Saturday, April 24, 2010

Fading Broadcast Channel Capacities II: Gaussian Broadcast Channel

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.

Wednesday, April 14, 2010

Fading Broadcast Channel Capacities I: Introduction

Broadcast multicast service (BMS) has increasingly been popular for delivering multimedia content to mobile users. BMS can be implemented through either a dedicated digital broadcast infrastructure like DVB-T/H/S2, MediaFLO and DMB or a 3rd generation and beyond radio access network like UMTS or cdma2000 network. Traditional digital broadcast air interface and network are designed with the tradeoff between the achievable capacity and intended coverage in mind. The actual throughput is limited by the maximum transmit power and the worst channel condition so that each user in the coverage area can reliably receive services. Therefore, all covered users share services with same quality. The users under good reception condition may not have advantages, even though their achievable throughput can be much higher. In addition, there are also rising interests in upgrading existing digital broadcast systems with more services for new users while be able to keep existing users unchanged, delivering additional or better QoS's to users with advanced receivers while still be able to guarantee other users' services, and providing unequal protection on digital contents with high spectral efficiency [ DVB Project 00, FLO Forum 07, Jiang 05, 3GPP2 07]. This is also encouraged by the recent advance in scalable video coding with an extension of the H.264/MPEG-4 AVC video compression standard, which provides possible adaptation capabilities for the BMS applications with no feedback necessary. Many technologies are under investigation for these goals, e.g., rateless coding, superpositioning precoding (SPC), multiple-input multiple-output (MIMO) and selective retransmission. Backward compatibility and implementation complexity are among the major concerns in upgrading existing systems. Among the candidates, SPC, e.g., hierarchical modulation, is one of the promising technologies for upgrading existing systems with larger coverage and more QoS options while maintaining strictly backward compatibility.

In system design, BMS is traditionally taken as a special case of the well-known broadcast channel model, in which one transmitter serves multiple receivers and the transmitter either know or has no prior information of the channels between itself and each receiver. For BMS, usually it is assumed that the transmitter has no such prior chanel information. So far, it is well-known that maximum sum rate of Gaussian broadcast channel is achievable with SPC, with which two independent signals are superimposed at the transmit side and successive interference cancellation is employed at the receive side. SPC has been widely proven and included in various standards, such as DVB-T, FLO, UMB, etc., and is under study for DVB-H. Currently, SPC is implemented with hierarchical modulation. In UMB and FLO, e.g., two bit streams, named enhancement layer and base layer, are coded, bit-mapped and modulated a QPSK/QPSK symbol stream, which is transmitted over orthogonal-frequency division multiplexing (OFDM). In this case, the channel fading is thought to limit the achievable spectral efficiency of the broadcast channel and complicate the receiver design. Therefore, a good channel coding is usually employed for compensating the impairments by the channel fading.

The recent progress in multiuser receiver design and CDMA has brought new attentions on how to broadcast layered contents over broadcast channels [Verdu 99, Shamai 01, Wang 08]. From an information-theoretic perspective, the channel fading on CDMA signals isn't so bad as we thought [Shamai 01]. The achievable capacity of a CDMA multiuser fading channel may be higher than a single-user channel corrupted by the same fading distribution. In fact, it asymptotically approaches the single-user Gaussian channel capacity if the system load, which is defined as the number of symbols per chip, is high enough and a optimum receiver is used. On the other hand from a signal processing standpoint, the performance of successive interference cancellation (SIC) is asymptotically close to optimum receivers while the power imbalance between the desired signal and interference is large enough [Verdu 98]. Bringing these two observations together, a precoded OFDM was proposed to broadcast layer-coded content through fading channels [Wang 08] with superimposing an additional layer of Multi-Carrier Code-Division Multiplexing (MC-CDM) signal on top of the existing OFDM transmission. The power setting of the two layers of signals are specially controlled so that a SIC receiver can be employed at the receive side to separate the signals. Here, the proposed precoded OFDM is generalized as an overloaded Quasi-Orthogonal CDM (QO-CDM) and the impact of fading on broadcast channel with QO-CDM is analyzed.