Recently, we have witnessed an increasing demand for ubiquitous and reliable wireless connectivity fueled by new applications such as remote office, immersive virtual reality, autonomous vehicles and telemedicine. To meet these challenges, the telecom industry is rapidly expanding the deployment of 5G technologies.

Fundamental technology drivers in RAN for 5G and beyond

Typically, the performance of wireless cellular networks is limited by the air interface provided by the Radio Access Network (RAN), comprised of antennas, radios and baseband processing units. In principle, the 5G RAN is significantly more capable than its predecessors due to the introduction of the following enhancements:

ⅰ) Wider channels in the sub-6 GHz bands.
ⅱ) Use of millimeter wave (mmWave) spectrum.
ⅲ) Large increase in sub-6 GHz spectrum efficiency.
ⅳ) Expanded cloud orchestration.

In the traditional frequency bands (hundreds of MHz to 6 GHz), known as Frequency Range 1 or FR1, channels as wide as 100MHz are now allowed in 5G; a 5X increase in channel bandwidth compared to 4G. This is possible in 5G because recent advances in semiconductor technology are being applied in the new wide-band radio components. A wider channel (bigger “pipe”) supports a higher rate of information flow, producing higher download/upload speeds in the cellular network.

5G also adds FR2 to the traditional frequency range, which spans much higher frequencies than FR1 (e.g., 15-70GHz). These are called millimeter waves (mmWave), supporting even wider channel bandwidths than FR1, such as from hundreds of MHz to GHz.  This increases proportionally the rate of information flow. However, mmWave bands suffer from very high propagation loss and even total link loss in non-line-of-sight transmissions. Fundamentally, the transmission range in FR2 is so much smaller than FR1 that using the conventional RAN design, as in FR1, is not suitable in practice. 

Beamforming and Phase Coherency – The range difficulty in mmWave systems can be mitigated by a classical technique called phase-array beamforming (not to be confused with DSP-based beamforming used in 4G). This technique uses many active antenna elements placed in a large dense array to transmit and receive wireless signals coherently (i.e., in a very precise phase and magnitude mutual relationship). By controlling the phase and magnitude of the signals at every antenna element, constructive and destructive electromagnetic (EM) interference patterns are created over the air, including the generation of physical, three-dimensional (3D) beams that act like spotlights. This technique has been used very successfully in radars and space exploration. The net result of using physical beams in 5G mmWave communications is that the transmitted/received signal strengths into/from the beam directions are multiplied by the large number of active antennas used. The range is extended correspondingly. In addition, user separation occurs naturally, as beams illuminate only narrow solid angles. Therefore, spatial multiplexing (transmission of multiple streams of data over the same bandwidth) is easily achieved with such phased array beamforming, a powerful way to dramatically increase system capacity. However, the key to obtaining these very beneficial effects in practice is to make sure all active antenna elements operate synchronously and are calibrated to high precision under all conditions. This is a rather challenging design specification, especially when low cost is also required.

RF Coherency in Massive MIMO Systems – It was mentioned earlier that 5G is targeting a large increase in the FR1 spectrum efficiency (number of bits per Hz). Similar to mmWave systems, a general approach for achieving this goal is with active antenna arrays called Massive MIMO (MaMIMO). The attribute “massive” refers to having 16, 32, or 64 active antenna elements rather than 2, 4, 8 as in 4G. There are two general use cases for MaMIMO systems. The first use case is intended primarily for time-division multiplexing (TDD) systems and is based on channel sounding and signal processing to obtain the beamforming effects of phased arrays. Channel sounding consists of transmitting overhead pilot signals to measure the channel characteristics including the radio chains (baseband-to-baseband estimations). Then, using these channel measurements/estimations, an appropriate computation can create constructive and destructive interference patterns just like phased arrays. Since channel sounding and the computations are done for receive and transmit paths respectively, this method is appropriate for TDD where the two paths are identical (channel reciprocity).  Typically, in practical TDD systems, signal boosting and range extension are achieved consistently but spatial multiplexing is more challenging due to practical errors in channel estimation and hardware impairments. However, frequency-division multiplexing (FDD) systems using this method have shown inferior performance to date due to lack of channel reciprocity.

The second use case of MaMIMO systems is based on phased-array physical beams and is appropriate for both TDD and FDD. As per discussion above on mmWave systems, the use of physical beams naturally increases the signal strength and range and allows for easy spatial multiplexing. However, the practical MaMIMO performance is determined by the quality of its implementation. When the array is roughly synchronized/calibrated, only moderate range extension and little capacity increase are achieved. The largest range extension and capacity increases are only possible when the array elements are very precisely synchronized and calibrated, such as within a few degrees in phase error and a fraction of dB in magnitude error. We call this level of precision “RF Coherency” because it produces results practically indistinguishable from ideal phased arrays (zero phase/magnitude errors). 

Performance Improvements with Full Connectivity – Blue Danube, an NEC Company, has developed a breakthrough technology to achieve RF Coherency in 4G and 5G active arrays at much lower cost than in traditional phased array designs. This technology relies on new patented RF synchronization and calibration methodologies, implemented with low-complexity, custom mixed-signal integrated circuits (ICs), printed circuit board (PCB) connectivity methods and software/firmware methods. While this RF Coherency technology is applicable to all MaMIMO systems (16-64 Tx/Rx), it also enables the fabrication of low-cost MaMIMO arrays with reduced radio chains, where each radio chain is connected to the entire active aperture. The latter feature known as “Full Connectivity” is more advanced than that of conventional MaMIMO arrays where each radio chain is connected only to a small portion of the active aperture. RF Coherency and Full Connectivity provide excellent MaMIMO performance at reduced cost. Blue Danube has validated this concept in multiple extensive field trials over several US and international networks operating in FDD 4G bands. Notably, the Blue Danube RF Coherency and phase-array beamforming technology can be applied to any number of radio chains.

Advanced AI/ML Techniques for Spectral Efficiency  – Cloud orchestration in traditional RAN, also known as Self Optimized Network or SON technology, has been mostly limited to simple configuration updates and occasional RF coverage redistribution with Remote Electrical Tilt (RET) antennas. The introduction of MaMIMO in 5G provides an opportunity to significantly enhance Cloud orchestration to deliver great network performance improvements. For example, for the second use case of MaMIMO, assuming RF Coherency, the precise shape and placement of the 3D physical beams can be controlled dynamically and automatically from the cloud based on closed loop artificial intelligence (AI) / machine learning (ML) techniques. The net result is a drastic reduction in cell-to-cell interference and optimum RF energy match to user traffic demands, resulting in large capacity increase and better user experience (higher rates, lower dropped calls, etc.). 

The “Super-SON” capability described above can be fully provided via interfaces specified in O-RAN, a technology in which NEC is an established leader. Clearly, by combining their respective strengths in O-RAN and RF Coherency technologies, NEC and Blue Danube NEC are in a unique position to provide carrier customers with the best-in-class, highly advanced 5G RAN solution.

Synergies of Beamforming Improvements and Open RAN Adoption  – The improvement in spectral efficiency and beam agility using Blue Danube phase coherency and NEC massive MIMO Open RAN technologies create new synergies and open opportunities in the global markets addressing the use cases that were previously not possible. This combination can enable a variety of deployment models in very dense and low economy markets like India and LATAM countries. On the other hand, it can help the government and defense industry that often require much better performance and precision for their applications.

Beyond 5G, these kinds of technologies will be necessary to achieve the 6G key performance goals for enabling applications using ultra-dense deployment topologies, and more challenging spectrum situations.