We consider a set of small-cells deployed over a macro-cell. To coexist under the same spectrum the two systems must cooperate so that one system can adapt to the other. In this letter we assume that the macro-cell has accurate channel information (about its channel and the cross-tier channels), which is quantized and then forwarded to the small-cells. The aim is to minimize the required cross-tier information exchange rate and at the same time guarantee that the macro-cell performance is not affected by the small-cells.
If the information exchange rate is adjusted as a function of the average SNR to achieve a BER target, then the required rate increases with the log of the average SNR. Therefore, we propose two adaptive approaches where the exchange rate adapts to the instantaneous channel conditions and show that the requirements can be exponentially reduced, i.e. only a log[log[:]] increase with the SNR is enough.