As well known, wireless channels are characterized by temporal and spatial variations due to different reasons such as multipath propagation of the signals, and mobility of the communicating devices or their surrounding environment. This has two consequences. First, the channel quality (i.e. amplitude) varies resulting in changes in the amount of data rate (in bits/sec) that can be received reliably over the channel. Second, the channel “phase” varies, which necessities the ability of the receiver to track these changes reliably in order to demodulate the transmitted signal correctly. This is called “coherent” transmission. If the receiver is unable to track the phase variations, then the transmitter should use “non-coherent” modulation schemes, which can be detected without phase knowledge at the receiver. However, this will be at the cost of significant degradation of the data rate that can be transmitted reliably. Therefore, modern communication systems are supported with channel estimation algorithms in order to enable coherent reception. However, this is not always feasible. Channel estimation is usually accomplished by transmitting pilot signals with some frequency. Depending on the frequency of pilot transmission and the channel coherence time, some receivers might have reliable channel estimates and other receivers might not have that reliable channel estimates. This is one reason why each mobile wireless standard supports some maximum velocities for the mobile users, limited by the frequency of pilot transmission. Mobile users moving at higher speeds might not have reliable channel estimates and this means that they will not be able to receive any information via coherent transmission.

Having this in mind, we are mainly interested in this work in broadcasting systems like mobile TVs. These systems are usually transmitted using coherent modulation schemes in order to enable good quality of reception which cannot be maintained using the “low-rate” non-coherent modulation schemes. Therefore, mobile users with unreliable channel estimates will not be able to receive such applications, while the users with reliable channel estimates can receive the transmitted stream reliably. Therefore, broadcasting applications are characterized by “all or nothing” reception depending on the mobility and the channel conditions of the receiving terminals.

Alternatively, we propose a layered coding scheme from a new viewpoint that has not been addressed before in the literature. We propose a layered transmission scheme with two layers, one layer, base-layer (non-coherent-layer), that can be decoded by any receiver even if it does not have reliable channel estimates, and the other, refining-layer (coherent-layer) that can be only decoded at receivers with reliable channel estimates. The basic bits could be transmitted on the first layer and the extra bits that improve quality could be transmitted on the second layer. Therefore, receivers with unreliable channel estimates can decode the non-coherent layer and the receivers with reliable channel estimates can decode all the information and experience better service quality.

This proposed scheme can be designed using multi-resolution broadcast space-time coding, which allows the simultaneous transmission of low rate (LR) non-coherent information for all receivers, including those with no channel state information (CSI), and high rate (HR) coherent information to those receivers that have reliable CSI. The proposed scheme ensures that the communication of the HR layer is transparent to the underlying LR layer. We can show that both the non-coherent and coherent receivers achieve full diversity, and that the proposed scheme achieves the maximum number of communication degrees of freedom for non-coherent LR channels and coherent HR channels with unitarily-constrained input signals.


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