What is CINR (Carrier to Interference plus Noise Ratio)?

CINR, or Carrier to Interference-plus-Noise Ratio, is a key metric used in wireless communications to measure the quality of a received signal in relation to the background interference and noise. It is an important factor in determining the performance and reliability of wireless communication systems, including LTE (Long Term Evolution) networks.

What is CINR in LTE?

LTE is designed to function in RF environments where co-channel interference exists. The link performance degrades as C/I (i.e. Carrier to Interference Ratio) is reduced, yet can remain acceptable in locations with a zero or negative C/I. Every site and sector of the LTE system operates on the same channel; therefore they can potentially receive interference from all other sectors within the system. Because of this, the RF boundaries between sites and sectors that have overlapping coverage will exhibit 0 dB or lower CINR.

One of the system design goals is to minimize the interference within the system. The desire is to keep the CINR below a defined acceptable criterion. (The acceptable criterion for CINR varies from one MCS level to another.) The CINR requirement would also need to account for the fast fading environment.) The carrier to interference plus noise ratio (i.e. CINR) is the ratio of the best serving carrier signal strength to the interference being presented by all other sites/sectors plus all noise.

This ratio provides insight into where sites/sectors may be producing excess interference so that a design can be adjusted to correct these issues. Following the 1x3x1 site placement reuse pattern along with following the 1x3x3 Resource Block Group reuse pattern minimizes the amount of coverage area experiencing increased co-channel interference. Selecting appropriate antenna down tilt adjustments also helps toward this goal.

Even though a link budget may be created for a given MCS level (e.g. 16QAM) to achieve a high target edge data rate, the CINR requirement needed to achieve this MCS level may not allow for this rate to be achieved in site coverage boundary areas where low CINR conditions exist. Therefore it is important that a CINR study is performed to ensure the desired results can be achieved, just looking at the received signal strength (i.e. RSS) is not sufficient.

LTE Methods for Reducing Low CINR Impact.

1. Resource Block Group Assignments.

The assignments of initial resource block groups to sectors within the system can be
done to resemble a 1x3x3 reuse sub-pattern. In so doing, the system will encounter less
incidences of co-sub-channel interference.

2. Frequency Selective Scheduling.

Frequency Selective Scheduling (FSS) on the forward link provides potential throughput capacity increase. With FSS, the scheduler governs transmission resources to a user using the resource block(s) that will offer the best performance. This requires knowledge of the channel associated with each frequency band normally obtained through feedback from the UE via frequency selective PMI CQI.

Frequency selective scheduling can be done for the reverse link. The scheduler will need the reverse link SRS channel to find out which subchannels are optimal for which UE.

3. Inter-Cell Interference Coordination (ICIC).

Inter-cell Interference Coordination (ICIC) will manage radio resource blocks specifically for the purposes of minimizing inter-cell interference. Interference rejection combination (IRC) on the reverse link can be employed to aid MU-MIMO on the reverse link to reject the strongest interference.

The receiver uses a proprietary algorithm which needs no signaling or parameter setting. The link performance is significantly improved compared to MRC receiver when strong interference is present. More advanced receiver like Turbo-equalizer and SIC are being investigated to further improve the performance.

Importance in Wireless Communications.

1. Signal Quality: CINR is a direct indicator of signal quality. Higher CINR values mean better signal quality, leading to more reliable data transmission and fewer errors.
2. Network Performance: CINR affects the throughput and capacity of a network. High CINR values allow for higher modulation and coding schemes, which increase data rates.
3. Coverage: Understanding CINR helps in optimizing network coverage. Areas with low CINR might require additional infrastructure, such as more base stations or better antenna positioning, to improve signal quality.
4. Interference Management: CINR helps in identifying and mitigating sources of interference, which can degrade network performance. This is particularly important in dense urban environments or in networks with a high number of users.

Factors Affecting CINR.

1. Interference: Interference from other wireless devices, overlapping frequency bands, and other sources can significantly lower CINR.
2. Noise: Background noise, including thermal noise and environmental factors, contributes to the overall noise level and affects CINR.
3. Distance from Transmitter: As the distance between the receiver and the transmitter increases, the signal strength decreases, which can lower CINR.
4. Obstructions: Physical obstructions like buildings, trees, and terrain can attenuate the signal, reducing CINR.
5. Antenna Quality and Placement: The type, quality, and placement of antennas can significantly influence CINR. Proper antenna design and placement can help maximize CINR.
6. Network Load: High network load and congestion can introduce additional interference, negatively impacting CINR.

Optimization.

1. Network Planning: Proper network design and planning, including cell site placement and frequency allocation, can help improve CINR.
2. Interference Management: Techniques such as frequency reuse, interference cancellation, and dynamic spectrum management can help reduce interference.
3. Antenna Technology: Using advanced antenna technologies like MIMO (Multiple Input Multiple Output) and beamforming can enhance signal quality and improve CINR.
4. Power Control: Adjusting the transmission power of base stations and user equipment can help manage interference and improve CINR.

CINR is very important in wireless communications that impacts signal quality, network performance, and user experience. By understanding and optimizing CINR, network operators can ensure more reliable and efficient communication, leading to better overall performance of wireless networks of LTE.