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Interleaving
Broadcast also allows use of interleaving, which increases robustness further by spreading packet losses evenly along a transmission stream rather than congregating in clumps as tends to happen otherwise with the effect of overwhelming FEC mechanisms. Interleaving comes at the price of increased latency, which renders it unsuitable for interactive and unicast applications requiring two-way communications, but can be tolerated for broadcast. Interleaving works by separating adjacent units of information to disperse the impact of lost packets and then reassembling them in the correct order at the destination. (See Figure 2.)
The second killer punch for DVB-NGH is its support for larger cells than the LTE/3GPP Evolved Multimedia Broadcast Multicast Services (E-MBMS) being adopted for broadcast within 4G/LTE cellular services. This follows simply because DVB-NGH supports larger guard intervals than the 33.33µs limit for LTE/4G. This equates to a maximum cell radius of 10km for LTE/4G, while DVB-NGH allows up to 107km, which enables a single high-powered transmitter to serve the whole area much more cost-effectively than multiple lower-powered ones as in 4G/LTE. Equally, the largest cells work well with single-frequency mode (SFM) digital terrestrial transmission, where adjacent transmitters share the same frequencies, enabling more channels to be delivered within a given spectrum allocation because neighboring transmitters no longer need to send each channel over a different frequency to avoid interference. SFN mode also enables power to be reduced and can increase robustness through multipath transmission from more than one transmitter.
While DVB-NGH also caters well for unicast transmission for on-demand content, here it will not scale well for large volumes. In that event, cell sizes have to be reduced to avoid the number of unicast sessions congesting the available bandwidth within each cell, and this has, of course, happened in the case of cellular services driven by increased user numbers and traffic levels. This points to the potential synergy between cellular and digital terrestrial services, which the DVB is trying to foster, looking perhaps to some future overlay infrastructure where on-demand and interactive mobile applications are served by small cell LTE/4G-type networks, and broadcast via a large cell SFN.
