Mono Interference Cancellation (MIC) is NXP's implementation of Single Antenna Interference Cancellation, a software algorithm that can boost cellular network capacity in densely populated areas like big cities by as much as 40%. It's a technology that has attracted the interest of major operators worldwide, including Vodafone in Europe and Cingular Wireless in the US. It works by cancelling out the signal from the basestation that is causing the greatest interference while processing the signal from the serving basestation.
The origins of SAIC
Users of GPRS, for example, often see their bit-rate fluctuate between low and high throughput. This is a symptom of interference from other basestations in the area, which are operating in the same frequency range. In order to eliminate this interference, SAIC passes the received signal through a filter that recognizes each signal by analyzing its strength, cancelling out the one causing the most interference. SAIC not only cancels co-channel interference, but also addresses adjacent-channel interference, which is caused by the spectral overlap of neighbouring frequency channels. In this way the carrier-to-interference ratio at the handset can be improved significantly, leading to increased data rates, a reduced number of call drops and better voice quality. By promoting SAIC-enabled handsets operators can serve more subscribers on their network and increase quality of service.
With returns on 3G infrastructure investments only now beginning to be realized, operators around the world are looking for ways to improve the performance of GSM network infrastructure. Conveniently, SAIC is only implemented in handset software, and has the potential to achieve a 30-50% increase in network capacity. And, although all handsets in a cell would need to be equipped with the technology to realize that figure, lower penetration would still have a significant effect on network efficiency.
Interference in cellular systems
As a low-cost software solution built into the Digital Signal Processor (DSP) of the handset, SAIC offers the practical advantages of using a single state-of-the-art antenna and RF circuit. The new algorithms in the handset's DSP support operators using the Radio Frequency (RF) spectrum as efficiently as possible.
To support the multitude of phone calls in a cellular system, the GSM standard combines Frequency-Division Multiple Access (FDMA) with Time-Division Multiple Access (TDMA) techniques to provide five communication channels per MHz bandwidth and eight time slots. In a 1:1 frequency reuse scheme, which is applied in more and more networks, every cell in the network can transmit on every available frequency channel.
This is a challenge because the signals from a basestation propagate well past the cell boundary, resulting in co-channel interference. This occurs when a handset in one cell receives a signal from an adjacent cell that is broadcast on the same channel and in the same TDMA timeslot, albeit destined for another handset. If the strength of this interfering signal is not well below the strength of the local signal, the handset will experience degraded audio quality and may even drop the call.
Due to the irregular positioning of cells and the impact of local geography on radio-wave propagation, co-channel interference has the potential to affect a significant portion of any GSM network, often causing critical levels of interference. This can occur even if frequencies are only reused in cells that are separated by two or more other cells. Thus co-channel interference potentially affects the majority of networks in principle, and poses a challenge to operators, who have to tighten frequency reuse in order to boost network capacity.
DARP standards in GSM
The advantages of SAIC in various network scenarios are proven in the feasibility study on SAIC published by 3GPP. In addition, 3GPP concluded the standardization of SAIC by extending the GSM standards with interference cancellation requirements entitled "Downlink Advanced Receiver Performance" (DARP) and by defining the corresponding test cases.
Award-winning implementations
NXP's implementation of SAIC, called MIC, was the first available and continues to lead the market. Implemented in the company's Nexperia cellular system solutions, it has been being tested by operators and handset manufacturers around the world and is now ready for the mass market. As recognition of the technology's importance the inventors of MIC received a prestigious award for innovation from Vodafone's Stiftung für Forschung.
SAIC/MIC offers a way for operators to use the available bandwidth more efficiently to increase capacity in usage hot spots, and is therefore an important addition to an armoury of tools that can be used to optimize network performance. As the market penetration of SAIC-enabled handsets increases, operators can extend the benefits of better quality of service and increased capacity to larger areas of their networks as part of their routine frequency-reuse partitioning.
Further enhancements under study
Recently a feasibility study has been started in the GERAN standardization bodies with the aim to identify and investigate technologies which can further enhance existing GSM/EDGE networks in terms of spectral efficiency average, data rates, coverage and latency. Inspired by the success of SAIC, a successor technology for mobile phones with receive diversity is now going to be standardized. Dual antenna interference cancellation (DAIC) is enabled by joint processing of the RF signals from two different receive antennas in the phone. In this way, DAIC offers similar interference suppression gains for EDGE as SAIC does for GSM, it enhances the interference robustness of GSM and, as side effect, it increases the coverage for both systems.