5G networks will bring solutions to technologies and standards for the next generation of mobile communication infrastructure. The capacity of wireless networks is skyrocketing, quadrupling the number of people who can use the network at the same time and interconnecting billions of devices. 5G technology will change the way modern networks exist. 5G networks will take advantage of a series of different frequency bands to transmit information at different speeds and with entirely different transmission characteristics. The network will have to adapt according to the data requirements of each service and device. Compared to other generations of networks, 5G technology will have to find ways to utilize the network and spectrum resources in a flexible manner. This means that the new systems will have to support new intelligent ways to improve and provide further enhancements on 5G spectrum efficiency.
5G Massive MIMO & MU-MIMO
Massive MIMO technology uses additional antennas to help increase efficiency and transmitted power. This increase of antenna elements increases the overall spectrum efficiency. Massive MIMO with beamforming is implemented first at the gNB and later on the UE side. There is UE specific beamforming for data and control channels. TDD beamforming enables fast UE specific beam tracking, while multi-user MIMO (MU-MIMO) is expected to boost resource utilization. Further advantages of massive MIMO include the extensive use of low-cost, low-power components, the reduction of space, the simplification of medium access control (MAC), and the robustness of intentional interference.
However, although massive MIMO technology cancels out existing problems, it also reveals completely new ones that need to be tackled immediately. These challenges include the setting up of low-cost, low-precision components that work together efficiently and the connectivity and synchronization of ever-expanding newly integrated terminals.
Another great challenge is internal power consumption to achieve an overall reduction in the 5G system and device energy efficiency. Another crucial area is the effective support of the large number of connected devices and the number of new applications. These applications include critical alarm messaging or traffic safety, which lead to reduced latency and high reliability.
OFDMA for Uplink
This system provides higher efficiency of OFDMA over DFT-Spread-OFDM. This happens because it is more robust against multipath, even without an equalizer. Moreover, by exploiting frequency selectivity and diversity, there is an overall increased scheduling efficiency. This technique does not apply to cell edge UEs due to power back-off because of switching between OFDMA and DFT-S-OFDM at the macro site level.
Ultra Lean Carrier
With ultra-lean design, 5G carriers can minimize the always-on transmissions. In this approach, cell specific reference symbols are removed. This results in significant power savings and, at the same time, the reduction of interference. MIMO demodulation reference signals (DMRS) are only sent when required for the MIMO scheme. Dynamic sharing of PDCCH control and PDSCH data region takes place. The control overhead is significantly reduced since the synchronization signal is brought down from 5ms periodicity to 20ms. At the same time, the Physical Broadcast channel from 10ms periodicity is down to 20ms.
5G Enhanced Spectrum Usage
The overall guard band of a 5G signal is reduced at the edge of the carrier. The system operates with the maximum carrier size mapped to the specific band. This is achieved by removing the guard band between multiple carriers, for example, 5 x 20 MHz, and at the same time by adapting the operating bandwidth to each UE capability and the actual transmitted traffic. Compared to carrier aggregation, there is less control channel overhead with single PSS/SSS/PBCH and less PDCCH overhead by larger allocations. Fast load balancing is achieved across the whole bandwidth.
Dynamic TDD & Full Duplex
Fast UL/DL slot adaptation results in up to 50% efficiency gain. Faster UL/DL switching times (intra-slot switching) are achieved by improving link adaptation accuracy with TDD channel reciprocity and providing higher throughput for bursty traffic at low and medium load. With future full-duplex systems, the expectation is for an efficiency increase of up to 100%. The gain is limited to clustered small cell deployments using frequencies of more than 3 GHz unless inter-cell interference coordination is done, which also limits the gain.
ITU-R Spectrum Efficiency Requirements
ITU defines the minimum 5G performance requirements, according to ITU-R report published in 2017, entitled “Minimum requirements related to technical performance for IMT-2020 radio interface(s)”. Among other issues, spectrum efficiency is defined in this documentation. For indoor hotspots for eMBB communications, the spectral efficiency is defined at 9 b/s/Hz for the downlink and 6.75 b/s/Hz for the uplink. In a dense urban environment, the downlink is defined at 7.8 b/s/Hz, while the uplink at 5.4 b/s/Hz. Finally, for the rural application environment, the downlink is set at 3.3 b/s/Hz and the uplink at 1,6 b/s/Hz.
Proposals will be evaluated by several independent evaluation groups from all over the world. Within 3GPP, some first simulation calibration is already done, while 5G-PPP will perform simulations within the EU.
The results from the performed 3GPP RAN1 calibration were, for the example of SINR (Geometry) Distribution. The simulation included an eMBB dense urban scenario at 4 GHz with an eNB antenna element at 128 Tx/Rx and 4TxRx per transmission point. Moreover, the UE antenna elements were 4 Tx/Rx with 4TxRx per transmission point with an inter-site distance of 200 m. The initial observations were a large range of geometries, with some UEs with an extremely good SINR and almost no extreme bad UEs.
In an eMBB rural scenario at 700 MHz, an eNB antenna element of 64 Tx/Rx with 8TxRx per transmission point was used, together with UE antenna elements 2 Tx/Rx with 2TxRx per transmission point, on an inter-site distance at 1732m. The first observations were SINR geometry more distributes across UEs with some UEs with an SINR less than 0 dB and no extremely good UEs, for example, with more than 30 dB signal.
Overall, with all the above-aforementioned system enhancements, a notable gain of 5G network performance over LTE performance will be observed. It is also expected that significant spectrum efficiency enhancements are possible if MU-MIMO is used to further enhance resource allocation. Another final key aspect for providing spectrum efficiency in 5G systems is to perform the appropriate frequency spectrum allocation per service and reliability needs. It would be wise to reserve the 3.5 GHz frequency spectrum for the 5G macro cell operation. The spectrum above 6 GHz, including the mmWave in 26/28 GHz bands, will be used for small cells and radio backhauling purposes.
