There is a general view that we are rapidly approaching the capacity limits of the physical layer. But as demand for capacity continues to grow, the supply of capacity is tapering off. What to do about this and how to continue to inject capacity is being addressed at standardization meetings. Let’s take a quick look at some of these techniques and some of their challenges:
1- Carrier Aggregation: LTE-Advanced allows for aggregation of up to 100 MHz of spectrum. The cost of spectrum is certainly a major expense, but increasingly finding suitable spectrum is a major problem. Carrier aggregation in one band or across bands will also add cost and complexity to the handset.
2- Enhanced MIMO: There are many different multiple antenna systems that fall into this category, so a good analysis would break down the pros and cons of each implementation. However, in general, more antennas (up to 8 in LTE-Advanced) will add cost and not always feasible especially when the user equipment size is small while crowded towers and building rooftops will make it difficult to add more antennas on base stations.
3- Coordinated Multipoint (CoMP): Transmitting and receiving signals from a single mobile unit to multiple base stations requires good synchronization at much lower tolerances that what’s required in today’s 3G base stations. The backhaul would have to feature low latency and jitter to combine packets from different cells with minimal delays.
4- Small Cells: Low power, low-height base stations are good for boosting network capacity. It will be required to coordinate their Tx/Rx operation with the macro cells especially when using the same spectrum as interference would potentially lead to diminishing marginal return on capacity. The backhaul is a challenge for the small cell business case that must be resolved.
5- Relays: Relays operate in-band and are used mainly for coverage enhancements especially at cell edge although there are relay techniques used to enhance capacity. The challenge is to maximize the efficiency of relays since they operate in-band and therefore cut into the bandwidth available for the access network.
Small cells and relays combine under the heterogeneous network concept to provide a new topology for enhancing the capacity of wireless networks. This next generation topology will roll in the other features described above such as carrier aggregation, enhanced inter-cell interference coordination (eICIC) and coordinated multi-point to increase the capacity of future networks. The significance of this is whereas until now we have relied heavily on increasing the capacity of the physical layer (MIMO, larger channels); next generation networks will use more techniques to maximize the efficiency of the physical layer as to squeeze more capacity from a constrained resource.