In his latest post for the Backhaul Forum, Yoav Mor analyzes the most frequently asked questions he receives about Line of Sight MIMO (LoS MIMO). Yoav addresses new deployment scenarios that make use of the latest multi-core radio technology, the technical aspects of the technology and the various operational and financial benefits MIMO brings!
MIMO is typically used for non-line-of-sight (NLoS) applications. What is its connection to this line-of-sight version?
NLoS MIMO (Multiple Input, Multiple Output) is widely used in access networks like WiFi, WiMAX and LTE where user equipment connects to the network. It exploits signal multi-path caused by reflections from different physical obstacles, using multiple transmitters and receivers to increase spectral efficiency by spatially multiplexing multiple bitstreams over the same frequency channel.
In line-of-sight microwave, the non-LoS multipath signal is weak and unusable for the purpose of MIMO. Instead, LoS MIMO achieves the spatial multiplexing by creating an artificial multi-path not caused by physical objects, but rather by deliberate separation of antennas in such way that a deterministic and constant orthogonal multi-path is created.
How does LoS MIMO work?
A 2X2 MIMO microwave link comprises two transmitters and receivers connected to two antennas on each side.
Spatial separation between antennas is denoted respectively, and the different signal path lengths are denoted for the length of the path between transmitter and receiver.
Separation between signals is achieved by having them arrive at a specific and constant phase difference at the different antennas. Since the two different signals are transmitted on the same frequency and polarization, the “interfering” signals ( and ) cannot be higher, and should rather be of equal power to that of the “desired” signals ( and respectively). A signal processing algorithm is then applied to cancel cross-interference and to separate the signals. Control of the phase at which signals arrive is achieved through the length of the paths over which the signals traverse. The path length can be controlled by the distance of separation between antennas on each side of the link.
The use of this new deployment scenario effectively doubles the capacity over the given frequency channel. 4X4 MIMO is also achievable by using four transmitters and receivers in both H and V polarization effectively quadrupling capacity.
How do you know by how much to separate the antennas?
The following equation formulates the antenna separation distance required for optimal LoS MIMO operation:
where h1,h2 denote the respective antenna separation distances on either side of the link (in meters), D denotes the overall link distance (in meters), c denotes the speed of light (3x108 meter/sec) and f denotes the link frequency (in Hz).
Taking the symmetrical special case where antennas are equally and optimally separated on both sides, we achieve the optimal separation distance:
No, 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 or wind load, etc.) can be compensated by adjusting antenna separation on the other side of the link. You can see that in the equation.
What if my installers don’t get the antenna separation exactly right?
One of the main considerations with LoS MIMO is the sensitivity of the link to the accuracy of the installation: how does inaccurate antenna separation affect the quality of the MIMO link?
In the case of optimal installation, we achieve a 3dB MSE improvement compared to a 1+0 SISO link (single input, single output, the classic microwave installation). But there is substantial tolerance for inaccurate installations. For example, with Ceragon’s implementation of LoS MIMO, in cases where deviation in antenna separation is as much as 10% on each side, we forfeit only about 1dB in MSE from the 3dB gain we achieved in the optimal installation (compared to a 1+0 SISO link).
Does the antenna separation have to be vertical, like on towers?
No, antenna separation does not have to be vertical. On masts and poles, it is convenient to separate the antennas vertically, but horizontal separation (e.g., a rooftop installation) will also deliver the full benefit of LoS MIMO as long as the axis of separation is consistent on both sides of the link.
What do I need to look for when evaluating LoS MIMO equipment in order to make full use of this technique?
- You shouldn’t implement LoS MIMO as an experiment. Look for a vendor of microwave equipment who has experience operating successful LoS MIMO links. Your network is not a learning experience but an important part of your business.
- Make sure your LoS MIMO vendor can give you a large degree of tolerance of antenna separation deviation. In our installations, even with 10% deviation, you get 100% of the total capacity of the link. With 20% deviation, you still get 95% of full capacity. And if you really go crazy and install with a 40% deviation, you get 80% of full capacity. (But, in this case, maybe you need a new installer!)
- You want a simple installation and not one that you will have to re-visit time and time again in order to keep it running smoothly. Ceragon’s all-outdoor FibeAir IP-20C is perfect for this sort of deployment. Small in form and light in weight, it mounts directly to the antenna for successful deployment and operation. FibeAir IP-20C, is a multi-core radio that gives you 4x4 LoS MIMO capability with only two units and two antennas on each side of the link.
What operational and financial benefits do we get with MIMO?
Microwave radios operating in LoS MIMO present a new set of benefits to the user, including the obvious capacity benefit, lowering TCO, and higher immunity to noise and interference.
- Multiplying 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. Using both polarizations of a frequency channel, i.e., employing a 4X4 MIMO scheme, enables transmission of four independent bitstreams over the same frequency channel, and effectively provides 300% more capacity than a standard 1+0 SISO link and 100% more capacity than a 2+0 SISO XPIC link.
- Reduced spectrum licensing fees - MIMO isn’t just about multiplying capacity, it’s also about multiplying spectral efficiency. With more data transmitted over less spectrum, operators spend up to 50% less on frequency licensing fees.
- Space Diversity Protection - Spatially separated antennas employed in MIMO also give the benefits of space-diversity link protection and result in higher immunity to dispersive fading. Networks enjoy higher availability.
- Improved system gain - Combining received signals from both antennas boosts system gain by 3dB, similar to that achieved by space-diversity systems with IF combining. Further improvement to system gain can be achieved at the expense of the capacity boost by splitting a bitstream between transmitters operating in MIMO, thus enabling reduction of modulation scheme and, in turn, increasing system gain (both Tx power and Rx sensitivity). This can help in achieving longer link distances, smaller antennas, or spectrum decongestion by utilizing higher frequencies for long-distance links. An improvement of as much as 20dB can be reached.
Yoav Mor has been a Product Marketing Manager at Ceragon Networks for the past two years, bringing with him seven years of experience in optical and wireless network engineering for the defense industry. He holds a B.Sc. in Electrical Engineering and an M.B.A., both from Tel-Aviv University. Yoav can be contacted directly at email@example.com and his past Backhaul Forum posts can be viewed here.