The third Generation Partnership Project (3GPP) is examining further upgrades to radio base stations to enable services with higher volumes and higher speeds. LTE-Advanced Release 13 specifications (hereinafter referred to as “Release 13”) prescribe Elevation Beam Forming/Full Dimension-Multiple Input Multiple Output (EBF/FD-MIMO) to expand antenna port mapping on base stations from one dimension to two dimensions, and increase port numbers. These can be achieved using Active Antenna System (AAS) technology in which antennas are combined with transceiver units to make base stations that are more.  According to  meeting of 3GPP TSG RAN #65, the Study aims to understand performance benefit of standard enhancements targeting two-dimensional antenna array operation (including a single column of cross-poles) with 8 or more transceiver units (TXRUs) per transmission point, where a TXRU has its own independent amplitude and phase control. The study project consists of two phases:

Phase 1 

- Identify antenna configurations for 2D antenna arrays with {8, 16, 32, 64} TXRUs and evaluation scenarios, including homogeneous and heterogeneous scenarios, for feasibility study, taking into account the outcome of 3D channel model SI.

- Decide antenna element spacing, number of antenna elements per TXRU, polarization, etc.

- Decide how to model virtualization of antenna elements per single TXRU. 

- Identify target operating frequency range considering practical antenna size limitations.

- Evaluate the performance of Rel-12 downlink MIMO (including both SU- and MU-MIMO) using 3D-UMa and 3D-UMi channel models.

- Number of TXRUs for evaluation is 8, where each TXRU is connected to an antenna port and the antenna ports constitute a horizontal array. 

Phase 2 

- Evaluate performance benefits of standard enhancements targeting two-dimensional antenna array operation (including a single column of cross-poles) using 3D-UMa and 3D-UMi channel models, taking into account the discussion and findings of the 3D channel model SI.

Antenna Array Model (Need):

The antenna array size is proportional to the number of antennas in the array, so the deployment of massive linear arrays in the limited installation space at the top of BS towers is impractical. For example, a macro-cell BS has a form factor of 1430 × 570 × 550 mm, while pico-cell and femto-cell BSs have even smaller rooms available for the deployment of antenna arrays. Presently think about the 64 radio wires in a uniform direct array (ULA) with 0.5λ separating, where λ is the bearer wavelength at the run of the mill LTE working recurrence of 2.5 GHz. This would require a level room of about 4m at the highest point of BS tower. Contrasting it and the structure factor of a full scale cell BS, unmistakably introducing 64 radio wires straightly at existing BSs is unreasonable.

To adapt to this impediment, full measurement (FD) MIMO was recognized as a promising applicant innovation for advancement towards the next generation LTE frameworks amid the 3GPP LTE Release-12 workshop in 2012.  FD-MIMO uses a 2D active antenna array (AAA) that incorporates a 2D planar detached receiving wire component exhibit and a functioning handset unit exhibit into a functioning radio wire framework (AAS). The 2D array structure permits an expansive number of reception apparatus components to be pressed inside doable BS structure factors. The 2D array structure allows a large number of antenna elements to be packed within feasible BS form factors. As an example, again consider the deployment of 64 antennas but now in an 8 × 8 2D planar array with 0.5λ inter-antenna spacing. This would require an array of dimensions ∼ 50cm × 50cm, which can be readily installed at existing BS. 

Fig1Comparison of 1D and 2D array

2D Planer Array:

A 2D planar uniformly spaced antenna array model is used. The configuration of a 2D planar uniformly spaced antenna array model is represented by (M, N, P) where,

M is the number of antenna elements with the same polarization in each column

N is the number of columns and 

P is the number of polarization dimensions

The antenna element spacing is given by d_H in the horizontal direction and by d_V in the vertical direction. This model including indices for co-polarized antenna elements is shown in the Figure 2. Antenna numbering below assumes observation of the antenna array from the front (with x-axis pointing towards broad-side and increasing y-coordinate for increasing column number).

Fig. 2: Antenna array model represented by (M, N, P) 

FD MIMO

FD stands for Full Dimension. Therefore, FD-MIMO stands for Full Dimension MIMO  means in both horizontal and vertical direction so that it can cover (focus on) anywhere in 3D spaces. Following illustration would shows you a comparative picture between FD and conventional multi-antenna system.Conventional BS can only do beamforming in one direction (horizontal) and  3D Beam forming  or (Elevation beamfroming and Azimuth beamforming).

Rel-13 FD-MIMO specification primary includes the following parts:

Non-precoded CSI-RS

This class contains plans where different CSI-RS ports have the equivalent wide beamwidth. This category comprises schemes where different CSI-RS ports have the same wide  width and direction and hence generally cell wide coverage and increase CSI-RS port to 16 and 64.

Beamformed CSI-RS

This category comprises schemes where CSI-RS ports have narrow beam widths and hence not cell wide coverage, and at least some CSI-RS port- resource combinations have different beam directions. 

CSI reporting enhancement

Release 13 grows the downlink information DeModulation Reference Signal (DM-RS) capacities to help MU-MIMO with up to eight streams (max. eight terminal). In particular, It guarantees up to four symmetrical layers by utilizing a code multiplexing sequence length of four for DM-RS.

Reference: 3GPP TR 36.897 V13.0.0 (2015-06), “Study on elevation beamforming / Full-Dimension (FD)”