Optical Fronthaul Options for Meeting 5G Requirements

Abstract

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

“…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.…”

“…A fronthaul for such an AAU needs to transport different streams for the sub-arrays, referred to as layers, plus some control signals for amplitude and phase weights. With the new DU-RU digital functional splits, with 5G bandwidths up to 400 MHz and eight layers, bit-rate requirements are expected to be between 16 Gbps and hundreds of Gbps, as the selected split point varies from the top to the bottom of the RAN PHY layer [9]. Both bandwidth and number of layers are expected to increase in the future.…”

“…Compared to digital transport, analog transport, although susceptible to impairments of distortion and additive noise from RF and optical components, achieves higher spectral efficiencies and alleviates the latency constraints [9], [11][12][13][14][15][16][17][18][19][20][21][22][23]. Analog transport is also capable natively of supporting multiple RAN technologies from different vendors, and for different mobile operators, without interoperability issues, sharing antennas and a single fiber infrastructure (as used in Distributed Antenna Systems) [11], [12].…”

“…Analog transport is also capable natively of supporting multiple RAN technologies from different vendors, and for different mobile operators, without interoperability issues, sharing antennas and a single fiber infrastructure (as used in Distributed Antenna Systems) [11], [12]. Thus, analog transport has gained renewed interest for the fronthaul of the 5G (and beyond) RAN [9], [16][17][18][19][20][21][22][23][24]. Analog Radio-over-Fiber (RoF) can transport multiplexes of the radio signals' bandwidths, using their different carrier frequencies, or through translation to different intermediate frequencies (necessary for MIMO, when the radio signals are at the same frequency), by Subcarrier Multiplexing (SCM).…”