The fourth generation wireless communication systems
have been deployed or are soon to be deployed in many countries. However, with an
explosion of wireless mobile devices and services, there are still some
challenges that cannot be accommodated even by 4G, such as the spectrum crisis
and high energy consumption. A question to be raised as the pending standardization of 5G wireless communication networks looms is what would be the possible challenges such networks might faces. Here is a list of these;
OPTIMIZING PERFORMANCE
METRICS
The evaluation of wireless communication networks has been commonly
characterized by considering only one or two performance metrics while
neglecting other metrics due to high complexity. For a complete and fair
assessment of 5G wireless systems, more performance metrics should be considered.
These include spectral efficiency, energy efficiency, delay, reliability, fairness
of users, QoS, implementation complexity, and so on. Thus, a general framework
should be developed to evaluate the performance of 5G wireless systems, taking
into account as many performance metrics as possible from different perspectives.
There should be a trade-off among all performance metrics. This requires
high-complexity joint optimization algorithms and long simulation times.
REALISTIC CHANNEL MODELS FOR 5G WIRELESS SYSTEMS
Realistic channel models with proper accuracy- complexity
trade-off are indispensable for some typical 5G scenarios, such as massive MIMO
channels and high-mobility channels (e.g., high-speed train channels and
vehicle-to-vehicle channels). Conventional MIMO channel models cannot be
directly applied to massive MIMO channels in which different antennas may observe
different sets of clusters. Massive MIMO channel models should take into account
specific characteristics that make them different from those in conventional MIMO
channels, such as the spherical wave-front assumption and non-stationary properties.
Also, 3D massive MIMO models, which jointly consider azimuth and elevation angles,
are more practical but more complicated.
REDUCING SIGNAL PROCESSING COMPLEXITY FOR MASSIVE MIMO
One technical challenge in developing massive MIMO
systems is the signal processing complexity. As transmit and receive signals
are quite lengthy, the search algorithms must be performed over many possible
permutations of symbols.
In the current literature, massive MIMO research is
often treated as a detection problem based on a search motivated by the
well-known ML criterion. The existing detection algorithms assume that the
channel has been perfectly estimated, which appears to be an unreasonable assumption
given the size of the channel matrix and thus amount of channels to be tracked.
INTERFERENCE MANAGEMENT FOR CR (Cognitive Radio) NETWORKS
A major issue in interference-tolerant CR networks in
5G is how to reliably and practically manage the mutual interference of CR and
primary systems. Regulating the transmit power is essential for the CR system
to coexist with other licensed systems. An interference temperature model is
introduced for this purpose to characterize the interference from the CR to the
licensed networks. Interference cancellation techniques should also be applied
to mitigate the interference at CR receivers. Another issue in interference-tolerant
CR networks is that a feedback mechanism is important to periodically inform
the CR network about the current interference status at the licensed system.
