section, indicated as OFDM Front-end, includes all the conversion functions from the antenna to OFDM symbol after FFT. The following section, Equaliser, estimates the channel response, by means of a complex division of the incoming pilot tones and those stored in a look-up table (LUT), and performs the time/frequency interpolation of the result. The upper complex divider is the equaliser, in charge of restoring correct amplitudes and phases of all the OFDM constellations. A further block evaluates the channel reliability, to optimise the Viterbi decoding performance, by estimating the C/N ratio of each constellation. C is basically the square magnitude of the channel response; the evaluation of N involves averaging of the noise and interference components, also on a component can be only extracted if the transmitted signal is known, this operation is done only on the pilot tones, and the result, again, is interpolated in the frequency domain to get the missing values. The last section is the decoding and demodulation, which is fed by the equalised vectors together with the related channel reliability factors. Such signals are used to calculate the metrics for the Viterbi decoder. The CD3 receiver architecture can be viewed as a superset of that of the conventional receiver. This can be easily seen in fig.3. The first (OFDM front-end) and last ( Decoding and Demodulation) sections are exactly the same as in the conventional receiver. The Equaliser subset has the same basic building blocks, although used in a somehow different way. The CD3 section includes the functional block to be added. HARDWARE OVERHEAD FOR CD3 The introduction of CD3 requires some new hardware blocks; let us see what it means in terms of complexity. In the feedback branch, the encoder and mapper are composed by very simple combinatorial and sequential logic. The Interleaver requires a one-symbol memory, plus some address generation logic. The block is similar to the De-interleaver of the Decoding path, but is simpler, because here the RF tuner, IF conv, A/D, FFT CPE estim. & removal Demapper Deinterleaver decoder Time and Frequency Synchronisation reliability eval. Decoding and Demod. interpol. OFDM Front-end Pilot tones LUT Equalizer frequency by frequency basis. Since the N Figure 2: Structure of the conventional DVB-T receiver

3 RF tuner, IF conv. A/D, FFT CPE estim. & removal Time and Frequency Synchronisation D D e e l l a a y y Open-loop channel estimation domain domain S2 Demapper reliability eval. Pilot tones LUT Deinterleaver decoder Decoding and Demod. (standard) Equalizer subset (standard) S1 OFDM Front-end (standard) CD3 Extras mapper Interleaver encoder Figure 3: Structure of the CD3- DVB-T receiver parallelism is just three bits instead of nine. The Delay block is a FIFO memory, one-symbol long, and with a depth of 24 bits. The Open-loop estimation block (see Appendix for the algorithm description) requires a one-symbol deep FIFO memory to perform the averaging. The parallelism adopted is 22bits per axes. Arithmetic operations (accumulators, look-up tables, etc.) are also required. The required computational speed is similar to that the conventional receivers. In the prototype, the maximum possible speed in the loop has been adopted, to maximise the tracking capability in time variable channels 1 in presence of Doppler shifts. In facts, the equalisation of symbol n is based on the estimation of the channel response on symbol n-1. To achieve this minimum delay, a FIFO buffer has been adopted in front of the equaliser 2, which is read it at twice the sampling speed (about 9MHz) of the receiver. All the loop operations are carried out at clock speed of 18MHz apart from the serial Viterbi decoder, operating at 36MHz. Although this speed is easily achievable by today technology, the 1 At the moment of editing this paper, the non-static channel tests are still in progress. 2 In an optimised receiver this FIFO could be avoided, recycling the already present buffering ahead the equaliser. operation at lower (e.g. half) clock frequencies is also possible, but at a cost of an accordingly lower Doppler tracking capability. Of course, the opposite approach may also be adopted in a single-chip design, by increasing the clock speed and reducing the hardware resources. LABORATORY TEST The CD3 receiver has been tested in the laboratory using a hardware Multipath Simulator, with and without Gaussian noise. The test set-up is shown in fig.4. The used Multipath Simulator has two identical channels, with the full control of the delay, amplitude, and phase of the echo. Unfortunately, the largest allowed delay is 186µs, that is far less than the guard interval duration. To overcome this problem, a direct connection (cable) was used as the direct path, and the two channels of the simulator have been cascaded, with variable delay and amplitude. The intrinsic processing delay of the instrument, usually not relevant, in that common to every path, has been estimated in 4µs, and taken into account in the results presentation.

4 DVB-T DVB-T Modulator Modulator IF, 36MHz Long Echo Long Echo Simulator Simulator (see details) (see details) Direct Path Delayed Path Noise & Interference Test Set IF 36MHz OFDM OFDM Frontend Frontend DVB-T receiver Equalyser Equalyser classic/cd3 decoding classic/cd3 decoding Spectrum, Constellation and Response monitoring facilities BER BER Meter Meter Yφ Multipath Chann.Simul (ch#2) Multipath Chann.Simul (ch#2) Multipath Chann.Simul (ch#1) Multipath Chann.Simul (ch#1) Convertion Local Osc. ~ dual channel Multipath Simulator Long Echo simulator Delayed Path Figure 4: Laboratory test set-up for long-delayed echo test The echo delay values over the normal DVB-T operating range (i.e. over Tg) has been tested; however, even with the two cascaded simulator channels, the maximum delay achievable is =380µs, that is about 1.7Tg. Hence, the limit of 2Tg could not be investigated. A Noise and Interference Test Set allowed to introduce calibrated IF gaussian noise, to achieve the required C/N values. The second bandpass was needed to remove residual local oscillator carrier and image signal resulting in the upconversion operated into the channel simulator. The DVB-T receiver was operated alternately with CD3, and with the conventional channel equaliser. All the relevant signal information, such as spectrum, constellation, and channel response, was monitored; however, the figures are not reported here, since, in case of very long echoes, the frequency notches are so dense, that they are nearly invisible on a spectrum analyser, and the (magnitude of) channel response looks awful, due to the vicinity to the sampling limit of the reconstruction D/A converter (2Tg limit is at F sampl /2). The constellation looked noisy as if the receiver were operated in AWGN (Additive White Gaussian Noise) channel, at the same bit error rate. The following results were obtained: On AWGN channel, to achieve BER= after Viterbi decoding, the CD3 prototype requires 18.8dB versus the 19.4dB for the conventional receiver. This slightly (0.6 db) better performance is due to the time domain noise ing on the channel response estimation; adding a single echo with delays exceeding the guard interval (delays between τ g and 2 τ g ), CD3 demonstrated to be capable of gradual performance degradation according to the theory. Figure 5 compares the performance of the CD3 prototype to that of the conventional receiver, in terms of echo C/I to achieve BER= after Viterbi decoding, without noise. In the delay region 1.2 T g (270 µs) to 1.5 T g (340 µs), the gain of CD3 in terms of protection ratio is of the order of 9-10 db, and at 1.7 T g (380 µs) it is of about 7 db. C/I [db] dttb CD3 Limit of the Laboratory Multipath simulator 1,0 1,2 1,4 T/Tg 1,6 1,8 2,0 Figure 5. Single echo outside the guard interval: gradual performance degradation of CD3 prototype compared to the steep degradation of a conventional receiver

5 CONCLUSIONS The CD3-OFDM algorithm, now tested on the hardware prototype, allows to achieve the gradual performance degradation for echoes outside the guard interval. This offers significant coverage improvements in Single Frequency Networks, especially in the cases of portable reception, medium to high bit-rates, high transmitter power. Since the lock-in algorithm is slow (see Appendix), CD3 is not suitable for mobile reception, although, once locked, CD3 can effectively track non-static channels. Nevertheless, since CD3 is a superset of the conventional equaliser, sharing most blocks, a single-chip may be easily developed to operate both on static and rapidly varying channels. The hardware prototype implemented by the RAI allowed to perform laboratory tests on long echoes, to validate the computer simulations, and will enable future field trials. APPENDIX - CD3-RECEIVER OPERATION As mentioned above, the CD3-OFDM algorithm allows to estimate directly the channel transfer function for every frequency position. This is possible because of the feedback loop exploiting the error correction of the native decoder: the re-encoded, re-interleaved, re-modulated bits deliver regenerated constellations, that, after complex dividing the (delayed) incoming ones, deliver the channel response H(ω), that is further frequency and time domain ed, to reduce noise. Being evaluated in every frequency position, instead of just the pilot position, the estimation of is not affected by frequency-domain sub-sampling. Therefore, echoes exceeding the guard interval duration no longer produce aliases in the H(ω). In the conventional receiver such aliases corrupt the correct equalisation and drive it in sync-loss condition, unless large echo protection ratio is allowed. To start the CD3 loop, the first symbol must be equalised in a open-loop fashion. In presence of reference OFDM symbol this is straightforward. In DVB-T, where such symbol is missing, the same (extraction and interpolation of pilot tones) algorithm as in conventional receiver is appropriate. However, if CD3 is started in presence of long (T>Tg) echoes, as already explained such algorithm won t work, due to undersampling. Hence, a special lock-in algorithm (fig.3, block Open-Loop Estimation) has been developed. It operates into two consecutive phases: 1. Conventional pilots-based lock-in is tried, and CD3 loop is operated with narrow frequency domain (bandwidth T f =T g ), for maximum noise rejection. In case (presence of echoes outside T g ) it fails, the second phase is performed. 2. H(ω) is obtained merging the channel information from the pilot carriers (as above) and from the data carriers. To get the channel response for the data carriers, amplitude and phase of the H(ω) corresponding (only) to the data carriers are estimated by applying a non-linearity (e.g. elevation to the fourth power) and a time-domain (e.g. average) over L consecutive OFDM symbols, to reduce the data noise (in the prototype, L=50 has been used). The phase ambiguity of multiples of π/2, due to the symmetry of the m- constellations, is solved by adopting carrier-by-carrier the phase which is closer to that of the channel estimation obtained in phase 1. A frequency domain with bandwidth T f equal, for example, to 2 T g, is then applied to the merged estimated channel frequency response (i.e. the pilot and the data carriers). In phase 2, the π/2 phase ambiguity resolution is based on the hypothesis that the echoes outside the guard interval are constrained in amplitude. Therefore they produce only small additional phase rotations (i.e. less than ±π/4) compared to those produced by the echoes inside the guard interval (as estimated in phase 1). As a first approximation, confirmed by the simulation results, it is possible to say that echoes attenuated by more than 3-4 db produce phase rotations smaller than π/4 and therefore compatible with the proposed algorithms. When the CD3-OFDM lock-in is achieved, the channel tracking speed of the receiver becomes that typical of CD3-OFDM, allowing to maintain a reliable reception also in the presence of moving reflectors around the portable receiver. REFERENCES [1] European Telecommunication Standard: "Digital broadcasting systems for television, sound and data services; Framing structure, channel coding and modulation for digital terrestrial television", ETS February 1997 [2] V. Mignone, A. Morello, "CD3-OFDM: a novel demodulation scheme for fixed and mobile receivers", IEEE Transactions on Communications, COM-44(9), pp , September 1996 [3] V.Mignone, A.Morello, M.Visintin: Advanced algorithm for improving DVB-T coverage in SFN, IBC 97, Amsterdam