The introduction of 5G NR has an impact on the RAN hardware design as well as the overall RAN architecture. Some of the areas affected are the RF modules where now OFDM capable RF hardware is required. Furthermore, Active Antennas must be deployed to provide mMIMO technique. Finally, new Baseband units are needed to offer higher capacities and support the necessary changes in the RAN architecture.
Architectural aspects
The different functional split options between a central (CU) unit and the distributed units (DUs) are handled by 3GPP specifications. Depending on each actual use case, the Operator will have to decide whether a split makes sense for the network and which architecture provides the most advantages according to the network needs. It is commonly agreed that moving the NR functions between central and distributed units will provide flexible hardware implementations. This flexibility will further allow for scalable and cost-effective solutions. Furthermore, it will be possible to have better coordination of performance features, load management, and real-time performance optimization. A new flexible architecture based on a functional split will allow the Operators to introduce NFV (Network Function Virtualization (NFV) and SDN (Software Defined Network) functionalities in their networks. This, in turn, will give the ability to have variable latencies on the transport domain and will make it possible to support services with different demands.
CU-DU Separation is defined in 3GPP TR 38.801. The Distributed Unit (DU) is a logical node that includes, depending on the functional split option, a subset of the gNB functions. The operation of the distributed unit is controlled by the control unit. A central control Unit (CU) will be a logical node that includes gNB functions similar to E-UTRAN. These functions include the transfer of user data, ciphering and deciphering, header compression mobility control functions, load balancing, and synchronization. Obviously, the functions allocated exclusively to the DU will not be part of the control unit. Furthermore, a CU logical unit can handle Network Slicing, tight Interworking with E-UTRA, E-UTRA-NR handover, and session management. The control unit also handles the contact with UEs in inactive mode and controls the operation of DUs.
Deployment scenarios
For the early deployment scenarios, a standard distributed base station architecture is assumed. This means that a baseband unit is present at the site, together with an RRH and a passive or active antenna on the mast. The initial deployment of the n78 band is based on this legacy baseband solution. On the pole, an RRH and a passive 8×8 antenna or an active antenna with 16TRx, 32TRx, or even 64TRx paths can be present. Higher TRx paths will provide higher capacities. However, the Operator must be aware that these are relatively more expensive solutions and will consume significantly more output power.
In the near future, when more small cells are deployed, the overall RAN architecture has to be reconsidered. A split between the CU and the DU may provide extra advantages, but a more detailed analysis of the CU concept might be required by each Operator. A multi-supplier scenario will be possible, especially with the use of open protocols and interfaces. However, at least for the early deployment period, a single supplier upgrade scenario will be the norm in most cases.
Antenna Hardware configurations
Currently, different antenna types are offered from different suppliers. The various antenna options include 64TRx active antenna, 32 TRx active antenna, 16TRx active antenna, and 8TRx passive multiband antenna. 16TRx, 32Trx, as well as 64TRx antennas, typically have the same panel setup, but the number of separated active elements are different.
All these antenna types have a typical 8 column in the horizontal domain, cross-polarized. However, the number of vertical elements (overall 12 vertical antenna elements) which are fed by the same Power Amplifier (PA) is either 4 for 64TRX (=8 horizontal x 4 vertical x 2 cross-polarized) or 2 for 32TRX (= 8 horizontal x 2 vertical x 2 cross-polarized) or 1 for 16 TRX (= 8 horizontal x 1 vertical x 2 cross-polarized).
The gain of the vertical split is dependent on the planning assumption, but it might rather be limited for standard urban deployments.
One must also be aware that the power consumption for the 64TRx antenna is nearly two times higher than the one for the 16TRx antenna, even though the same maximum RF output power is used. This is most likely also valid for the antenna cost, which increases exponentially according to the number of antenna TRxs used.
f-OFDM capable Hardware
With the introduction of 5G NR, RF modules and Active antennas need to be f-OFDM capable. F-OFDM is based on the existing OFDM numerology and leverages all benefits of the OFDM technique. Furthermore, in f-OFDM, a filter is applied to the time domain of OFDM symbols leading to an improved out of band emission of sub-band signals. Therefore, the ISI (Inter Signal Interference) can be minimized. This way, sub-bands with different parameters can be allocated to different users and services, to be able to support ultra-low latency and high-reliability communications. Compared to LTE, where about 90% of the allocated BW can be used, f-OFDM does not have such limitations and benefits from a better spectrum utilization where significant reductions on the usage of guard bands are present. This leads to higher throughput gains compared to the conventional OFDM technique. Filter length can be designed to exceed the cyclic prefix length.