More and more devices are being equipped with wireless capabilities, causing wireless systems to move towards the commodity
market. This means that the price pressure on wireless products is high, creating a drive for low-cost solutions. This is
especially an issue for multiple antenna systems, since they require multiple radio frequency (RF) frontends. Furthermore,
the growing number of wireless standards forces for flexible solutions, which can support several standards.
The concept of direct-conversion[118] for frequency translation is promising to fulfill these requirements, since it does not need external intermediate frequency
(IF) filters and image rejection filters [119]. Instead, the image rejection is provided by the signal processing in the in-phase
(I) and quadrature (Q) arm. Therefore, this architecture opens the door to monolithic integration of the analogue front-end
and, thus, low-cost implementations.