A question regarding cooperative diversity systems

[David83]

Hi,

Can a relay transmit using a subset of sub carriers, while receiving on another orthogonal subset of sub-carriers at the same time? In another word, can it transmit on a frequency and receive on another frequency at the same time like using FDMA?

Thanks

 

[Mityan]

The answer is likely to be yes. The only problem is transmitter decoupling from receiver. You can even use the same channel carrier but upper and lower side bands.

 

[David83]

In the literature, has it been done like this? Traditionally, TDMA is used: one time slot for the source and one time slot for the relay. Now, suppose we have 2N orthogonal subcarriers and adopt the following transmission scheme:

1- S transmits N signals using the first N subcarriers to R and D.
2- S transmits another N signals using the second N subcarriers to R and D, while R transmits at the same time the received signals in the previous stage using the first N subcarriers to D.

Note that R has to transmit and receive at the same time but over different subcarrier sets that are orthogonal. It is like the principle of OFDMA. I have a concern from the practical point of view about the transmission model, and I am not sure if it is implementable and can be used in practice.

Thanks

 

[Mityan]

I am not sure if it is implementable and can be used in practice.

Can you specify your doubts?

FDMA is implementable. It was the first generation of radio systems. Many users transmit and receive at the same time. For implementation in one device there is a problem - decoupling transmitter and receiver or use a diversity tx and rx antennas. Orthogonality is not the point, it only further imploves performance. The example of radiosystem with simultaneous tx and rx is radar with continuous signal. It has separate antennas.
For one antenna directional-phase shifter may be used. I took a participation in design of communication over high power electric line. It works in both directions over one cable simultaneous. About 40+ dB analog decoupling were enough (the rest was digital)

 

[David83]

Usually OFDMA is used as a multiple access technique of a multi-user system, where the channel is divided into U subcarriers and each user assigned N=U/M subcarriers where M is the number of users. In this system, all user transmit at the same time to the base station, and each user is separable from others, thanks to the orthogonality of the subcarriers. Note that here the base station just receiving.

I am thinking if we can use the same concept on a relaying system consists of one source, one relay, and one destination the way I described it earlier. It is the same as multi-user OFDMA except that the relay will have to transmit and receive at the same time slot. My concern is here: Is this practically implementable efficiently and without extra complexity requirements than the TDMA counterpart?

Thanks