Instead of the traditional single-coverage model, a layered broadcast model with a two-layer coverage is considered here. In this model, the broadcast station (BS) broadcast two layers of signal to all mobile stations (MS) in the covered area. The signal for the inner coverage has the achievable rate of R1 and the achievable rate for the outer coverage is R2, where R1 > R2. The MS's located near to the outer coverage edge may only be able to reliably decode the data stream of a low rate R2 while the MS's close to the BS can decode both data streams with a high sum rate R1. There many ways for achieving this two-layer broadcasting, including frequency-division multiplexing (FDM), time-division multiplexing (TDM) and superposition precoding (SPC).
|Figure 1. Achievable capacity region of broadcast channel. The coverage difference is 6dB.|
One key aspect of studying two-layer broadcast is the finding of the achievable broadcast channel capacity, which states that a little throughput sacrifice on the users near to the coverage edge may lead to a big increase for the users with good reception [Cover 72]. This concept is illustrated in Figure 1, where the achievable rates of FDM, TDM and SPC are compared. It shows that SPC can outperform FDM and TDM most of the time. With moving up the network operation point, e.g., the single-coverage operation point (r2, r2), a little bit along the SPC curve a higher throughput r2+ \Delta > r2 is achievable. There are many methods implementing SPC. The most popular one is hierarchical modulation. More generally, SPC can be implemented by overloaded CDM.