Cooperative Communications

General wireless communications need to modulate input data to the given carrier wave. When the carrier frequency is higher, we can set a wider frequency bandwidth for transmission, which enables us to achieve a higher data rate. However, a higher carrier frequency causes severe propagation loss, so that in such a case the transmitter needs higher transmit power to maintain the required data rate. In particular, a mobile terminal such as a cellular phone, of which power source is a battery, has a power constraint. Therefore, it is difficult to perform high-quality communications especially in the uplink, where the mobile terminal transmits its signal to a base station.

o solve the above issue, recently a technique called "cooperative communication" has drawn attention. The technique enables the mobile terminal to steadily communicate to the base station with low transmit power by cooperating with its neighboring terminals. A typical scheme in cooperative communications is "multi-hop" transmission. This is the scheme relaying the signal over multiple terminals from the transmitting terminal to the base station when the distance between them is too long to send directly, as shown in the figure below. Since each terminal sends the signal to a neighbor, we can keep the transmit power low. Meanwhile, we must raise the following issues to be addressed; route selection and relay schemes under multiple relay terminals, and also transmission delay or packet loss especially when the number of relay terminals is large.

Concept of multi-hop transmission.

On the other hand, for more reliable transmission, our laboratory is aiming to form a "virtual antenna array" by an inter-terminal communication, where terminals share the transmitted data. The following figure illustrates the concept of the virtual antenna array. In this transmission scheme, we can apply the smart antenna technique to the antenna array, i.e., beamforming. Moreover, when multiple antennas are installed also at the base station, namely at the receiver, the system can be assumed as a virtual MIMO system, where we can perform multi-beamforming such as E-SDM for more improvement in transmission efficiency. Since each transmit antenna is actually an independent terminal, however, we have to synchronize their carrier frequencies, phases, and transmit timing among the terminals for accurately operating them as an antenna array. Moreover, practical schemes sharing the transmitted data among the terminals should be examined. We have studied virtual array systems and techniques to solve these issues.

Concept of a virtual antenna array.