Field of the Invention
The invention relates to methods for efficient beam training, and more particularly to methods for hierarchical beam training.
Description of the Related Art
The term “wireless” normally refers to an electrical or electronic operation that is accomplished without the use of a “hard wired” connection. “Wireless communications” is the transfer of information over a distance without the use of electrical conductors or wires. The distances involved may be short (a few meters for television remote controls) or very long (thousands or even millions of kilometers for radio communications). The best known example of wireless communications is the cellular telephone. Cellular telephones use radio waves to enable an operator to make phone calls to another party, from many locations worldwide. They can be used anywhere, as long as there is a cellular telephone site to house equipment that can transmit and receive signals, which are processed to transfer both voice and data to and from the cellular telephones.
There are various well-developed and well-defined cellular communications radio access technologies (RATs). For example, the Global System for Mobile communications (GSM) is a well-defined and commonly adopted communications system, which uses time division multiple access (TDMA) technology, which is a multiplex access scheme for digital radio, to send voice, data, and signaling data (such as a dialed telephone number) between mobile phones and cell sites. The CDMA2000 is a hybrid mobile communications 2.5G/3G (generation) technology standard that uses code division multiple access (CDMA) technology. The UMTS (Universal Mobile Telecommunications System) is a 3G mobile communications system, which provides an enhanced range of multimedia services over the GSM system. Wireless Fidelity (Wi-Fi) is a technology defined by the 802.11 engineering standard that can be used for home networks, mobile phones, and video games, to provide a high-frequency wireless local area network. The LTE (Long Term Evolution) and the LTE-Advanced evolved from the LTE are the 4G mobile communications systems, which provide high-speed data transmission over 2G and 3G systems.
The millimeter-wave band has the available spectrum and is capable of providing significantly higher-level throughputs than the microwave frequency band. Due to significantly higher attenuation levels and the directional nature of millimeter-wave signals, millimeter-wave devices (i.e., stations) generally employ highly-directional antennas as well as beamforming techniques for communicating.
Beamforming is a signal processing technique which allows to combine signals received from multiple antenna branches for special purpose, e.g., for SINR maximizing or for interference suppression.
By analog beamforming, the signal combination is performed in analog domain (before ADC) and is usually less flexible. The combined signal passes through ADC and at the digital domain, there is simply one branch of signal.
As for digital beamforming, the signal combination takes place in digital domain. For this to happen, the signals received from individual antenna branches go through individual ADC. With this approach, it is more flexible as we have multiple signals in digital domain for us to play with. However, in order to achieve this, it requires multiple processing chains in order to convert analog signals to digital domain. For example, multiple ADC is needed.