Decoupling Uplink-Downlink Associations in 5G Cellular
In existing 3G and 4G cellular networks, mobile devices associate with the same base station(BS) in the downlink(DL) and uplink(UL) directions. A key reason for this is that the overhead and control channels in each direction help inform communication in the other direction. For example, in LTE, the resource block assignments for both the DL and UL are given in DL control messages. In macrocell-based networks, there didn't appear to be any obvious reason to associate with different base stations in the two directions, since the same BS would generally provide the best performance in both directions. In the increasingly dense heterogeneous cellular networks(HCNs) of tomorrow, however, rate is largely determined by the load which can vary dramatically from BS to BS and between the DL and UL. Furthermore, there are very large received power disparities in the DL depending on the type of BS.
In this work WNCG students Sarabjot Singh and Xinchen Zhang, with faculty advisor Prof. Jeffrey Andrews, provided the first complete modeling and analysis of rate in the event of decoupled downlinks and uplinks in an HCN. The WNCG research team proposed an accurate and tractable model to characterize the uplink SINR and rate distribution in a multi-tier HCN as a function of the association rules and power control parameters. Using the developed analysis, the researchers proved that the optimal degree of channel inversion (for uplink power control) increases with load imbalance in the network. In sharp contrast to the downlink, minimum path loss association is shown to be optimal for uplink rate. Moreover, with minimum path loss association and full channel inversion, uplink SIR is shown to be invariant of infrastructure density. It is further shown that a decoupled association--employing differing association strategies for uplink and downlink--leads to significant improvement in joint uplink-downlink rate coverage over the standard coupled association in HCNs. In summary, there appear to be important advantages to decoupling the downlink and uplink in future 5G cellular networks, and this work should encourage further exploration in this direction and in the development of new control channel mechanisms that allow such decoupling.
For more details, read the full paper HERE.