Transport Evolution for the RAN of the Future [Invited]

Comparison of digital signal processing approaches for subcarrier multiplexed 5G and beyond analog fronthaul

“…Digital transport techniques in the fronthaul section of the Radio Access Network (RAN), traditionally based on the transportation of digitized radio samples [1], cannot scale to the signal bandwidths employed by the 5 th generation (5G) mobile network [2], [3]. Furthermore, the use of multiple antenna techniques such as massive Multiple-Input Multiple-Output (mMIMO) means that bit rates over the fronthaul not only scale with the number of radio access technologies, signal bandwidths, carriers and sectors but also with the number of (spatial) radio streams, making the use of such transport techniques exceptionally challenging [2][3][4][5][6][7][8].…”

“…The Core Network (CN) is connected through the backhaul to a Central Unit (CU) where some higher-layer RAN protocols are executed. The 3GPP has standardized the F1 interface between the CU and a Distributed Unit (DU), which connects the packet data convergence protocol (PDCP) to the radio link control (RLC) layers of the mobile stack [7], [8]. This transport network section is often termed the mid-haul.…”

“…The remaining RAN protocol stack processing is split between the DU and a Radio Unit (RU). 3GPP has not reached a consensus on the split point in RAN functions between the DU and RU, but possible options within the baseband Physical (PHY) layer have been proposed [4][5][6][7][8][9][10]. The fronthaul is the transport network for this F2 interface between the functionally decomposed DU and RU.…”

“…As stated, an important aspect for 5G and beyond systems will be the increasing use of MIMO and, specifically, mMIMO antenna systems, as portrayed by the active antenna unit (AAU) in Fig.1, The complexity of handling within the AAU, and transporting to it, signals for each antenna element [8], [9], means, that current arrays are partitioned into sub-arrays, with a requirement to transport a pre-coded signal for each subarray, only. Thus, hybrid beamforming is used with some analog phase/amplitude control creating the different possible beam directions for each sub-array.…”

“…This lacks deployment flexibility and adaptability. The 5G and beyond RAN will need to be flexible and scalable, able to meet differing requirements across a wide range of use cases such as AR/VR, gaming and immersive applications for Industry 4.0 [8], [25]. Part of these requirements emanates from the need to accommodate variable 5G bandwidths, latencies and numerologies 1 .…”