Upper Layer Fec for Dvb-sh: Technical Solutions and Performance

A new system for hybrid satellite and terrestrial broadcasting of multimedia services to mobile handheld receivers has been recently standardised by DVB, namely DVB-SH. Direct mobile reception by S-band satellites is affected by obstacles (trees, buildings, bridges,..) blocking the Line-of-Sight (LoS) propagation, producing deep signal fading events lasting up-to several seconds or even more. To guarantee service continuity under such critical conditions, two approaches are defined by DVB-SH and, correspondingly, two classes of receivers are considered: Class 1 receivers support short fading protection at Physical Layer (PL), while long fading events are managed at the Upper Layer (UL); Class 2 receivers support long echo protection at physical layer, for a better exploitation of the PLFEC (Forward Error Correction) capability, at the expense of huge memory requirements (still challenging for cost effective hand-held devices). The standard adopts the 3GPP2 turbo code at the physical layer, together with a channel interleaver. The choice on the upper layer FEC is not yet taken, and conventional structures, such as an evolution of the MPE-FEC of the DVB-H standard, as well as more innovative solutions are currently proposed. The paper presents the system performance, in several DVB-SH scenarios and for different Upper Layer structures, then the results are compared to the corresponding Class 2 configurations. The long UL interleaving memories required to operate on mobile satellite channels may imply unacceptably long service switching times (zapping). Therefore a new method is proposed, allowing immediate zapping in any LoS propagation condition, and seamless transition to the steady state, where the full interleaving/ULFEC error protection is guaranteed.