1. Field of the Invention
The invention relates to the field of communications and, more particularly, to transmitting multi-layered source coded information using multiple transmit antennas.
2. Description of the Related Art
During the last decade, wireless systems with multiple transmit and receive antennas have been studied extensively, and the performance of such systems has been proven to be extremely promising. The propagation channel associated with a system with multiple transmit and receive antennas is sometimes called a multiple-input multiple-output (MIMO) channel, and the associated coding and signal processing is referred to as space-time coding. Using proper space-time coding, it is possible to use the degrees of freedom of the MIMO channel both to increase the throughput and to counteract fading. For this reason, MIMO technology is believed to become a major cornerstone in many future wireless communication systems.
Loosely speaking, communication links can be classified into two categories: point-to-point links and point-to-multipoint links. In the former case, there is exactly one transmitter and one receiver which communicate with each other at a given time, whereas for a point-to-multipoint link, a transmitted message is aimed at multiple different recipients simultaneously. A cellular communication system with mobile users is an example of a point-to-point communication system, whereas a radio/TV broadcast is an example of a point-to-multipoint link, also referred to as broadcast channels.
Point-to-multipoint links are becoming increasingly important. For instance, the introduction of Digital Audio Broadcast (DAB) and High-Definition Television (HDTV) has pioneered a whole new field of digital broadcasting applications. As a further example, it is widely believed that much of the next generation's wireless networking will be based on so-called ad-hoc networks, where it may be necessary for multiple units to listen to one message at the same time. Also, in conventional cell communication systems using directional or adaptive antennas, it is sometimes necessary to broadcast a message to the entire cell.
There are two major differences between point-to-point and point-to-multipoint communication links. First, a point-to-point connection can be optimized for a given transmitter-receiver pair. For instance, a cellular system usually employs power control techniques that adjust the transmitted power to minimize the power consumption, reduce the amount of co-channel interference, and at the same time ensure that the received signal strength exceeds a certain threshold. Second, in contrast to a point-to-point communication link, for a broadcast transmission all receivers have different capabilities to decode the transmitted message. This is so because the different receivers experience in general very different radio link qualities (for instance, due to varying propagation conditions). Moreover, since quality can usually be traded for cost, the receivers themselves may have different inherent abilities to decode the transmitted message.
The fact that distinct receivers have different capabilities for decoding a message suggests that the transmitted signal should consist of several components which are of different importance for the reconstruction of the message (and therefore have an inherently different vulnerability to transmission errors). This idea has lead to the concept of layered source coding which is now a mature technique employed in many multimedia standards. For instance, the image coding standard JPEG-2000 and the video coding standard MPEG-4 use what is sometimes referred to as “fine granularity scalability,” which enables a gradual tradeoff between the error-free data throughput and the quality of the reconstructed image or video sequence.
While such progressive source coding methods have been used in Internet applications where data rate can be traded for quality, these techniques have yet to be applied to wireless communications, particularly with respect to streaming video and audio.