1. Technical Field
The present disclosure relates to beamforming and more specifically to providing improved communication using frequency sub-units which can be chosen based at least in part on a level of mismatch between an amplitude differential and a group delay differential within a given frequency sub-unit. The disclosed concepts will provide a large number of orthogonal channels that can be assigned to a mobile communication network, and support a large number of users. The disclosed techniques are well-suited for point-to-multipoint cellular communications, and are applicable to one or more of a base station, remote radio head (RRH), Evolved Node B (eNB), mobile devices, user equipment, or other devices.
2. Introduction
Beamforming is a known feature within communication systems in which an array of transmission antennas are used to form respective beams or transmitted signals such that, in given directions, the signals experience constructive interference while in other directions, destructive interference cancels the signal. The traditional concept for Massive Multiple-Input and Multiple-Output/Beam Form Nulling (MMIMO/BFN) is to handle the signal processing over the composite spectrum such as 200 MHz or 1 GHz. The composite beam is targeted for a unique direction. For the traditional MMIMO/BFN technique, the MMIMO device generates N beams; each beam addressing a group of user equipment terminals (UE's) at a location or area.
The beamforming device, such as a base station, mobile station, remote radio head or any other device, transmits one of the N beams of focused energy in a particular direction rather than sending an omnidirectional signal in all directions. The traditional MMIMO/BFN beams can be processed with either digital or analog phased array techniques. The MMIMO/BFN technique, when used for wideband signals in a Point to Multipoint communication system, faces many limitations. For example, with respect to the accuracy and controllability of a beam or a null, the traditional approach can provide high accuracy if implemented in the digital domain but it has a low accuracy if an analog phased array is used. With a wideband composite signal, the system has difficulty with transmission and antenna calibration. The system experiences gain and group delay variation over the wide bandwidth as shall be explained below.
Channels can be dependent on cell frequency and location. One challenge is that multipath characteristics can be different for each cell area and the composite spectrum solution and/or phased array antenna schemes will have a low signal-to-noise ratio. With wide bandwidth beams, interference at surrounding cells is a strong possibility and in an effort to suppress such interference, the system may create a wideband null that also can cause unintended interference. The number of orthogonal channels available in a traditional system is bounded to be a small number and is based on the number of antennas. Further, the antenna gain is low and it is difficult to do gain control over the wide spectrum.
In another aspect of beamforming, the data to be transmitted is meant for a particular device at a known location. Transmitting an omnidirectional signal thus wastes much energy as it projects electromagnetic signals in all directions, where the device which is to receive and decode the signal is only at one location in a known direction from the transmitting device. The transmitting device, by using beamforming, can control the phase and relative amplitude of the signal from each transmitting antenna in order to create a wavefront with the energy focused in a particular beam at a chosen direction (to the receiving device) and not in all directions.
Beamforming has been applied in various forms in standards such as 2G, 3G, 3G Evolution (LTE). It is expected that beamforming will be part of more advanced standards such as the 4G and 5G standards.