MEDIA TECHNOLOGY & INNOVATION. General issues to be considered when planning SFNs

Transcription

1 EBU TECHNICAL MEDIA TECHNOLOGY & INNOVATION 13/03/09 General issues to be considered when planning SFNs 1. SFN networks In a Single Frequency Network (SFN), all transmitters in the network use the same channel. Thus they possess a common coverage area and cannot operate independently. They require a high degree of synchronisation; the emitted signal from different transmitters must be identical in content, signal emissions must take place at the same time (or with precisely controlled delays) and the RF carriers must comply with stringent frequency precision requirements. Spectrum usage, network design and the shape and size of coverage are all inter-related. For example, a SFN using one frequency designed to achieve nationwide coverage will probably use a different network structure than a network chain that serves the same nationwide area by means of several SFN using several frequencies on a regional basis. SFN coverage, in general, could be considered in three different sizes: Firstly, a national SFN, where all the stations use the same channel. Complete implementation of such a network using the same frequency depends on the co-ordination of the frequency being agreed everywhere on the country borders, which may be difficult. A national SFN must broadcast the same programme everywhere. National SFN providing complete coverage of a country may be generally difficult because of selfinterference effects, depending on the chosen system variant. Secondly, a regional SFN, where all the stations in a region use the same channel, but neighbouring regional SFN use different channels. Sites associated with different regions broadcast different programmes some of the time or all of time. Most of the European countries currently planning for SFN foresee regional SFN with areas of up to 200 km diameter (though the size of the regions is very different from one country to another, even for countries of comparable size). Thirdly, a sub-regional SFN configuration, where the main station and its relays use the same channel, but the neighbouring main station within the same region uses a different channel. Sites associated with different sub-regions broadcast different programmes some of the time or all of time. On a European scale, for all cases more than four channels - contrary to the suggestion of the pure "map colouring" theory - are needed for full area coverage. For a national or regional SFN coverage 5-6 channels are needed, in general. For sub-regional SFN coverage this figure is increased by about 2. SFN based on the main transmitter infrastructure of existing analogue networks are likely to be well suited for fixed roof-level reception. For networks that are intended for portable or mobile reception the largest guard interval should be chosen and in many cases, in particular for portable indoor reception, a higher transmitter density is needed.! "#$"% &' ()*#+,,-.# +, '/'&'

2 Although possible in principle, it is not advisable in an SFN to introduce local windows that is areas where some transmitters of the SFN radiate a slightly different multiplex to achieve local programme variation. Local windows cause problems for the receiver in the overlap area of the differing programmes of the multiplex since it cannot determine which programme to select. The gain in planning flexibility for local programmes would not compensate for the overall loss of coverage in the network. 2. Limitations on SFN performance 2.1 Self-interference The power of all signals in an SFN received within the time width of the guard interval is treated as useful, and contributes to the total available signal power. Outside the guard interval, only a part of the echo power is associated with the same OFDM symbol as the primary signal, and therefore contributes positively to the total useful signal power. The other part of the echo power is associated with the previous or subsequent OFDM symbol and produces inter-symbol interference. Therefore, as the signal delay is progressively increased beyond the guard interval, the useful contribution decreases and the inter-symbol interference increases. 2.2 The guard interval length In an SFN each transmitter is required to radiate the same OFDM symbol at the same time. This comes from the fact that echoes (natural or artificially generated by co-channel transmitters) shall be confined in the guard interval period. The OFDM receiver has to setup a time- window during which it samples the on-air OFDM signal. The objective is to synchronize this time-window with the useful period of the OFDM symbol. Accordingly, it will ignore the signal during the guard interval period where the receiver signal is made of a mixture of two or more OFDM symbols. If the transmitters deliver the same OFDM symbol at the same instant, or with a negligible sufficiently small time delay of few µs, the differential propagation path delay to the OFDM receiver will remain inside the guard interval period. Accordingly, the sum of the received signals will be constructive because they constitute the same OFDM symbol (no inter-symbol interference). The DVB-T specification offers a selection of system guard intervals, i.e., 1/32, 1/16, 1/8 or 1/4 times the duration of the active useful symbol duration. For 8K(2K) mode this represents a permitted guard interval duration of 28(7) µs, 56(14) µs, 112(28) µs and 224(56) µs, respectively. The selection of the appropriate guard interval parameter for digital terrestrial television affords resilience against delayed, interference causing signals in television reception. Moreover, the guard interval value chosen to operate an SFN has a major implication on the topology of the SFN network: as the guard interval duration governs the maximum echoes delay admissible by the system, it governs accordingly the maximum possible distance between co-channel transmitters (producing active echoes sources). Some modes allow setting up large SFN networks having a great distance between high and medium power transmitters sites. Some others allow smaller service areas with a greater density of low power transmitters Maximum transmitter separation distance and maximum allotment area size Inter-symbol interference gives rise to two restrictions imposed on SFN. Firstly, for a given receiving location, the main contributing signals in an SFN come from the nearby transmitters. In order to keep these contributions constructive the time delay between them must not exceed the guard interval remarkably, which means that neighbouring transmitters have to keep a certain upper limit for the distance between them. "$!

3 The recommendation given in ETSI TR (Implementation guidelines for DVB terrestrial services; Transmission aspects) is that guard interval selection for DVB-T should be based on the distance between the transmitters. The spacing between adjacent transmitters in an SFN should not be significantly greater than the propagation time permitted in the guard interval: In a 2K-FFT system the guard interval values are: 7 µs, 14 µs, 28 µs, 56 µs. These values translated into distance give respectively: 2.1 km, 4.2 km, 8.4 km, and 16.8 km. In an 8K-FFT system the guard interval values are: 28 µs, 56 µs, 112 µs, 224 µs. These values translated into distance give: 8.4 km, 16.8 km, 33.6 km, and 67.2 km. For an 8K-FFT system and guard interval of 1/4 it means that the permissible signal delay times are outside the signal delay between adjacent transmitters, when these transmitters are situated less than 67.2 km apart. Studies on the maximum distance between transmitters in theoretical SFN for DVB-T and T-DAB systems have shown that together with the guard interval the maximum inter-transmitter distance is influenced by the system variant required and the effective radiated power of the transmitters in the network. Secondly, even if the maximum separation distance for neighbouring transmitters is kept, more distant transmitters in the network may contribute destructively in such a way that a maximum extension of the SFN service area must not be exceeded in order to keep the number of relevant self-interfering transmitters small. The significance of self-interference, the resulting maximum separation distance between neighbouring transmitters and whether there is an overall maximum extension of the SFN service area depends on the chosen guard interval, the sensitivity of the system with regard to selfinterference, indicated by the relevant C/N value, and the density of the transmitters in the network. In a large SFN, it may be difficult to plan the network so that signals from transmitters a long distance away from the receiver are always of an insignificant level compared to those from nearby transmitters. This difficulty is increased because the signal levels from distant transmitters have to be calculated for small percentages of the time (typically 1%) to ensure that reception is protected for high percentages of the time (typically 99%) and the receiving aerial for portable and mobile receivers is non-directional. For DVB-T, only the most rugged system variants allow for a national extension of the SFN coverage area (for larger countries). These rugged system variants, however, provide only restricted data capacity (typically, 5-6 Mbit/s). For more practical system variants, with a data capacity between 13 and 24 Mbit/s, the size of the SFN coverage area is restricted to a diameter of km. 3. MFN versus SFN The advantage of the multi-frequency planning approach is that a large part of the existing analogue network infrastructure may be re-used. This has obvious cost-saving implications for the broadcaster but should also provide benefits for the viewer. The latter will arise in any case where channels for the digital transmissions from a particular site are the same or close to the channels used for the analogue transmissions from the same site, especially if the same polarization can be used. This allows viewers to re-use their existing receiving antenna and feeder system. "0$!

4 The introduction of SFN-based DVB services is faced with the problem that, in countries that have to switch to digital, the television spectrum is occupied by analogue services which use a MFN structure. Even if some free channel assignments exist for digital services, this is only of limited use for the introduction of an SFN-based large area service since a network can only operate in the SFN mode under the condition that its channel is cleared for the entire service area. If there are still analogue services using this channel - and this is probable as long as there is any national or regional analogue service in operation - the affected analogue transmitters would have to be shifted in frequency. Among these transmitters there will be main stations with a considerable amount of population coverage. It is questionable whether it makes sense, from the accompanying large cost efforts for broadcasters and consumers, to re-arrange an analogue service which will shortly be phased out. However, there may be suitable channel configurations that make this transformation practicable. In particular, for smaller area networks comprising only two or three high power transmitters the SFN approach may be applicable and attractive. In some countries the possibility exists that one or more channels can be released for the implementation of digital services on a nation-wide scale. These channels are either not yet allocated to television broadcasting, or they are already allocated but not used by television services. These countries are offered a good chance to implement an SFNbased digital service on a national or regional scale, which potentially represents the introduction of an attractive long term scenario from the very beginning. In general, the use of these channels it is difficult on an entire nation-wide basis because of neighbouring countries which probably use these channels for analogue television or other services or the new digital services. Co-ordination will be almost impossible in these cases, as long as the neighbours operate these channels on an MFN basis. 4. DVB-T2 and SFN planning There are important developments taking place that will provide for an increase in the DTT platform s capacity. These relate to improvements in the standards used for coding (compressing) information, to squeeze as much as possible into a given amount of spectrum, and in its physical transmission: MPEG-4 AVC (or H.264 AVC) is an improved video and audio coding compression standard. This is expected to operate at up to double the efficiency of the coding standard that is used at the moment on DTT, MPEG-2. DVB-T2 is a new transmission standard in development. Early estimates of performance of the baseline specification suggest up to 50% 1 bit rate capacity gain for a typical application for the same reception conditions, i.e. at the same C/N ratio. DVB-T2 gives the possibility to use 16k or 32k carrier modes. Due to the longer symbol lengths of the 16k and 32k modes, with the same fraction of guard interval (1/32, 1/8, 1/4,...) the guard time expressed in µs increases. This can be used to increase the possible SFN size, by keeping the guard interval as fraction of the symbol length unchanged, or it can be used to increase the capacity, by changing the guard interval as fraction of the symbol length in such a way that the guard time expressed in µs is kept unchanged. By carefully choosing the parameters, also a combination of both is possible: a (more moderate) increase of the SFN-size combined with an (more moderate) increase of capacity. 1 This value still needs to be confirmed by field trials under real conditions for different reception modes (fixed and portable reception). "$!

5 The 16k and 32k modes are feasible for fixed, portable and pedestrian mobile reception. However, they are not suited for mobile reception at higher speeds due to the increased impact of the Doppler shift of the carriers. However, it is to be noted that DVB-T2 and MPEG4 AVC requires all existing DVB-T MPEG2 settop boxes to be replaced. The biggest cost in changing broadcast technologies is the cost of replacing the consumer s STBs. In any countries, adopting a complete and instantaneous switch from DVB-T to DVB-T2 operation, there is the risk that consumers will object to being forced to buy yet more new technologies soon after the first digital switchover. To be attractive to consumers, DVB-T2 can be used to introduce new (potentially HD) services alongside, and in addition to, existing DVB-T services. Once adoption of DVB-T2 receivers has become almost universal, an upgrade of existing DVB-T multiplexes to DVB-T2 operation could be relatively painless to consumers. In addition, in terms of the introduction of new technologies, whilst MPEG-2 and MPEG-4 AVC services can co-exist within a multiplex, the introduction of DVB-T2 requires its own channel slot independent of DVB-T. Consequently, a migration strategy is required that minimises the displacement of existing DTT services - so that viewers with existing equipment are not disadvantaged. "!$!