This application claims the benefit of Indian provisional application number 202041046686, filed on Oct. 26, 2020; which hereby incorporated by reference.

TECHNICAL FIELD

Embodiments of the present disclosure are related, in general to communication, but exclusively related to massive multiple input multiple output (MIMO) base station (BS).

BACKGROUND

FIG. 1shows a traditional Radio Access Networks (RAN) architecture, typically deployed as a co-located architecture. As shown inFIG. 1, the RAN architecture includes Base station (BS), backhaul and core. The BS comprises remote radio head (RRH)/radio unit (RU) and base band unit (BBU). The RU comprises amplifiers, filters and an analog frontend components of a transceiver, and the BBU with the baseband modem were typically co-located either at the top or bottom of the cellular tower. When located at the bottom of the tower, RF cables from the antenna feeds at the top of the tower were used to connect to the units at the bottom of the tower. These methods typically had significant losses due to these RF cables and warranted special solutions.

FIG. 2shows a centralized RAN architecture, wherein common public radio interface (CPRI), a protocol for IQ data transmission is used between the RRH and BBU via optical fiber. This architecture allowed the BBU unit to be located at a distant location from the cell tower in which case the CPRI cables had to be long such as few kilometers (kms) to support a centralized deployment

A centralized RAN deployment further evolved to support a cloud-RAN deployment, which allowed the BBU to coordinate radio units across multiple sites, better coordinate interference across multiple sites and improve overall network throughput. The front haul (FH) was supported initially supported by CPRI. However, it was soon realized that CPRI has too much overhead and lot of bandwidth wastage even during idle times. Quickly, this was replaced by eCPRI standard (enhanced CPRI) which supports Ethernet based IP packet flows only when traffic is present, which is not point-to-point like CPRI. This architecture also allowed for the BBUs to be virtualized to be able to support on-demand services.

A cloud-RAN architecture evolved which could support more splits. The BBU itself is split into a real-time distributed unit (DU) and a non-real-time control unit (CU), where a single DU can source multiple RUs and a single CU can source multiple DUs. However, there is not a single place where the split between RU, DU and CU can be done in the entire protocol stack of a wireless network such as 4G and 5G.

FIG. 3shows block diagram of possible splits using the various components of a wireless stack. Depending on the splits between the CU-DU and DU-RU, the bandwidth and latency requirements change in the entire design. This problem especially aggravates with the introduction of the massive MIMO solutions to enhance capacity and coverage.FIG. 4shows DU-RU split with 5 potential split points. The bandwidth requirements on the DU-RU interface change from a mere 100 Mbps at split 6 to 250 Gbps at split 8 when multiple antennas, higher order modulations and 100 MHz bandwidth systems are considered. Hence, this requires a careful analysis and design of the entire wireless system.

The split is also commonly referred to as the low-PHY and high-PHY split. The low PRY is the portion closer to the radio and the high PHY is the one closer to the baseband. The low PHY portion is typically referred to as the RU and the high PRY portion as the DU. The high PHY does the baseband processing at bit level including FEC, and modulation mapping. The output of this is passed to a precoder module. The precoding module can be either in low-PHY or the high-PHY depending on the split used. Depending on where the precoding module is present the two nodes i.e., the DU and RU may have to communicate with each other the precoding vectors for the DL MIMO operation. Similarly, in the uplink the beamforming on the received vectors can be performed either in the high PHY or the low-PHY portions. Depending on where the split is and what operations are performed in each block, the DU-RU or the low-PRY and high-PHY blocks exchange information necessary for precoding and beamforming operations via the interface. If the beamforming is done in low-PHY, then the high-PHY must in advance send these beamforming vectors to the low-PHY which can be based on some reference signals. However, the low-PHY itself can design its own beamforming vectors then there is no need for any message exchange. The low-PHY then sends the beamformed received signals to the high PHY for further processing.

Modern designs have baseband and RF units split out in the form of distributed unit (DU) and radio unit (RU). The DU handles the baseband processing part and the RU handles the radio part including antennas, filters, ADCs and DACs. The output of the RUs is fed to the DUs for further processing. The DU and RU communicate with each other and typically the RU acts as a slave to the master DU. The DU may in turn communicate to the CU (central unit) which can house the scheduler in some designs or the DU itself may house the scheduler. The control from DU to RU may be sent via ORAN messages which includes the control plane and user plane messages among others.

Another RAN architecture that supports a vertical split along the various protocol layers and horizontal splits within a protocol layer to support control plane and user plane traffic separation is proposed by the ORAN alliance. This alliance narrowed down to split, between DU-RU and split 2 between CU and DU. They allow 2 variants of the devices in the 7.2 split based on where digital beamforming is performed. This architecture depends on the deployments chosen by a telecom operator. To support this deployment, eCPRI protocol is used and has been created various types of messages.

To accommodate this split, new interfaces defined as control plane, user plane, synchronization plane and management plane are defined. The CUS Plane is at least one of Control, User and Synchronization Plane which is configured to:Covers real-time control-plane communications between the DU and RUCovers user plane traffic in DL and UL between DU and RUCovers synchronization of the RU generally sources from the DUUses Ethernet transport and eCPRI/IEEE 1914.3 radio application transport

M-Plane: Management PlaneCovers management of the Radio Unit (RU) as governed by the DUProvides all non-real-time control of the RU (Real time control uses the C-Plane)Uses Ethernet (UDP/IP) transport

Different blocks such as modulation, scrambling, precoding, rate matching and the reverse operations for the uplink are split between the DU and RU as per the split option.

Depending on the number of the antennas at the RU side which is connected to the antennas, the DU can send beamforming vectors to the RU. The RU can apply beamforming vectors by itself as well. The DU-RU can exchange their capabilities at the startup. There is a S-plane based synchronization via PTP module on both sides and the grand-master clock to which the nodes synchronize. Based on the timeline, messages are exchanged. The messages between DU and RU can be compressed and sent to save BW. This is called IQ compression. Then the other node has to decompress before using the information. The messages are exchanged both DL and UL directions. In either case DU is the master and RU will be the slave.

A single user (SU)/multi user (MU) multiple input multiple output (MIMO) in uplink, a single user may employ multiple transmit antennas and transmit either single or multiple layers. The multiple layers mean multiple modulation data streams that are transmitted simultaneously. Such multi-layer transmission increases the total data rate per user when the channel conditions are favorable to allow reliable decoding of each layer.

In Single-layer SU-MIMO, single layer transmission is preferable under low SINR conditions while multiple layer transmission is desirable when SINR is high. The multiple antennas available at the BS will be used to coherently combine the signal outputs of the receiver antennas. Such receiver is commonly referred to as Maximal Ratio Combining (MRC) or matched-filter receiver. In some implementations, the interference that arises from the transmissions of users located in other cells can be mitigated/reduced by using interference suppression receivers such as IRC. These conventional receivers rely on an estimate of the CSI and an estimate of the noise and interference covariance matrix (NICM). Such estimates of CSI and NICM are generally acquired by using the reference signals that are transmitted along with the data. The proposed embodiments are applicable for 4G LTE and 5G NR cellular systems and the like.

In 5G NR, a slot typically comprises data/control information carrying OFDM symbols as well as reference signals that are also known as DMRS. The number of DMRS symbols are typically configurable. Two DMRS symbols configurations are common and even 3 symbol DMRS transmission is permitted by the 5G specifications. The duration of a slot is a function of subcarrier width and other system requirements. A slot of 0.5 ms duration with 30 KHz subcarrier spacing with a bandwidth of up to 100 MHz is commonly used in deployment scenarios. The carrier frequency is a configurable parameter. The specification supports FR1 frequencies in the sub 6 GHz range and use of above 6 GHz is designated as FR2.FIG. 5shows a Frame structure for 5G NR.FIG. 6shows a representation of a slot structure with SS block reference signals in a particular location as per the band, numerology, etc.

FIG. 7shows an example DMRS configurations to support multiple layers in a slot. As shown inFIG. 7, which is an example DMRS configurations for 5G NR system in a slot. The DMRS can be configured in multiple locations in slots. The frequency domain tones are loaded based on the number of layers supported by the user in SU-MIMO and the number of users in MU-MIMO situation.

Common channels like SSB are beam formed using a common beam. User specific signals such as PDCCH, PDSCH, DMRS can be sent using user specific beams in the downlink. There are other channels such as CSI-RS, which may be sent in broadcast or user specific manner based on the configurations. The common beams can be sector beams or pre-defined beams or dynamically configured based on user locations among others.

FIG. 8shows a block diagram representation of Multi-layer SU-MIMO. In Multi-layer SU-MIMO, the UE is allowed to transmit multiple layers only when the channel between the user and the BS is such that mutual interference between the layers can be eliminated at the receiver by use of receivers with user interference suppression ability. Examples of such receivers include IRC, MMSE, and MMSE-IRC, and non-linear receivers etc. When transmitting multiple layers, it uses a DMRS structure that facilitates estimation of CSI corresponding to each layer at the BS. The multiple layer DMRS configurations are shown previously. The multiple layer DMRS can be configured on different odd and even tones or on the same tones using CDM method. Depending on the same, the receiver must do the channel estimation.

FIG. 9shows a block diagram representation of Multi-user MIMO. The Multi-user MIMO refers to a method where multiple users communicate with the BS simultaneously on the same time-frequency resource. Each user may use a single layer or multiple layers. In scenarios where the BS has a large number of antennas, it is possible to eliminate both the self-interference that arises due to the use of multiple layers by a given user and inter-user interference that is caused by the transmission of signals by multiple users at the same time. Examples of such receivers include IRC, MMSE, and MMSE-IRC, and non-linear receivers etc. that are specifically tailored for MU MIMO operation.

In current systems that use up to 8-antennas at the receiver the existing receiver techniques are capable of supporting SU or MU MIMO. However, when the number of receiver antennas becomes very large, use of standard receivers results in either very computational complexity or in some cases sub-optimal performance. There is a need to develop receivers with low-implementation complexity and high performance.

One embodiment of the present disclosure relates to a receiver processing in massive MIMO UL. Signals are received at the antenna array on NRx (for example 64) antennas. Depending on the number of ADCs/RF chains available in the receiver, the digital processing is performed in the base station. When 64 RF chains are available, the DU will have to send the baseband I/Q samples corresponding to the all the received signals to RU. The amount of fiber bandwidth (BW) required to send such large number of baseband I/Q samples becomes so high that the cost of the DU-RU interface becomes a limiting factor.

Therefore, there is a need for RU to condense the 64 antennas data into a lesser number before the I/Q samples of the condensed information is sent back to the DU. When the number of streams to be sent to DU is less than 64, then there are multiple ways to do this. However, an intelligent approach is required to ensure that the overall system performance is maximized. Some common approaches available to condense 64 antennas data into “L” streams are:Down-select L out of 64 streamsCombine or add the 64/L antennas data akin to equal gain combiningDFT weights are applied on a subset of 64/L antennas and combines these signals to obtain “L” streamsIntelligently select or combine using groups of antennas using various signal processing methods and send the resulting data to the DU

For the above process, some of these combinations may be done in analog domain and some in digital domain. Alternatively, combining may be implemented in time domain or frequency domain. In case of frequency domain processing, the RU must at least perform the CP removal and FFT operations on the incoming streams of data.

A condensing process is also referred as the filtering process interchangeably may be identified by the RU itself or be indicated by the DU. Once the filtered signals at RU are sent to DU, the DU may filter them again for further processing. That is, the L streams in DU may be further filtered/processed using linear or non-linear processing to recover the actual necessary data streams which can be anywhere from 1 to L Conventional MMSE type equalizers process all 64 received streams into a single effective stream by maximizing the signal power interference and minimizing the noise-plus-interference.

The main challenge in condensing is to make sure that when received signals are mapped to streams, data and reference signal characteristics are not distorted to the extent that jeopardizes the data reception quality. Alternatively, the condensing is performed to ensure that the receiver system performance is maximized. This can be done by constructively combining data and reference signals and mitigating intra-cell (or other-cell) and inter-cell (or self) interference.

SUMMARY

The shortcomings of the prior art are overcome and additional advantages are provided through the provision of method of the present disclosure.

Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure.

In one aspect of the present disclosure a method of received signal processing by a massive multiple input multiple output (MIMO) base station (BS) is disclosed. The massive MIMO BS comprises a plurality of antennas, at least one radio unit (RU), at least one distributed unit (DU), an interface configured for a communication between the at least one RU and the at least one DU. The method comprises receiving, by the at least one RU, a plurality of signals corresponding to the plurality of antennas. The plurality of signals comprises at least one of data signals, demodulation reference signals (DMRS) and sounding reference signals (SRS). Also, the method comprises grouping, by the at least one RU and the at least one DU, a subset of the plurality of signals corresponding to a subset of antennas to a group, to generate a plurality of signal groups. The signals associated with each group is at least one of the data signals, the DMRS and the SRS. Further, the method comprises performing a first stage filtering, by the RU, on the plurality of signal groups associated with each group using one or more group specific filters to obtain one or more group specific filtered signals. The group specific filters are determined using at least one of the signals associated with the group, and a set of values communicated by the DU to the RU, wherein said set of values are determined by the DU based on SRS or predetermined values. Furthermore, the method comprises performing a second stage filtering, by the DU, on the one or more group specific filtered signals associated with a plurality of the signal groups to obtain one or more second stage filtered signals.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

