Software-based receivers for DTV and mobile

Software modems are becoming essential for multimedia content delivery in portable devices. December 1, 2009

The integration of multiple wireless technologies within portable consumer electronic devices has been gathering pace in the last five years; devices such as cell phones, portable media players and portable PCs now incorporate multiple wireless technologies including Bluetooth, Wi-Fi and 3G cellular. A key requirement of wireless technology within a portable device — irrespective of the delivery mechanism, whether it is IP, cellular or broadcast — is to provide user access to a wide range of multimedia content for entertainment purposes.

Broadcast is the most effective method of delivering live TV content such as news and sports to a vast number of simultaneous users. Conversely, IP and cellular are better suited to “catch-up” TV and video-on-demand. These three wireless technologies, therefore, complement each other by enabling the user access to multimedia content anytime, anyplace and anywhere.

The silicon challenge

The multitude of terrestrial analog and digital TV standards creates significant technical challenges for wireless technology semiconductor vendors and portable CE device manufacturers — the original equipment manufacturers (OEMs). The key reason for this, as shown in Figure 1 on page 52, is due to the differing spectral allocations and channel bandwidths required to support the various broadcast standards. As a result, to date, broadcast receivers have typically only been implemented as single-standard solutions, or at best, a hybrid of one analog and one digital standard. Due to the lack of truly multistandard broadcast receivers, rather than implementing solutions utilizing multiple silicon receivers and thereby violating the two key CE market requirements of small size and low cost, OEMs have not integrated broadcast reception at all. As a result, OEMs have not benefited from economies of scale that a global platform solution could potentially enable.

Meeting the requirements of small solution footprint and low cost is essential if broadcast receivers are to become an integral part of enabling a variety of content on next-generation portable CE devices. From a semiconductor point of view, the challenge remains of how to deliver an efficient solution that can receive global broadcast standards.

Software-defined modems

Broadcast reception is traditionally implemented as two key silicon blocks: an RF tuner that selects (tunes to) the required broadcast channel, and a demodulator that post-processes the selected channel to extract the encoded broadcast information. While recent integration advances have seen the RF tuner and demodulator integrated onto a single system-on-chip (SoC), reducing cost and solution size to a degree, a truly universal receiver based upon silicon has yet to be developed. This is largely because the demodulator architectures required for differing broadcast standards (e.g. single-carrier vs. multicarrier) do not readily lend themselves to circuit reuse, thereby leading to a multistandard size — and hence, cost — penalty. However, there is an innovative solution now possible by the recent advances in low-cost powerful processing: software-defined modems.

The software-defined modem lends itself perfectly to the challenges of enabling low-cost wireless convergence by combining a multistandard RF front end with a back-end modem implemented as algorithmic code running on an existing system host processor. A software modem therefore obviates the need for one or more silicon blocks with the readily apparent cost and size benefits this brings. By redefining the conventional boundary between functions implemented in silicon hardware and those implemented in software, software modems deliver a step-change in flexibility and cost.

Figure 2 depicts a multistandard broadcast receiver. This is a commercial implementation of a software-based modem targeting portable CE devices featuring a host CPU. This soft modem solution uses a reconfigurable multistandard RF tuner and combines it with demodulation algorithms implemented entirely in software running on the system processor. Note that the analog output of the RF tuner must be converted to a suitable digital form for host CPU interfacing, which this solution does via USB interface.

Figure 3 depicts an implementation of a multistandard RF tuner capable of receiving all commercially available terrestrial TV broadcasts. This reconfigurable RF tuner architecture overcomes many challenges, including support for multiple bands, bandwidths and different modulation methods.

The challenges for the software-based demodulator are similarly complex, and the algorithms must deliver demodulation performance as good as any silicon-based solution, and meet all appropriate industry requirements such as the NorDig test specifications applied to terrestrial DVB-T receivers.

Today, a typical mass-market notebook PC with a dual core x86-based processor incorporating to receive a SD DVB-T terrestrial TV broadcast would require less than 25 percent of the total available CPU load. As the processing power available within these devices continues to increase with time, and as dedicated graphics processing resource in the form of GPUs is added to address HD video decoding and 3-D gaming, the total proportion of CPU load needed to implement software modems will rapidly decrease in relative terms.

Chet Babla is marcom director at Mirics Semiconductor.

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