Introduction
The wireless broadband access systems wide spread has caused a number of problems that impede the existing systems efficient operation and the scaling of wireless networks. Within this article two main problems will be reviewed:
- High interference - the influence of third-party wireless data transmission systems that use same frequency channels.
- Inefficient use of the input power - often the input power is used inefficiently due to the antennas weak directional properties. This factor also leads to interference.
Communication systems without beamforming technology
The antenna's radiation pattern without beamforming technology support is fixed and cannot be changed during data transfer. An example of the base station sector with three subscriber devices is shown in the video 1.
The base station sector consistently exchange data with the subscriber devices bidirectionally: the sector transmits data to the subscriber device 1, waiting for a response from it, then performs data exchange with the third and second subscribers. Downlink messages transmitted from the base station to the subscriber are received by all subscriber devices, regardless of the recipient, i.e. message sended to subscriber device 1, will be received by subscriber devices 2 and 3, but will not be processed.
Video 1 - System without a beamforming technology operation
This approach has several disadvantages:
- Inefficient use of radiation power (the sector performs data transmission to all subscriber devices that are within the coverage area, regardless of the recipient device), i.e. the link power with a specific subscriber unit can be higher.
- Low security level (by gaining access to one of the subscriber devices, the attacker can intercept whole outgoing traffic of the sector for all subscribers).
- The appearance of local interference on the sector affects all subscriber devices operation connected to this sector.
Video 2 shows the operation of the base station sector with three subscribers, under the local interference conditions. At time intervals allocated for uplink data transmission, the sector receives the sum of the subscriber signal and interference. In case of high interference level, the sector cannot separate the useful subscriber component from the sum of received signals and the data transmission must be repeated. It will take extra time, i.e. system resources will be used inefficiently. Note that the interference signal in the sector coverage area affects the operation of all subscribers, since the wide radiation pattern main lobe does not allow selective spatial suppression. In other words, any signal into the main lobe of the sector radiation pattern will influence the entire system operation.
Video 2 - A local interference influence on a sector without beamforming technology.
Beamforming technology
Description
The outlined problems can be solved by using the antennas with the beamforming technology support.
The beamforming technology is the ability to control the antenna's radiation pattern, i.e. depending on the situation, an antenna narrows radiation pattern main lobe and orients it in the required direction. Thus for each subscriber devices its own radiation pattern can be formed and an individual configuration will be used for data transmission to this subscriber. Note that the selected radiation pattern will be used for bidirectional exchange, i.e. both for reception and transmission.
An example with three subscribers connected to the same sector with beamforming technology is shown in video 3. In comparison to a conventional antenna, the antenna's radiation pattern has become narrow, and the main lobe is orienting towards the subscriber unit.
Video 3 - An example of sector with beamforming
The beamforming technology advantages
Advantages of devices with the beamforming technology support:
- The link energy increasement. Figure 1 shows a comparison of the antenna's radiation pattern with and without beamforming support. Note that the radiation power of the device has not changed, and due to the fact that the energy supplied to the antenna is radiated only in the direction of a particular subscriber, the subscriber received signal level increases significantly. A similar effect is appeared in the uplink. This advantage leads to the following scenarios implementation:
- Performance increasement (subscribers performing data transmission not on the maximum bitrate can use higher modulation-code schemes determining the link bandwidth, due to an increase in the signal-to-noise ratio).
- Higher devices lifetime (due to link energy increasement, the radiation power can be reduced without loses in performance, it will lead to device’s radio transmitter lifetime increasement).
- Sector coverage range increasement (higher link energy allows connection of new subscriber devices at a greater distance from the sector).
Figure 1 - Radiation pattern of the sector with the beamforming technology support and without it
- Improved antenna sector directional properties. Allows to increase security by reducing the risks of all sector's subscribers data interception. If case of beamforming technology support, the data transmission is performed only in the subscriber direction, if an attacker performs similar actions, he will be able to access only one subscriber data.
- The ability to select the radiation direction automatically, reduces the requirements for the alignment and allows to determine the azimuth of the subscriber location, which can be used in some scenarios (ex. with mobile objects).
- Using the BS sector with beamforming antenna reduces the local interference influence. Video 4 shows the interference effect in case of beamforming antenna using. The noise falls into the side lobe of the radiation pattern, when the second and third subscribers are servicing, and does not have a significant effect on the useful signal. Since the data transmission direction to the first subscriber and the interference source coincide, then the use of beamforming technology does not allow to reduce the local interference influence for this device. Although the beamforming technology support does not allow to completely neutralize the influence of interference, its use can significantly improve the performance and reliability of communication in some scenarios.
Video 4 - An example of the local interference effect on sector with beamforming technology
Implementation
Modern devices have several emitters. One of the structures used in modern radio communications is the phased antenna array (see Figure 2), which is a matrix of conducting elements. Their shape and relative position determine the frequency selectivity and directional properties of the antenna.
A copy of the modulated signal from the radio module is supplied to each antenna element. In addition to the antenna array design, the radiation pattern is determined by the signals characteristics from each of the radiating elements: by default, the signals are in-phase, but by consciously introducing a phase delay, the shape and direction of the main lobe can be changed.
Figure 2 - An example of a phased array antenna layout
The effect described above is used in devices with beamforming technology. The approach to the radiation pattern control divides such devices into two large groups:
- Devices with a set of radiation pattern templates (devices have several radiation pattern templates in the memory that usually differ azimuth of the main lobe. Interaction with a specific subscriber device implies one of the templates using, which is best suited for a given location. The disadvantage of this approach is that the final set of patterns does not always allow to choose the most efficient radiation pattern for the subscriber device).
- Devices without radiation pattern templates (in accordance with the signal received from the subscriber, the device forms an individual radiation pattern, using one of the embedded algorithms. The disadvantage of this approach is the high computing resources requirements).
Implementation in the InfiNet devices
In the InfiNet company's product portfolio, the beamforming technology is implemented in the R5000-Qmxb base station sector models of the InfiMAN 2x2 point-to-multipoint family.
The R5000-Qmxb device integrated antenna includes 15 built-in templates of the radiation pattern with a main lobe width of 20º (gain 21 dBi) located with an offset of 5º from each other and one template with a lobe width of 90º (gain 15 dBi). Depending on the chosen template, the device can operate in two modes:
- Broadcast mode - using radiation pattern with a main lobe width of 90º. This mode is used to exchange service information, when the narrowly focused templates is not yet assigned to the subscriber or it is necessary to update the data on the selected template.
- Directional mode — using one of 15 radiation pattern templates with a main lobe width of 20º. This mode is used for direct data transfer.
Video 5 demonstrates the establishing of correspondence between the subscriber station and the radiation pattern template. The base station sector broadcasts requests addressed to all subscribers, using each of the narrowly directed templates in turn. The request contains information about the template number. The subscriber estimates the level of each received signal, selects the maximum and sends the template number to the sector. After receiving information from the subscriber, the sector fills in the correspondence table and uses a template from the generated table , to interact with a specific client device. During operation the sector initiates the update of the correspondence table, adapting to possible changes in the external environment.
Video 5 - Template selection mechanism for subscriber device
Migration to the system with the beamforming technology support is not laborious - it is enough to perform the base station sector replacement, the previous device configuration is compatible with new sector, the beamforming technology does not need any additional settings and is performed transparently for users and network administrators. There is no need to replace subscriber devices - the devices are compatible with each other. However, there are two limitations to deal with:
- The power consumption of devices with the beamforming technology support is higher than for devices without it, therefore, IDU-BS-G (60W) power supplies are used to power the R5000-Qmxb devices.
- The R5000-Qmxb devices can only be used with a software version with TDMA technology (the differences between two types of software are described in the TDMA and Polling: Application features article). Software with Polling technology support can be upgraded to software with TDMA technology support in accordance with the instruction. In case of adjacent sectors, for example, the implementation of the ABAB scheme, it is recommended to use the AUX-ODU-SYNC synchronization device.
Additional application scenarios for beamforming technology
Described above beamforming technology advantages can be used in mobile wireless communication systems. The mobility of subscriber or base station has a strong influence on the link parameters, which can be neutralized by using devices with beamforming technology. The radiation pattern of such devices will adapt to changes in the external environment, trying to keep the link radio parameters at the maximum level.
Note that the implementation of such projects is a complex task that requires a unconventional approach and ingenuity. Possible solution is to use devices with beamforming technology on the client side, or both on the subscriber side and on the base station side.