Every day, critical signals are sent utilizing wireless microwave technology. The use of microwaves is a conventional method for Utilities to contemplate when connecting two or more sites. However, the vast proliferation of wireless technology has posed challenging circumstances for your technical staff when implementing a trustworthy, qualified and cost-effective installation for a new wireless link. There are so many broadband wireless links being deployed by so many different users, that the quasi exclusiveness once enjoyed by Utilities is being lost and the free space environment is becoming as congested as an expressway at rush hour.
So, what is an RF engineer to do? How can you deploy systems and still be sure that they will perform at the levels that are needed to meet your operational and business objectives? The tried and true approach is to leverage the physical aspect of microwave systems design to enhance the link performance. Soon, an engineer will have many new tools available to improve link performance. There are some emerging technologies that are about to hit the marketplace, which will advance microwave technology further in the next two years than it has moved in the past 50 years. However, it is important not to forget the basic techniques, so the following is a quick refresher of some of the tricks of the trade. Here is a brief introduction to these emerging technologies. When these approaches are combined, communication network engineers will be empowered to maximize microwave performance and deploy economical solutions wherever they choose.
Implementation Strategies to Mitigate Interference
Antenna Polarization – Change the polarization by aligning to an opposite polarization to the interfering signal. Choices include vertical, horizontal, left-hand circular and right-hand circular. Out of phase polarization may reduce interfering signals by as much as 40 dB.
Antenna Beamwidth – Change the antenna in order to narrow the antenna beamwidth in order to no longer “see” the interfering signal. A larger antenna will provide increased gain and narrow the beam. This is similar to blinders on a horse. If the antenna can not see the interfering signal within its bore sight, then it may no longer pose a problem.
Time Division – Leverage time division as a means to transmit out of synchronization with an interfering signal. Time division multiplexing has become more popular in recent years as a means to enhance spectral utilization. Data is assembled into thousands of discrete time slots and the transmit radio and receive radio exchange their data within these time slots.
Frequency Division – Apply a frequency division strategy to make use of multiple carriers within the channel so as to assemble the channel in a far more robust manner. Many new radio technologies take the source signal and then transmit it simultaneously in numerous pieces and at different frequencies within the channel. This way, if one carrier is lost, then other will likely get thorough. With the use of redundant transmission, the likelihood of the signal being delivered is very high.
Frequency Band – Change the frequency of the signal to move further out of band compared to the interfering signal. This may seem like such an obvious statement, but you might be surprised how few people pursue this approach. For example, virtually all 802.11b/g Wi-Fi links are delivered set to channel 6. So, the vast majority are installed at this channel. By simply changing the channel to 1 or 11 you will immediately reduce interference. Channels 1, 6 and 11 are the only three non-overlapping frequencies in the North America Wi-Fi plan (see Figure 1). The rest have some level of impairment so they should be avoided if possible.
Figure 1: 2.4 GHz Channel Plan
Bandwidth – The narrowing or reduction of bandwidth by the use of fixed filters is another often overlooked and simple trick to help mitigate an interfering signal. All too often, engineers assume that the radio’s built-in filters are sufficient to eliminate undesired out-of-band carriers. Typically, these filters are cut much wider than needed or are frequency agile, which also results and excessive bandwidth. An external fixed filter precisely cut for above, below or as a window through the channel is a cost effective means to block out unwanted RF interference. RF absorbing materials may also be used to blind one or both sides.
Tilt and Azimuth – Mechanical beam tilt and azimuth beam tilt are things that you can do that cost nothing and might add noticeable improvements. There are two contrasting concepts when it comes to physical alignment – perfect alignment and deliberate misalignment. For the first idea, we often find that antenna installation is poor and that a professional mechanical realignment to bore sight the transmitting and receiving antennas goes a long way to improve overall link performance. Over time, antennas can shift out of alignment due to a variety of factors – wind, snow-loading, fatigue, tower twist, etc. The second idea is useful to mitigate interference. If you have sufficient gain for the link, then a deliberate antenna misalignment may permit you to shift one antenna so as to place an interfering signal into a deep side lobe null. This trick can effectively reduce the interfering carrier by 50% or more.
Power Levels (EIRP) – Adjust the EIRP by either decreasing the power levels. All too often, design engineers aim to implement a link that operates at the perfect power level. The perfect power level is thought to be the maximum limit authorized by the regulator. However, this is a misconception. By adjusting the power level to operate at lower radiated energy may go a long way to solving a problem. Finding the sweet spot for the perfect power level, while still not violating the government’s specifications, is a smart strategy that may also save you electrical costs. EIRP is derived from amplifier gain, cable loss and antenna gain. So, do not forget the golden rule of RF design – passive gain before active again! That means, buying a larger antenna and reducing the amplifier output may serve to improve the link performance while saving you money.
Antenna Diversity – Once in a while, the best idea may be to employ a redundant antenna and diversity switching system to mitigate multi-path interference. This is especially true over paths with bodies of water within them, but can also be helpful in the flat plains found in the Prairies. The thinking here is that as the surface or atmospheric attributes bend the transmitted beam, it falls out of optimized alignment with the primary receive antenna. With good engineering, a secondary receive antenna is positioned to recover the signal as its misalignment shifts its focus towards the secondary. Electronics determine which antenna is utilized based on best receive signal level and then switch to that antenna.
Spatial Path Diversity – If a signal is critical to your operation, then perhaps you need to deliver it to the destination via two separate pathways. This is a horizontal perspective to the Antenna Diversity (vertical) strategy mentioned previously. Two discrete paths to the site can provide a vast improvement in reliability. But, they need to be separated by sufficient distances to be effective and you need extra spectrum to eliminate the possibility of being your own source of conflict.
Installation and Mounting Challenges – It is not unusual to visit a tower site and then discover that the source of interference is self inflicted. After investigation with a spectrum analyzer, we have discovered sources of interference on the property originating from motors, heat-pumps, exterior lighting, and even rider lawnmowers. Another source of concern is heavily populated towers where one antenna is creating interference for another on the same tower. It is vital to separate antennas by at least ½ lambda of their operating frequency. Not only do you need to be concerned about vertical population, but also front-to-back issues for horizontally mounted antennas. Cables and connectors can be a source of great concern, especially near coastal areas where salt in the air accelerates the deterioration of even the best installer’s craftwork.
Path Line of Sight – There is a great deal of confusion concerning line of sight or LOS. You must ensure LOS and Fresnel zone clearance on the path. Too many assumptions are made on this topic. Only a thorough analysis (paper study) followed by a detailed review (site survey) can validate the suitability of the path. Depending on the frequency used for the link, Fresnel zone clearance is also mandatory for a reliable connection. Paths can change over time. How? Well in one case a few years ago, a customer complained about ill-performing equipment that was intermittent and getting worse week by week. Once on site, we realized that a major condominium building was being constructed in the pathway. The reason the path was deteriorating was to due to the addition of each new floor to the building as it was being erected. A mid-path relay tower was added to bypass the condo and the path was restored.
Utilize Terrain Obstructions to Advantage – Often, customer’s complain that they have difficult local terrain that makes the use of microwave challenging or perhaps, even impossible. Likely, the opposite is true. A smart designer will take advantage of the local physical terrain attributes to block interfering signals. Terrain is a natural RF filter. It may permit excellent spectrum reuse techniques that might otherwise never be possible in a smoother terrain environment. So, do not be so quick to dismiss the terrain as an insurmountable problem. It may just be the ideal answer to a problem.
Grounding – There is a much debate about what is and what is not proper grounding. There are clear rules and best practices to be followed as well as lots of experience to fall back upon. Yet, grounding seems to be a reoccurring theme when resolving problems in the field. By ensuring a superior grounding strategy, you can go a long way to mitigate RFI and EMI ingress. Lightning ground is a second major safety concern. Yet, customers always debate whether or not they want a rooftop antenna grounded from lightning. This issue is compounded by different rules that are often misunderstood. The smart recommendation is to not fool around with lightning. Lightning demands the straightest and best path to earth. Yet, it is highly unpredictable, so you can only help to manage it better if you plan for it, rather than ignore it. Yes, it will cost your project more money to do it right, but consider the consequences if you do not.
Within the next few years, we will see the several order of magnitude of technical improvement in the world of RF systems increase several times. Of course, the physical world will not change, but the manner in which antennas and radios exploit the physics of our world will change. As an example, the IEEE has just approved the new 802.11n standard that will offer between four to twelve time increases in data rate – up to 600 Mbps! Here is a list of four primary emerging technologies that you will want to keep your eye on:
Sectorization – While not a technological solution per sey, it is an emerging and popular approach to designing base stations for point to multipoint networks. By using several radios to radiate in multiple directions instead of just one omnidirectional antenna, several important goals can be achieved – improvements to coverage, increased range, reduction of interference, greater node capacity, and more. Dividing the coverage into sectors is similar to slicing a pie into pieces, it is a divide and conquer strategy. The cover image of this article is representative of this design feature. Also see: Sectorization Article
MIMO – This is the single most significant RF technology advancement in the past 50 years. It will turn the RF world upside down and greatly boost signal propagation characteristics.
- MIMO means “multiple-inputs-multiple-outputs”.
- Recently emerged as one of the most significant technical breakthroughs in modern communications.
- Poised to penetrate large-scale, standards-driven commercial wireless products and networks such as broadband wireless access systems, wireless local area networks (WLAN), fourth-generation (4G) networks, and beyond.
- The idea behind MIMO is that multiple, parallel signals from an array of transmit antennas reach an array of receive antennas, which are then “combined” in such a way that the quality (bit-error rate or BER) or the data rate (bits / sec) of the communication for each MIMO user will be improved.
- A core idea in a MIMO system is space–time signal processing in which time (the natural dimension of digital communication data) is complemented with the spatial dimension inherent in the use of multiple spatially distributed antennas.
- MIMO systems can be viewed as an extension of the so-called smart antennas, a popular technology using antenna arrays for improving wireless transmission.
- A key feature of MIMO systems is the ability to turn multipath propagation, traditionally a pitfall of wireless transmission, into a benefit for the user.
- MIMO effectively takes advantage of random fading and when available, multipath delay spread for multiplying transfer rates.
- The prospect of order of magnitude improvement in wireless communication performance at no cost of extra spectrum (only hardware and complexity are added) is largely responsible for the success of MIMO as a topic for new research.
Figure 2: Example of a MIMO Channelization Model
Dynamic Modulation and FEC – Most broadcasters a well acquainted with modulation and forward error correction (FEC). What is new is the dynamic, real-time manipulation of these two RF techniques for the purpose of further enhancing a link. With the migration from analogue to digital, we can now leverage the time domain to throttle or rapidly adjust the modulation between 64 QAM, 16 QAM or QPSK and the FEC from 3/4, 2/3 or 1/2. The data rate will be reduced as we throttle the link down, but the link will be preserved. Video requires a stable throughput. Therefore, if Quality of Service (QoS) attributes are added to recognize the demands of “contiguous” traffic, then less essential “bursty” traffic may be shed to preserve the video traffic.
Smart Antennas – We will see classic single antennas make way for phased arrays and adaptive arrays. An array is a series of antennas that operate in concert with one another. Electronic management makes them smart. They are smart when they can be electronically tuned, and aligned without physical movement, while optimization adjustments are made from the ground by an engineer rather than by a rigger climbing the tower. Adaptive arrays can dynamically adjust to mitigate undesired interference while maintaining desired signal levels. Smart antennas can track a vehicle or a nomadic source signal. MIMO uses these smart antenna features and further processes them to enhance data rate.
Beamforming – This is a new approach to combining some of the features of MIMO and Smart Antennas into an array that dynamically reshapes the RF coverage pattern for each node in real-time. Therefore, it optimizes the coverage on a node by node basis. It can be used for receiving and for transmitting signals. Interference can be mitigated using this technique.
Software Defined Radios – While this approach is not exactly a new enhancement to the RF propagation methods, it has many special advantages that support these features. A SDR uses software instead of hardware for the core functions of the radio. So, this allows optimization and changes to the radio design, features, functions and frequency after installation. This way, some of the new emerging capabilities like MIMO and Beamforming can be added if they were not fully developed when the radio was delivered. Added capabilities for firewalls, switching, authentication, and analytics can all be nested into the SDR for even greater functionality.
Meshed Networks – A meshed network is a new topology that utilizes several radio / antennas and straps up multiple paths back to the base station. Traditionally, broadcasters have used just point to point connections. However, multipoint to multipoint networks will greatly enhance the robustness and performance of a link. This path diversity will extend the range and enhance network throughput with a meshed architecture.
Figure 3: Mesh Network example Architecture
There are now many new tools for an engineer to leverage RF as a data connectivity methodology. Some are “tried and true” and others are emerging. When integrated with enhancements in security and QoS, these methodologies will permit combined, multiple traffic types to share the same connections. Third-party service providers will soon be offering ubiquitous networks and bandwidth on-demand business models and services that might mean that you can outsource your microwave links rather than building and capitalizing your own. RF is coming of age and will become an even more valuable resource for running your operations.
Michael Martin has more than 35 years of experience in broadband networks, optical fibre, wireless and digital communications technologies. He is a Senior Executive Consultant with IBM’s Global Center of Excellence for Energy and Utilities. He was previously a founding partner and President of MICAN Communications and earlier was President of Comlink Systems Limited and Ensat Broadcast Services, Inc., both divisions of Cygnal Technologies Corporation. He holds three Masters level degrees, in business (MBA), communication (MA), and education (MEd). As well, he has diplomas and certifications in business, computer programming, internetworking, project management, media, photography, and communication technology.