Introduction

Scaling and increasing the performance of wireless communication systems in the 5 GHz frequency range required an increase in the allowed frequency channels number by using channels for military and meteorological radars data transmission. To reduce the impact of wireless broadband systems on radars, the data exchange protocol included requirements for detecting radar and immediately leaving the frequency it operates.

The radar detection mechanisms implementation was connected with the air scanning methods and transition between frequency channels. These methods were included in the requirements for wireless systems and were called dynamic frequency selection (DFS).

Dynamic frequency selection and radar detection

It is necessary to separate the mechanisms of dynamic frequency selection and radar detection:

• The dynamic frequency selection implementation require the available frequency channels scanning and the selection of the least noisy channel. Thus, the wireless data transmission systems distribution over the frequency spectrum is performed.
• A radar detection mechanism is necessary to a ban the use of frequency channels that radars operate on. Thus, the DFS technology retains the possibility of using such channels by broadband systems, in case they are immediately released when radar operation is detected.

Regulatory requirements

Regulatory requirements for wireless communication systems vary by country. The most common requirements for the implementation of dynamic frequency selection and radar detection in devices, were formed by the Federal Communications Commission (FCC) and the European Telecommunications Standards Institute (ETSI).

The limitations in InfiNet devices can be performed by the license which includes the set of frequencies used in the configuration and the presence of the radar detection option. A license may be issued in accordance with the regulations for a particular territory.

DFS operation principles

The dynamic frequency selection algorithm consists of the following steps:

1. Turn on the device.
2. Sequential channel scanning in accordance with the frequency grid. The scanning result is a filled DFS table, in which each channel corresponds to the level of the detected interference signal.
   
   1. The maximum signal level for each frequency channel is recorded in the DFS table. To eliminate false positives, when assessing the signal level, its spectral density is also taken into account.
   2. By default, the scan duration on each frequency channel is 3 seconds, this parameter is configurable.
   3. The total scan duration depends on the number of channels in the frequency grid.
3. Analyze the DFS table and select the frequency channel with the lowest signal level.
4. Set the selected channel center frequency as the operating center frequency of the wireless device.
   
   1. Establishing a link with subscriber devices.
      
   2. Data transmission.
5. After 24 hours, rescan and reselect the frequency (steps 2-4). User can manually set the re-scanning time.

An example of dynamic frequency selection algorithm is shown in the video 1.

The base station (BS) sector and the subscriber (CPE) are installed and configured to establish a wireless link between the two buildings, and the dynamic frequency selection support is activated on the BS. After turning on, the BS starts a sequential available frequency channels scanning, fill in the DFS table the signal levels. The table has the following form:

The BS device analyzes the table and selects the frequency channel with the minimum signal level value, i.e. channel F3. The selected channel center frequency is set as operational.

The center frequency changing causes the wireless connection break. The CPE device starts to look over the allowed frequency channels list, while searching for a BS device. After the BS is detected, the wireless devices perform the association, establish a link, and the data exchange resumes.

The radar detection principle

The radar detection algorithm operation can be described by the following steps:

1. Turning on the device.
2. Sequential frequency channel scanning. The device scans the air in accordance with the set frequency grid.
   
   1. Signals on the air are checked for belonging to the known radars.
   2. By default, the scan duration on each frequency channels is 3 seconds, this parameter is configurable.
      
   3. The total scan duration depends on the channels number in the frequency grid.
      
3. Frequency channels with radar detected are marked as inaccessible for use as an operational.
   
   1. The channel is excluded from the table for 30 minutes.
4. The next scan repeats the procedure for radar detection (steps 2-3).
   
   1. The radar detection procedure is performed simultaneously with the DFS mechanism operation.
      

An example of the radar detection algorithm is shown in the video 2.

BS and CPE are installed and configured to organize a wireless link between two buildings, the BS supports the dynamic frequency selection mechanism with radar detection. A meteorological radar is installed on the building next to the BS and using frequency channel F3.

Same as in the previous example (see video 1), the BS starts a sequential available frequency channels scanning and fill in the table. During the F3 frequency channel scanning the BS detects the radar operation. Although the signal level in channel F3 is lower than in other channels, it is excluded from the decision-making process. Thus, the updated table looks as follows and the BS sets F1 as the operating frequency:

Instant DFS

The DFS mechanism disadvantage is the inability to quickly assess the radio and timely change the operating frequency in accordance to the situation on the air.First solution is to make scanning the air more frequent, however, it should be understood that the time spent on updating information about the radio parameters is not used to transmit data, i.e. communication system performance decreases.

InfiNet devices use proprietary technology called Instant DFS, which allows to obtain current information about the radio state, without breaking the connection.

Instant DFS operating principles

Instant DFS operation principle is similar to the described DFS algorithm, but it has the following differences:

• Sequential frequency grid channels scanning is performed continuously, not at selected time. This allows the device to operate with actual information about the radio parameters without stopping the data transfer process. For the InfiLINK 2x2 and InfiMAN 2x2 families devices this is implemented using an additional radio module, for the InfiLINK XG family devices - by scanning the air during guard intervals.
• In case of a freer frequency channel detection, a switching to another operating frequency is performed without interrupting the data transmission process.
• The InfiLINK 2x2 and InfiMAN 2x2 families devices with two radio modules after turned on distribute among themselves a set of frequency channels that need to be scanned, so scanning is performed two times faster.

An example of the proprietary Instant DFS mechanism is shown in the video 3:

• The BS and CPE devices are installed and configured to organize a wireless link between two buildings, the Instant DFS technology is activated on the BS. The initial operating frequency selection is similar to the DFS algorithm (see video 1), therefore, it supposed that the devices have already established a connection and started data exchange.
• Initially, the BS performs data transmission and reception in frequency channel F3 and scans channel F1. After the scan is completed, the BS device updates the radio status table. Since the F3 channel, which is used as an operational is the most free from interference, the BS does not make any changes.
• On the second step, the BS transmits and receives data in frequency channel F3 and scans channel F2. Interference in the F2 frequency channel has significantly improved: the signal level has decreased from -80 dBm to -90 dBm. Now the best performance is in channel F2, so the BS starts the process of changing the operating frequency.
• In order to perform the frequency change without breaking the connection, the BS generates a broadcast request with an overhead message, which indicates the new operating frequency F2. The frequency is changed on the BS after sending a broadcast message, and on the CPE after receiving this message.
• On the third step, the BS performs the transmission and reception of data in the frequency channel F2 and scans the channel F3. The obtained scan results do not lead to a change in the operating frequency; therefore, data continue to transmit through the channel F2. Scanning is repeated cyclically in accordance with the set frequency grid.

Схемы использования Instant DFS

Устройства с поддержкой Instant DFS могут использоваться в топологиях "точка-точка" и "точка-многоточка". Следует понимать, что результаты сканирования справедливы только для принимаемого сигнала, поэтому:

• В топологиях "точка-точка" рекомендуется использовать оба устройства с поддержкой технологии Instant DFS. В процессе работы устройства будут обмениваться результатами сканирования и решение о выборе частоты будет принято с учётом помеховой обстановки на двух устройствах.
• Если в топологии "точка-точка" нет возможности использовать оба устройства с поддержкой Instant DFS, то устройство с поддержкой Instant DFS необходимо устанавливать в месте с наиболее сложной помеховой обстановкой.
• В топологиях "точка-многоточка", если нет возможности использовать все устройства с поддержкой Instant DFS, оптимальным вариантом будет установка устройства с Instant DFS в качестве сектора базовой станции.

Поддержка Instant DFS в устройствах Инфинет

Поддержка фирменной технологии Instant DFS реализована в устройствах семейств InfiLINK 2x2 и InfiMAN 2x2, функционирующих в частотном диапазоне 5 ГГц, через установку дополнительного радиомодуля. О наличии второго радиомодуля можно судить по наличию строчной буквы "s" в названии устройства, например R5000-Mmxbs/5.300.2x500.2x16.

Среди устройств семейства InfiLINK XG поддержка технологии Instant DFS реализована на всех устройствах, функционирующих в частотном диапазоне 5 ГГц.

Дополнительные материалы

1. Настройка DFS в устройствах семейства InfiLINK 2x2 и InfiMAN 2x2 (web / CLI)
2. Онлайн-курс "Предварительная настройка и установка устройств семейств InfiLINK 2x2 и InfiMAN 2x2"
3. Настройка DFS в устройствах семейства XG (web / CLI)
4. Онлайн-курс "Устройства семейства InfiLINK XG"
5. Расшифровка обозначений устройств "Инфинет"