From single-user to multi-user communications
The last decade has witnessed an immense increase of wireless communication services to keep pace with the ever-increasing demand for higher data transfer rates combined with higher mobility. Several techniques were proposed to satisfy this demand for reliability in long distance communications, among which are multiple antenna technologies which have advanced significantly. However, when comparing their potential capacities with their achieved throughputs, large gaps are noticed. FLOWS project partners sought to understand more fully the effects of the presence of multiple users of the multiple-antenna transmission channel. Moreover, the assets of 'joint transmission' were explored with a view to minimise the computational complex signal processing. Deploying multiple antennas at the transmitter and at the receiver, independent streams can be transmitted simultaneously from all the antennas. The capacity of the designed multiple-input multiple-output (MIMO) channel grows at least linearly with the number of antennas at both ends without bandwidth expansion or increase in transmission power. Scientists at the laboratories of Siemens, Germany came up with a series of theoretical results concerning the capacity of the designated MIMO system. Their research was then focused on the flexibility of the 'joint transmission' scheme, which is inherently compatible across a wide variety of standards. This 'multi-standard' friendliness should maximise the opportunity to reduce the complexity through re-using hardware across multiple standards, from UMTS to HIPERLAN/2 and GSM. On the other hand, such simplified mobile terminals would offer the opportunity of a more unified approach to signal processing at the base station and across different air interfaces for each user. The attainable performance of utilising a transmission scheme that requires channel knowledge at the transmitter side was evaluated for realistic scenarios. 'Joint transmission', takes advantage of the fact that the channel responses between the mobile terminal and a base station are equal for the uplink and the downlink. Therefore, only simple linear filtering of the received signal at each mobile terminal is necessitated, offering a dramatic reduction in the channel estimation efforts.