MediaFLO® and Forward Link Only (FLO) is a digital wireless technology that has been developed by an industry-led group of wireless providers. The MediaFLO® technology was designed in one case for a mobile multimedia environment and exhibits performance characteristics suited for use on cellular handsets. It uses advances in coding and interleaving to achieve high-quality reception, both for real-time content streaming and other data services. MediaFLO® technology can provide robust mobile performance and high capacity without compromising power consumption. The technology also reduces the network cost of delivering multimedia content by dramatically decreasing the number of transmitters needed to be deployed. In addition, MediaFLO® technology-based multimedia multicasting complements wireless operators' cellular network data and voice services, delivering content to the same cellular handsets used on 3G networks.
It happens that when a signal is broadcast, it can propagate to a receiver by more than one path. For example, a signal from a single transmitter can propagate along a straight line to a receiver, and it can also be reflected off of physical objects to propagate along a different path to the receiver. Moreover, it happens that when a system uses a so-called “cellular” broadcasting technique to increase spectral efficiency, a signal intended for a receiver might be broadcast by more than one transmitter. Hence, the same signal will be transmitted to the receiver along more than one path. Such parallel propagation of signals, whether man-made (i.e., caused by broadcasting the same signal from more than one transmitter) or natural (i.e., caused by echoes) is referred to as “multipath”. It can be readily appreciated that while cellular digital broadcasting is spectrally efficient, provisions must be made to effectively address multipath considerations.
Fortunately, modulation techniques such as Quadrature Amplitude Modulation (QAM) in Orthogonal Frequency Division Multiplexed (OFDM) systems, which include MediaFLO® systems, are more effective in the presence of multipath conditions than are similar modulation techniques in which only a single carrier frequency is used. More particularly, in single carrier QAM systems, a complex equalizer must be used to equalize channels that have echoes as strong as the primary path, and such equalization is difficult to execute. In contrast, in OFDM systems the need for complex equalizers can be eliminated altogether simply by inserting a guard interval of appropriate length at the beginning of each symbol. Accordingly, OFDM systems that use QAM modulation are preferred when multipath conditions are expected.
A data stream to be transmitted is encoded with a convolutional or turbo encoder and interleaved, and then successive bits are combined in a bit group that will become a QAM symbol. Several bits are in a group, with the number of bits per group typically ranging from four to seven, although groups may include more or less bits.
Interleaving of the data before bits are grouped into multi-bit symbols helps to ensure that consecutive bits of the input data stream will not land on the same QAM symbol, which could cause an error in decoding of that symbol that could result in a burst of bit-errors. By “interleaving” it is meant that the data stream is rearranged in sequence, to thereby randomize potential errors caused by channel degradation. To illustrate, suppose five words are to be transmitted, and during transmission of a non-interleaved signal, a temporary channel disturbance occurs. Under these circumstances, an entire word can be lost before the channel disturbance abates, and it can be difficult if not impossible to know what information had been conveyed by the lost word.
In contrast, if the letters of the five words are sequentially rearranged (i.e., “interleaved”) prior to transmission and a channel disturbance occurs, several letters might be lost, perhaps one letter per word. Upon decoding the rearranged letters, however, all five words would appear, albeit with several of the words missing letters. It will be readily appreciated that, under these circumstances, it would be relatively easy for a digital decoder to recover the data substantially in its entirety. After interleaving the m-ary symbols, the symbols are mapped to complex symbols using QAM principles, multiplexed into their respective sub-carrier channels, and transmitted.
Recent developments in MediaFLO® system communications have sought to employ turbo encoders which encode each physical layer packet (PLP) separately. Additionally, developments seek to employ long turbo encoders with time diversity enhancements and for improved performance in fading channels. These evolutions to the MediaFLO® system are sometimes referred to as FLO-EV. Disclosure of long turbo encoders and encoding methods are disclosed in U.S. patent application Ser. No. 12/165,663, entitled “Wireless Communication of Turbo Coded Data with Time Diversity” filed Jul. 1, 2008 and published on Oct. 29, 2009 as U.S. Patent Publication No. 2009/0268694, the contents of which are hereby incorporated by reference in their entirety. However, use of long turbo codes with known encoder/interleavers has exhibited performance degradation issues. For example, the use of long turbo coders with conventional interleaving techniques have resulted in the biasing to particular packets or frames within an OFDM superframe. Accordingly, a need exists for interleaving in a FLO system that does not introduce biases and affords better performance with long turbo codes.