Spatial spreading by multi-transmit antennas allows high spectrum efficiency and low required transmit energy in multi-user mobile radio downlinks

Spreading of transmit signals in the spectral and/or temporal domains in combination with appropriate despreading measures on the receiver side dates back to the early days of spread spectrum techniques in the 1940s and since then had fruitful effects on many information technical fields as for instance communications, radar and satellite navigation. Spreading goes along with the consumption of additional resources, that is more bandwidth and/or more transmission time is needed as compared to the unspread case. In communications, such a resource luxury can be used to accommodate an increased number of communication links, that is to enhance capacity, which is for instance the crux of CDMA mobile radio systems. A promising approach to a further capacity increase of radio communication systems is the application of multiple-input-multiple-output (MIMO) antenna arrangements. In this paper we interpret the use of such antennas as spatial spreading/despreading, that is as an investment of additional spatial resources, a view somewhat contrasting the conventional perception of MIMO. With our interpretation we can explain the capacity gain enabled by such antennas analogously to the capacity gain offered by spectral and/or temporal spreading. As an asset of MIMO antennas, the investment of additional resources in the spatial domain does not stress the spectral and temporal domains. In the paper, after the presentation of our basic considerations, these are applied to OFDM MIMO multi-user mobile radio downlinks (DL) and illustrated by simulations