Boosting Microwave Spectral Efficiency and Capacity with Line-of-Sight MIMO Technology
Line-of-Sight (LoS) MIMO (Multiple Input Multiple Output) revolutionizes microwave communication in terms of capacity and spectral efficiency while providing many additional benefits for mobile network operators. What is this technology and how does it work? How much can it boost capacity? How can it benefit mobile network operators?
100% Capacity Boost
The last major advance that boosted spectral efficiency by an order of 100% was co-channel dual polarization (CCDP) with XPIC technology. Since then, incremental improvements such as high modulation schemes (1024 QAM, 2048 QAM), packet compression, asymmetrical bandwidth delivery and Layer 1 link-aggregation techniques have continued the procession of improvements in spectral efficiency and capacity.
We are upon the next great leap in microwave technology that will improve spectral efficiency by another 100%: LoS MIMO. Employing multiple transmitters and receivers to increase spectral efficiency with a combination of spatial and polarity multiplexing over the same frequency channel, LoS MIMO is an exciting new technology that can boost radio link capacity in backhaul networks. With LoS MIMO, mobile operators will be able to use cost-effective microwave for higher capacity applications such as super-size macro cell and aggregation site backhaul.
How It Works
A 2X2 MIMO microwave link comprises two transmitters and two receivers connected to two antennas on either side. Spatial separation between the antennas on each side creates different signal path lengths across the link. Separation between signals is achieved by causing the signals to arrive at a specific and constant phase difference at the receiving antennas. This phase difference is determined by the distance of separation between the antennas on each side of the link. A signal processing algorithm is then applied to cancel cross-interference and to separate the signals.
4X4 MIMO is achievable by using four transmitters and receivers in both H and V polarizations for each antenna. 4X4 MIMO effectively doubles the capacity of the link relative to an XPIC link, and quadruples the link capacity relative to a single carrier over the same single frequency channel.
Antenna separation does not have to be vertical. On masts and poles, it is convenient to separate the antennas vertically, but horizontal separation, as in a rooftop installation, will also deliver the full effect of LoS MIMO as long as the axis of separation is consistent on both sides of the link. Antenna separation for multi-km links is typically measured in meters. For example, at 13 GHz, a 3 km link requires 6 meters of antenna separation on each side of the link.
Antenna separation does not have to be equal on both sides of the link. Constraints which may limit antenna separation on one side of the link (tower space, mechanical load, etc.) can be compensated by adjusting antenna separation on the other side.
While LoS MIMO normally requires careful planning of antenna separation, Ceragon’s advanced implementation includes a highly sophisticated and unique algorithm that accommodates a generous tolerance of separation distances while still maintaining the quality of the MIMO link. Tolerating sub-optimal deployment, even as much as 50% off the optimal separation, the algorithm allows for simple installation and smaller antenna masts where necessary.
Benefits of Using LoS MIMO
Microwave radios operating in LoS 4X4 MIMO provide a new set of benefits including the obvious capacity boost, lower TCO, and higher immunity to noise and interference.
Multiplying microwave capacity
LoS MIMO enables transmission of two independent bitstreams over the same frequency and same polarization. This means 100% more capacity in a 2X2 MIMO configuration compared to a 1+0 SISO link without wasting additional spectrum resources. Using both H and V polarizations of a frequency channel, the 4X4 MIMO scheme, enables transmission of four independent bitstreams over the same frequency channel and an effective gain of four times more capacity than a standard 1+0 SISO link or two times more capacity than a 2+0 SISO XPIC link.
Improved system gain
Combining received signals from both antennas boosts system gain by 3dB, similar to that achieved by space-diversity schemes with IF-combining. Further improvement to system gain can be achieved at some of the expense of the capacity boost by splitting a bitstream between transmitters operating in MIMO, thus enabling reduction of modulation and, in turn, increasing system gain, both Tx power and Rx sensitivity. Mobile operators can enjoy longer link distances and reduced antenna sizes. They can also decongest their spectrum by utilizing higher frequencies for long-distance links. An improvement of as much as 20dB can be achieved.
Immunity to dispersive fading
Improving on space-diversity in SISO deployments, the spatially separated antennas employed in MIMO provide multipath protection with double capacity at times when multipath interference is not strong and with the same SISO capacity even at times when multipath interference is strong.
LoS 4X4 MIMO improves on standard 1+1 hot standby deployments. In normal operation, there is no need for an idle hot standby radio unit. The MIMO link uses both of the units for transmission achieving double the capacity. In case of single-unit failure, the link automatically falls back to normal SISO capacity.
Advanced MIMO technology boosts microwave spectral efficiency and throughput allowing microwave transport to scale with the escalating demand for capacity and achieving 2Gbps of uncompressed radio throughput per 56/60 MHz channel of 1Gbps over 28/30 MHz channel. With LoS MIMO, network operators can apply cost-efficient microwave to a new level of demanding high-capacity backhaul implementations.
Ron Nadiv is Vice President of research and development for Ceragon Networks.