ABS (almost blank subframe)

In wireless communications, an Almost Blank Subframe (ABS) is a subframe that is almost completely empty of transmissions, with only a few specific symbols or tones transmitted. ABS is commonly used in Long-Term Evolution (LTE) and 5G wireless networks to mitigate interference caused by satellite signals, such as those from Global Navigation Satellite Systems (GNSS), which can cause severe disruptions to wireless communication systems.

In this article, we will explain the concept of ABS in more detail, including the reasons for its use, its impact on wireless networks, and its implementation in LTE and 5G systems.

Background

Wireless communication systems operate in frequency bands that are shared by many different types of devices and services. As a result, wireless signals can often experience interference from other devices and services operating in the same frequency band. One of the primary sources of interference for wireless communications is satellite signals, particularly those from GNSS, which are used for navigation and timing applications.

The signals from GNSS can be very strong, and can easily overpower wireless communication signals, particularly when the wireless signals are weak. When this happens, wireless communications can become severely disrupted, leading to degraded network performance and reduced data rates. In extreme cases, wireless communications can be completely lost, which can be particularly problematic for safety-critical applications such as emergency services.

What is ABS?

To mitigate the effects of GNSS interference on wireless communication systems, an almost blank subframe (ABS) can be used. An ABS is a subframe that is almost completely empty of transmissions, with only a few specific symbols or tones transmitted. The main purpose of ABS is to reduce the amount of interference that is experienced by wireless communication signals, particularly those that are weak.

An ABS is defined in terms of the physical layer of the wireless communication system. In LTE, for example, an ABS is a subframe that consists of two consecutive slots, where one or both of the slots are completely empty of any transmissions. In 5G, an ABS is a subframe that consists of four or more symbols, where one or more of the symbols are completely empty of any transmissions.

How does ABS work?

ABS works by reducing the amount of interference that is experienced by wireless communication signals in the presence of strong GNSS signals. When an ABS is used, the wireless communication system can detect the presence of a strong GNSS signal and then instruct the wireless devices to stop transmitting during the ABS. This reduces the amount of interference that is experienced by the wireless communication signals, which can improve network performance and reduce the likelihood of dropped calls or lost connections.

In addition to reducing the amount of interference, ABS can also improve the accuracy of location-based services. When a wireless device is trying to determine its location, it may use GNSS signals as a reference to help improve its accuracy. However, if the wireless device is experiencing interference from GNSS signals, this can lead to inaccuracies in the location estimation. By using ABS, the wireless device can reduce the amount of interference from GNSS signals, which can help to improve the accuracy of location-based services.

Impact on wireless networks

The use of ABS can have a significant impact on the performance of wireless communication networks. By reducing the amount of interference that is experienced by wireless signals, ABS can improve network performance, reduce the likelihood of dropped calls or lost connections, and improve the accuracy of location-based services.

However, ABS can also have some negative impacts on network performance. Because ABS involves the use of subframes that are almost completely empty, this can lead to a reduction in the total amount of available bandwidth. This can be particularly problematic in networks that are already bandwidth-limited, as the use of ABS can further reduce the available bandwidth for data transmission.

Another potential issue with ABS is that it can introduce latency Another potential issue with ABS is that it can introduce latency into the wireless communication system. When an ABS is used, wireless devices are instructed to stop transmitting for a short period of time, which can result in a delay in the transmission of data. In some cases, this delay can be significant enough to cause issues for time-sensitive applications such as voice and video calls.

To address these issues, wireless communication systems must carefully balance the use of ABS with the need to provide sufficient bandwidth and low latency. This involves careful planning of the ABS schedule and the use of other techniques such as frequency hopping and adaptive modulation to further reduce the impact of interference on wireless communication signals.

ABS in LTE

In LTE, ABS is used to mitigate interference from GNSS signals in the L1 band, which operates between 1575.42 and 1577.42 MHz. The L1 band is used for GPS and other navigation systems, which can emit strong signals that can interfere with wireless communication signals.

In LTE, ABS is implemented in the physical layer of the network, where it is used to protect the uplink channel, which is used for data transmission from the wireless device to the base station. ABS is activated when the base station detects a strong GNSS signal, at which point it sends a message to the wireless devices instructing them to stop transmitting during the ABS. The ABS schedule is determined by the network operator, and can be adjusted based on the level of GNSS interference experienced in a particular location.

The use of ABS in LTE has been shown to be effective in mitigating interference from GNSS signals, particularly in areas where the strength of GNSS signals is high. However, the use of ABS can also result in a reduction in the total available bandwidth, particularly in networks that are already bandwidth-limited.

ABS in 5G

In 5G, ABS is used to mitigate interference from GNSS signals as well as other sources of interference such as other wireless communication systems and industrial equipment. ABS in 5G is similar to ABS in LTE, but with some key differences.

In 5G, ABS is implemented at the radio frame level, which is a higher level of abstraction than in LTE. This means that the ABS schedule can be adjusted more easily to accommodate different levels of interference and to balance the need for low latency and high bandwidth.

Another key difference in 5G is that ABS can be used to protect both the uplink and downlink channels, which is not possible in LTE. This means that ABS can be used to protect both data transmission from the wireless device to the base station as well as data transmission from the base station to the wireless device.

The use of ABS in 5G is still a relatively new area of research, and there is ongoing work to determine the most effective ways to use ABS in 5G networks. However, early results suggest that ABS can be effective in mitigating interference in 5G networks, particularly in areas where the level of interference is high.

Conclusion

In summary, an Almost Blank Subframe (ABS) is a subframe that is almost completely empty of transmissions, with only a few specific symbols or tones transmitted. ABS is used in wireless communication systems, particularly in LTE and 5G networks, to mitigate interference from satellite signals such as those from GNSS.

ABS works by reducing the amount of interference that is experienced by wireless communication signals, which can improve network performance and reduce the likelihood of dropped calls or lost connections. However, ABS can also have some negative impacts on network performance, particularly in terms of available bandwidth and latency.