In this series:
Part 1 of this series briefly compared the different approaches used by massive multiple-input/multiple-output (MIMO) versus cognitive radio in order to achieve efficient use of the spectrum.
One potential view of a future wireless system advocates for the use of orthogonal frequency-division multiplexing (OFDM) for modulation and multiple access schemes within a time division duplex mode (TDD). Unlike CDMA cellular systems which have robust interference capabilities, OFDM-based access (OFDMA) systems suffer from inter-cell interference at cell boundaries, especially in fully loaded systems where the frequency channel use is at maximum.
A review of the existing standards and literature shows that inter-cell interference mitigation techniques include:
- interference coordination
- interference cancellation
Of particular interest to this blog post is interference coordination, which includes schemes such as fractional frequency reuse (FFR), soft frequency reuse, and flexible FFR. All these techniques lend themselves to OFDMA systems. An FFR scheme uses a subset of subcarriers (a sub-band) with a frequency reuse factor of 1. These will be dedicated to users near the cell center, who therefore experience a high signal to interference and noise ratio (SINR). To avoid interference, a higher frequency reuse factor will be used for the other remaining sub-bands to serve users located near cell boundaries.
To improve the bandwidth efficiency of FFR schemes, a fractional reuse factor of 1 can still be achieved by allocating different power levels to the sub-bands as a function of user location (hence reducing inter-cell interference). A high power level is allocated to sub-bands dedicated to users at the cell boundaries while low power is assigned to the remaining sub-bands (mainly for users at the cell center). This concept is referred to as soft frequency reuse. Coupled with TDD mode, a cognitive radio can locate a given user by using the uplink information/channel state and rescheduling the user’s transmissions accordingly.
Bandwidth efficiency can further be improved by subdividing the whole bandwidth into many sub-channels. Each sub-channel will be assigned primarily to one of its adjacent cells. Depending on traffic demand, a cell can borrow some of the sub-channels reserved for the adjacent cell. This scheme is knows as flexible FFR. In fact, LTE adopts a special interface known as the X-interface to support its inter-cell interference coordination techniques by sharing information among them. Cognitive concepts of spectrum sensing and coordination can implement such an interface to some extent as well.
Nutaq’s Radio420X is a QAM64-capable radio transceiver that is ideal for experimenting with these schemes in a small area where several nodes (consisting of PicoSDRs, for instance) can be distributed. Interference coordination through the use of Gigabit Ethernet connectivity is relatively easy, especially when compared to the wireless coordination as advocated by cognitive radio.