As we know the concept of Far and Near field comes due to large antenna arrays, recently with large RIS, so it mainly depends on the antennas array characteristics and specifications, size, aperture area, etc.

1- So, let's assume that there is two scatters (RXs) associated with the array, one in the near field region and the others in the far field region. Hence, after channel estimation, some researchers proposed methods for that, and other operations, the array will produce two beams. These beam may overlap (interference is important at the RXs), i think with very low probability of overlapping because this event needs both RXs be at the edge of the far/near regions, associated at the same time to the BS/RIS, scheduled at the same time (considering MU-MISO/MU-MIMO), and so on. Of course the probability of this event occurrence will increase with increasing the number of the MU to 4, 8, etc. Further study and investigation is needed.

- Channel Estimation for Extremely Large-Scale Massive MIMO:

Far-Field, Near-Field, or Hybrid-Field?

In beamforming, the near-field and far-field beams can interact in a number of ways, depending on the specific design of the beamforming system.

One common interaction is beam split. In the far-field, the wavefronts of electromagnetic waves are planar, meaning that they have a constant phase over any plane perpendicular to the direction of propagation. However, in the near-field, the wavefronts are spherical, meaning that the phase varies with distance from the source. As a result, a beam that is focused in the far-field may split into multiple beams in the near-field. This can be seen in the following figure:

📷Opens in a new window📷www.researchgate.net

farfield and nearfield beam split

This effect can be mitigated by using beamforming techniques that are specifically designed for the near-field. For example, true-time-delay (TTD) beamforming can be used to create beams with planar wavefronts in the near-field.

Another interaction between near-field and far-field beams is mutual coupling. This occurs when the electromagnetic field emitted by one antenna element interacts with the other antenna elements in the array. Mutual coupling can affect the beam pattern of the array, and it can also lead to interference between the beams.

Mutual coupling is more pronounced in the near-field than in the far-field. This is because the electromagnetic field is stronger in the near-field. To mitigate mutual coupling, beamforming systems often use decoupling networks. These networks are designed to cancel out the mutual coupling between the antenna elements.

Finally, the near-field and far-field beams can also interact through the propagation environment. For example, reflections from objects in the environment can cause the beams to interfere with each other. This can lead to a reduction in the signal-to-noise ratio (SNR) and an increase in the bit error rate (BER).

To mitigate the effects of the propagation environment, beamforming systems often use adaptive beamforming techniques. These techniques can be used to steer the beams around obstacles and to null out interference.

Here are some specific examples of how the interaction between near-field and far-field beams can be exploited in beamforming designs:

• Near-field beamforming for wireless power transfer: Near-field beamforming can be used to focus the power transmitted from a wireless power transmitter to a receiver. This can be used to increase the efficiency of wireless power transfer systems.
• Near-field beamforming for millimeter wave communications: Millimeter wave communications are highly susceptible to attenuation and interference. Near-field beamforming can be used to mitigate these effects and to improve the performance of millimeter wave communication systems.
• Near-field beamforming for radar and imaging: Near-field beamforming can be used to create high-resolution radar images and medical images.

Overall, the interaction between near-field and far-field beams is a complex phenomenon that can have both positive and negative effects on beamforming systems. By understanding the interaction between near-field and far-field beams, engineers can design beamforming systems that are more efficient and effective.

A near-field channel is a linear combination of far-field channels. Any spherical wave can be expressed as a summation of plane waves.

Hence, when a signal is focused on a point in the near-field, it will lose its focus in the far-field but be distributed over a particular interval in the far-field. You can determine that interval by multiplying the precoding vector for near-field focusing with array response vectors for different angles and make a plot of the squared magnitude of the result.

I'll try to express quickly my point of view on the questions.

1) The far-field is a sort of limit of the field function for r->+inf; the two, near and far, are closely related, and any constraint on one will influence the other.

2) It is possible to evaluate this effect by calculating a near-far transformation, but according to my knowledge, no simple rules may apply. What you may see is a change in the beam-width of the pattern and the side-lobe level, as well as the directions of the maxima of the sidelobes, but - again - there are no simple rules.

Thanks for your great question.

1 Yeah, interference exits between antenna beams designed for the near-field and far-field regions, but the effects are typically minor due to large differences in field strength. For instance, a well-designed beamfocuser would focus energy to a focal point in the near field, while forming a uniform ray of light in the far field. However, the energy declines rapidly with distance. Therefore, the interference would diminish quickly beyond the Rayleigh boundary. The focused energy would be strong near the focal point but weaken significantly as you move further away.

2 Accurately characterizing these effects relates to both the array's pattern and the specific locations of the near-field and far-field users. However, it ultimately relies on the fundamental mathematical principle of coherent superposition. With this basic tool, we can theoretically analyze how a beampattern designed for a target point will impact another point for an arbitrary setup. From this standpoint essentially there is no real difference between the near-field and far-field. Conventional approaches focus on the far field just because a small antenna aperture provides low resolution along the distance dimension.

You may find more detailed explanations from

https://arxiv.org/pdf/2309.09242.pdf

All answers are consistent and complementary.

thank you all