TECHNICAL FIELD

The present invention is directed to methods employed in massive MIMO arrays and, more particularly, to methods using a dynamically configurable, active antenna array system (AAS) to sound channels to acquire channel state information.

BACKGROUND

MIMO is a wireless communication method that increases the capacity and quality of a wireless link by using a plurality of transmit and receive antennas to exploit multipath propagation. MIMO is used in various wireless technology standards. The 3rd Generation Partnership Project (3GPP) is one example of a global effort to determine the policy and strategy of several standards concerning wireless data communication technologies for mobile systems. 3GPP utilizes MIMO technologies in 4G standards such as Long-Term Evolution (LTE), LTE-advanced and 5G standard such as New Radio (NR). A massive MIMO system uses a very large number of transmit antennas (e.g., hundreds or thousands) to simultaneously send and receive multiple data layers to and from multiple receivers. MIMO systems depend on multipath signals to establish high data rate wireless data transmission.

The MIMO system implements a Downlink that conveys signals from transmission points such as a Base Station (BS), referred to as eNodeB in LTE and gNodeB in NR, to User Equipment (UE) terminals and an Uplink that conveys signals from UEs to BSs. The terminals can be cellphones, smartphones, tablets, or any devices with wireless capability communicating with the base station.

The base station, by utilizing the massive MIMO technology, has the capability to form a plurality of radio links with each of a plurality of terminals located in a horizontal and vertical space surrounding this base station. The horizontal space can, for example, be segregated into three 120° segments surrounding the base station with at least one MIMO antenna per segment or domain. The vertical space can be one contiguous segment. Other segregations in the horizontal, as well as vertical, are possible. Various digital signal processing (DSP) techniques are used in conjunction with the antenna arrays of a MIMO system to create a directional signal transmission or reception. The digital signals from the baseband are applied to transmitter/receiver units coupled to one or more antennas in the antenna array. The one or more antennas driven by the ports or transmitters combine the emitted signals from the antenna array such that particular directions of the emitted electromagnetic radiation constructively interfere while other directions destructively interfere thereby forming a directed radiation beam. This directional aspect is referred to as beamforming or spatial filtering.

The overall coverage of the beamforming of an antenna array, also referred to as the maximum scan angle or angular spread, is dependent on the individual antenna domain and the number of antennas driven by a port. If each antenna in the antenna array is driven by a separate port, the beams that are formed have a maximum angular spread or coverage over its domain. Larger antenna arrays are more desirable to increase array gain and multiplexing gain. However, a large antenna array would be costly if a port was used for each antenna. The number of ports implemented in the larger antenna array can be decreased. For example, the array can be partitioned into a plurality of sub-arrays made up of sub-sets of antennas, each driven by a corresponding one of the ports. One benefit of connecting each port to multiple antennas is an improvement in the power efficiency of the antenna array. Another benefit is a reduction in cost, since fewer ports requires fewer hardware components. On the other hand, reducing the number of ports reduces the maximum scan angle of the beams formed by the antenna array over its domain. Another downside of reducing the port count is the generation of sidelobes or spurs formed in the beam generated by the antenna array. Since each port drives multiple antenna elements, those antenna elements are controlled with the same digital weights, thereby reducing the resolution of the antenna array pattern and creating unwanted beam sidelobes. Sidelobes in an antenna array beam pattern in turn reduce the capacity performance of the MIMO system.

The base station strives to boost the capacity experienced by all the UEs in its coverage through various techniques such as spatial multiplexing using multiple antennas and precoding data streams to eliminate inter-stream interference. Precoding is a signal processing operation that is performed on the data streams to reduce interference among transmissions to multiple UEs and maximize the SINR at each of the individual UEs. In order to design the correct precoder for this purpose, a process referred to as channel sounding is used to estimate the channel characteristics of multipath signals. In other words, the BS typically relies on channel state information (CSI) obtained either from feedback given by the UEs based on their own downlink channel estimation performed with the help of downlink pilot signals transmitted from the BS antennas or from uplink pilot signals transmitted by the UEs (under TDD reciprocity assumption). For example, in an FDD (Frequency Division Duplex) system with ‘M’ antenna elements, the BS would transmit M downlink pilot signals corresponding to the M antennas. The pilot signals are also referred to as CSI reference signals, called channel state information-reference signals (CSI-RS) in Release 10 and beyond 3GPP. The UEs estimate the channel based on these M pilot signals and feed that information back to the BS.

The pilots are specific sequences of signals known to both the base station and the terminal. In the case of FDD, the terminals, after receiving the pilots, use them pilots to determine the characteristics of the channel and feed back that information to the BS. The pilots help to determine the direction, the quality, and the number of independent paths formed in the channel. These are correspondingly represented by, the Precoding Matrix Indicator (PMI), the Channel Quality Indicator (CQI), and the Rank Indicator (RI) and are reported back to the base station from each terminal. The base station uses this information to transmit a more reliable signal and transfer higher data rate to each terminal. More specifically, the BS selects the precoding matrix identified by the returned PMI and applies that precoding matrix to the layers to generate port signals. Alternatively, the BS runs signal processing algorithms, such as zero-forcing, on these estimates to generate the precoding vectors. The data streams that are precoded with these vectors are transmitted to the UEs in such a way that each UE receives its own stream without interference from other streams.

The process of sounding the channel scales linearly with the number M of antennas and if M is large (which is the case with Massive MIMO systems) the control signaling overhead becomes prohibitively large affecting adversely the downlink data rates/throughputs. In addition, for channel sounding with M pilots, traditional antenna array architectures require one RF chain/digital port for every antenna element. This raises the complexity and cost of hardware implementation since each TX/RX digital port involves power hungry components like ADC and DACs.

In view of these drawbacks, the embodiments described herein employ a method that acquires CSI in a system with a fewer number of digital ports by using dynamic subarray mapping thereby reducing both signaling overhead and implementation cost.

SUMMARY

When an active antenna array, or active antenna system (AAS), is used in cellular wireless communications, the AAS can shape or focus radio frequency (RF) energy in the downlink, and receive sensitivity in the uplink, by adjusting the magnitudes and the phase shifts of the transmit and receive signals at its plurality of antenna elements. In cellular systems, downlink (DL) refers to the transmit (TX) operation of the AAS, and uplink (UL) refers to receive (RX) operation.

It would be desirable to maintain the same maximum scan angle or coverage and improve channel sounding accuracy and capacity performance of MIMO systems for a given transmit antenna array when decreasing the number of ports to that antenna array. The present disclosure presents one or more methods of achieving this.

Embodiments of this disclosure include methods and systems to improve channel sounding measurements and capacity of massive MIMO antennas having a reduced port count. Reference signals sent from the MIMO antenna are used to sound the channel. For a MIMO antenna with a sufficient number of ports, the horizontal and vertical angular spreads (HVAS) of the PMIs can cover the desired domain of the base station. However, if the same MIMO antenna has a significantly reduced port count, the PMIs can experience a reduced HVAS, which causes a reduction in the coverage of the desired domain. Examples exist where the HVAS may be reduced by a factor of two or more in either the horizontal and/or the vertical directions causing some of the sub-sectors of the desired domain to receive an attenuated signal or no signal at all. If this occurs, the terminals (user equipment) in these sub-sectors of the desired domain of the base station may not return the accurate channel state information (CSI) feedback information to the base station. Thus, although the reduced port count helps to reduce the cost and complexity of the MIMO system, the reduced port count can directly impact the ability of the CSI-RS to aid in the sounding of the channel.

Some embodiments described herein utilize a digitally controlled analog phase shifter coupled to each antenna of the reduced port MIMO to increase the array's coverage over the desired domain. These phase shifters can be controlled to electrically steer all of the antenna's output beams independently in any direction desired in a matter of milliseconds (ms) or less. The analog phase shifters can also be used to steer all of the antenna's output beams in the same direction. A resource mapping and precoding block generates control signals, in a first time interval, to digitally control the analog phase shifters to steer all the reference CSI-RS beams together to cover a first sub-sector of the desired base station's domain. In a second time interval, all the reference CSI-RS beams together steer to cover a second sub-sector. In each successive time interval, each of the pluralities of sub-sector within the desired base station's domain is covered until the entire desired domain of the base station has been fully covered.

Another embodiment of this disclosure uses the resource mapping and precoding block to combine the PMIs returned from the CSI-RS measurements with the digital control values applied to the analog phase shifters for each of the pluralities of sub-sectors. The combination creates new set of digital control values for the analog phase shifters. These new digital control values are the values necessary to adjust the weights applied by analog phase shifters to steer the beams generated by the sub-arrays in the desired directions, i.e., the direction of the terminal with which the base station will be communicating. The combination of the PMI value with the initial digital value provides a single digital value which is applied to the analog phase shifters to direct the beam in the desired direction, e.g. This step of combining the PMIs with the digital control values that are applied to the analog phase shifters to produce a new set of digital control values for the analog phase shifters (and which eliminates the need for beam steering via the precoding matrices) removes the distortion of the side-lobes that occurs in a reduced port antenna system. Since each port is connected to multiple antennas, the combination of the multiple antennas driven by a single port creates an effective antenna that has spatial characteristics different than the placement of the individual antennas. The PMI value does not compensate for the new spatial characteristics of this effective antenna. However, the new digital value which combines the PMI value with the initial digital control value does compensate for the “effective antenna” effect. The single digital control weight is applied to the analog phase shifters to form “analog” control values. The “analog” control values steer the analog phase shifters for each of the pluralities of sub-sectors allowing the channel to carry data to the terminal within the domain of the base station in a substantially un-distorted (free of unwanted side-lobes) channel.

In general, in one aspect, the invention features a method involving a MIMO communications system including a phased array antenna for establishing communication with a terminal located within a domain covered by the MIMO communications system. The method includes: defining a plurality of sub-domains within the domain; for each sub-domain, defining a corresponding set of analog phase weights to be applied to the phased array antenna for directing beams towards that sub-domain; in succession, selecting each sub-domain among the plurality of sub-domains and for each selected sub-domain: (a) applying the set of analog phase weights for that selected sub-domain to the phased array antenna; (b) performing channel sounding with the terminal while that sub-domain is selected; and (c) receiving feedback from the terminal for that selected sub-domain; and after selecting all sub-domains of the of plurality of sub-domains and from the feedback received from the terminal, identifying among the plurality of sub-domains, a best sub-domain and identifying a best precoding matrix that in combination with the best sub-domain provides a best communication channel for the terminal.

Preferred embodiments include one or more of the following features. The method also includes using the best communication channel to carry out data communication with the terminal. Using the best communication channel to carry out data communication with the terminal involves employing the identified precoding matrix and applying the set of analog phase weights for the identified best sub-domain to the phased array antenna to direct beams towards the identified best sub-domain. Alternatively, using the best communication channel to carry out data communication with the terminal comprises deriving from the identified precoding matrix and the set of analog phase weights for the identified best sub-domain a revised set of analog phase weights and applying the revised set of analog phase weights to the phased array antenna. In that case, using the best communication channel to carry out data communication with the terminal does not involve using the identified (or any) precoding matrix to perform any beam steering. Performing channel sounding with the terminal while a sub-domain is selected involves sending reference signals to the terminal. It might involve non-precoded reference signal transmission (e.g. Class-A transmission) or it might involve precoded reference signal transmission (e.g. Class-B transmission). The feedback from the terminal includes a Precoding Matrix Indicator (PMI) and possibly a Channel Quality Indicator (CQI).

