I have a question related to CSI-RS channel estimation.
Let us assume that the base station has 64 antennas and we decide to use 32 antenna ports for CSI-RS transmission. Also, UE has 4 antennas.
My understanding is that UE can estimate 32 x 4 channel not 64 x 4 channel.
The question is how to map 32 antenna ports to 64 physical antenna when we simulate this system.
Does the standard define this mapping?
Hi, Following picture will explain how it is done. You need to like at the transition from C to D. More details at this link: 5G | ShareTechnote
When we talk about Antenna panel, there is physical concept called antenna elements. But AE (antenna elements) mostly won’t be directly concerned when talking about beamforming. But we need to understand one AE is usually a bi-polarized (or dual polarized, or cross polarized) which is 2T2R (using two orthogonal transmitter/receiver).
When talking about antenna ports, it’s virtual concept and related to beamforming. There is important concept called “sub antenna array”. Sub antenna array is the bridge connecting physical antenna elements to virtual antenna ports. We usually have 88 antenna array (physical concept) and then we can split to different antenna array called “sub antenna array” so that the sub set are formed into one beam. We use N1 and N2 to represents how many horizontal array group and how many vertical sub array group. And there are another parameters O1 and O2 (samples) defining the number of beams. The calculation is below: number of horizontal beams= N1O1, and number of vertical beams=N2O2. Total beams= N1O1N2O2.
We can see that with larger number of antenna elements of one sub array, we would achieve more beam gain. The beam is narrower and reaching further, vice versa for less number of antenna elements assigned to sub array.
Then how CSI-RS port is connected to antenna group? Since we treat one sub array group as one TRX (meaning there are two RF chains), so we have N1N22 CSI-RS ports. More Sub array groups, more TRX are required so it is more costly for designing the HW. This is related to codebook used for PMI feedback. Each sub array will be applied with one digital BF weight and one additional RF element called phase shifter for subarray’s steering angle adjustment.
3GPP defines specified number of CSI-RS ports (not randomly) which can be referred to following table:
From above we can see that CSI-Ports doesn’t equal number of beams. So when we talk about 32T32R, doesn’t mean we have 48 panel (32 antenna elements). It’s actually referring to N1N2=16 (162=32 ports). If we have 64T64R, then we have N1N2=32 (then it’s 64 ports). So far 3GPP only defines up to 32 CSI-RS ports, no more higher. We could have more antenna elements (16*8 panel but we have still up to 16 antenna sub arrays in horizontal and each array contains 8 antenna elements in vertical direction, or called Azimuth = horizontal, Elevation = Vertical)
Hi @Aaron_Shi , thank you for your answer, I find it really helpful. Just one quick question, if we have 64T64R and only 32 ports, we have the concept of sub arrays, does that mean it is hybrid beamforming?
Use port to antenna mapping. where 1 CSIRS port drives 2 antenna ports.