Monday, February 1, 2010

How to Broadcast Multimedia Contents? V Overloaded Transmission and Interference Cancellation

[How to Broadcast Multimedia Contents? I Introduction]
[How to Broadcast Multimedia Contents? II Lessons from The Channel]
[How to Broadcast Multimedia Contents? IV Hierarchical Modulation]
[How to Broadcast Multimedia Contents? VI Open-Loop MIMO for BCMCS]
[How to Broadcast Multimedia Contents? VII Network Layer or Steam Layer Design]
[Precoded OFDM for BCMCS, 3GPP2 TSG-C NTAH C00-20080218-006R1]

Though hierarchical modulations have been widely adopted for enhancing broadcast multicast services, several issues are still left for future enhancements. The first consideration is the inter-layer interference (ILI) between layers. The ILI from enhancement layer(s) to base layer(s) is not additive white Gaussian. The base-layer achievable spectral efficiency is actually dented by ILI more than expected. In addition, for example, when orthogonal frequency division multiplexing (OFDM) is employed on the carrier, there is a frequency selectivity issue on the layered transmission in fading channels, especially when the channel bandwidth is far more than its coherent bandwidth. With the combination of hierarchical modulation and OFDM, the base layer signal and the enhancement layer signal experience the same channel fading. There is no multi-layer diversity, which can help boost the achievable throughput.

Figure 1. An example of overloaded OFDM transmission for upgrading existing OFDM multicast/broadcast

One simple overloaded transmission solution to upgrade existing OFDM based broadcast multicast traffic channel is shown in Figure 1. With this scheme, legacy mobiles can seamlessly operate in the upgraded network without additional change. The control overhead signal part is same. The pilot part is reused. Only the traffic channel part is upgraded. The new traffic channel part is layer-modulated and transmitted with an additional pre-coded OFDM modulated enhancement layer, where the symbols are precoded with Walsh-Hadmard matrix before OFDM. In an additive white Gaussian channel, this scheme has the superposition precoding (SPC) gain since it essentially is an implementation of SPC. However, the interference from the enhancement layer is randomized due to additional Walsh-Hadmard spreading. In the fading channel, additional multi-layer diversity gain is achievable, since the base layer and the enhancement layer are operating in different signal spaces. A general overloaded OFDM transmission structure is shown in Figure 2.

Figure 2. A general overloaded OFDM transmission structure

Figure 3. An example of strictly backward compatible upgrade of existing OFDM based broadcast multicast network

Another advantage of this upgrade architecture is there is little interference between legacy air interface (AI) and new AI. The base-layer signals of the neighboring legacy local operating infrastructure (LOI) and new LOI still are soft-combinable. The enhance-layer signals of the neighboring legacy LOI and new LOI are not overlapped to each other due to its limited overage. Legacy mobiles can properly decode the base-layer of both legacy LOI and new LOI with no problem. The only degradation is that legacy mobiles may not be able to decode the enhancement-layer of the new LOI.

From an information theoretic perspective, the schemes shown in Figure 1 and 2 are interesting. However, the questions remain, can it be implemented in realities or are the overloaded signals be reasonably easy to demodulate and decode? The answer is yes and an interference cancellation receiver should be employed. A successive interference cancellation (SIC) receiver can be enough to do the trick. On the other hand, SIC is one of the advanced receivers which are widely discussed and implemented in commercial products in realities. The reason can be explained in the following and in Figure 4.

Figure 4. Asymptotic multiuser efficiency (AME) of various multiuser receivers

From a receiver design standing point, the effective energy of user 1 e1 is always upper bounded by the actual energy A21. This typically is quantified through a parameter called multiuser efficiency or asymptotic multiuser efficiency (AME). Multiuser efficiency or ratio between the effective energy and actual energy, e1/A21, of an user, depends on the signature waveforms, received signal-to-noise ratio (SNR) and the employed detector and is always not larger than 1. The AME of several popular advanced receivers are plotted in Figure 4. In Figure 4, it shows for a low signaling loading factor β = K/N, the linear multiuser receivers like decorrelator and MMSE can achieve near-optimum spectral efficiency. However, for a high loading factor β, nonlinear multiuser receivers can help obtain the optimal spectral efficiency. The loading factor β is a parameter what percentage of the system degree of freedom are used by the transmitter. For a CDMA system, it usually denotes the ratio between the number of active users and the spreading gain. However, additionally when the received signal power imbalance between desired signal and interference is large, the performance of SIC is asymptotically close to optimum receivers. For a two-user case, the imbalance requirement is A2/A1 > (1- ρ2)/|ρ|. Fortunately this imbalance requirement also is the prerequisite to SPC transmission.

From the above discussion, it is easy for us to find that the combination of overloaded transmission and successive interference cancellation can be right ingredients to achieve fading Gaussian broadcast channel capacity.

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