On the Sum-Capacity of Degraded Gaussian Multiple-Access Relay Channels

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The sum-capacity is studied for a <emphasis emphasistype="italic"><formula formulatype="inline"><tex Notation="TeX">$K$</tex></formula></emphasis>-user physically degraded Gaussian multiple-access relay channel (MARC). Decode-and-forward (DF) is shown to achieve the sum-capacity and capacity region for a subclass of degraded Gaussian MARCs in which the multiple-access link from the sources to the relay is the bottleneck link. For the remaining subclass, DF is shown to achieve the <emphasis emphasistype="italic"><formula formulatype="inline"> <tex Notation="TeX">$K$</tex></formula></emphasis>-user sum-capacity when the sources are symmetric, i.e., they transmit with the same transmit power. The optimality of DF is conjectured for the case of asymmetric sources. </para>