In an always-on, 4G/LTE world where mobile operators and service providers are challenged to deliver sufficient backhaul bandwidth as economically as possible, there is no sign that the skyrocketing demand for bandwidth is easing. With millions of smartphones and other devices that assume nearly constant connectivity to “the cloud”, microwave is a vital component in the transport infrastructure ecosystem.

Indeed, microwave radio has always been a solid option for mobile and wireless backhaul and other service provider and enterprise communications links. Thanks to the latest technology advances -- ever-growing capacity plus the ability to provide service level agreement (SLA) assurance, mitigate interference, and handle TDM traffic over packet-based networks -- microwave is now an even better choice.

Historically, microwave has had the flexibility advantage. Unlicensed microwave links can be set up in days rather than the weeks or months it might take to implement additional wireline capacity for backhaul. Even licensed links, while requiring some mildly time-consuming bureaucratic approval, still easily outpace the time-to-deployment of wireline alternatives. Fiber may offer unlimited bandwidth, but the tradeoff is in time and cost savings.

Microwave’s improvements in bandwidth, capacity, and reliability in the past few years have made it an ideal interim broadband connectivity solution during the often lengthy process of deploying fiber. In fact, these improvements go as far as to establish microwave as a legitimate permanent alternative to fiber.

That said, you can’t avoid the obvious. As operators work with sophisticated business customers, SLAs are key to gaining and retaining business. Customers have a lot at stake in their mobile networking, and they expect glitch-free operation with ample bandwidth nearly all the time. These expectations push operators into offering, and living up to, robust SLAs.

SLAs typically focus on Key Performance Indicators (KPIs) such as availability, capacity, delay, and delay variation (jitter) -- issues that are all but nonexistent in the wired world. But then, the wired network is rarely subject to the range of physical conditions that the wireless network faces, in which weather, distance, interference, and other hard-to-model conditions such as fading and multipath must be reckoned with.

Figure 1. A service provider application for a suburban, medium capacity corporate access environment. A single ODU covers 2 sectors, using 2 unipolarized antennas, 1 per sector.

Figure 2. A service provider application for medium capacity corporate access, with self-backhaul. This offers built-in backhaul from the base station site to the service provider premises.

Figure 3. A service provider application for an urban, high-capacity corporate access environment. Multiple sectors can be backhauled with a single system.

Figure 4. An urban corporate access environment in which a high-capacity SLA would be in place.

Running Interference On Interference
There are several sources of interference. For example, it can come from co-located equipment as the result of a physically short range installation (usually on the same pole) and demands for high frequency reuse. It can also be attributed to uncoordinated networks involving other equipment.

Most of the time, such problems are localized to a specific site or path. In a point-to-multipoint system, isolation of the interference to the individual link is critical so to avoid performance degradation for all other links in the sector.

While some sources of interference may be mitigated during installation and configuration on the day of deployment through manual selection of the “best” channel at the time, new sources of interference may arise later and impair service. These could include consumer and industrial products such as radio equipment, remote control units, motion sensors in security systems, radar installations, and various RF devices.

As microwave proliferates as a backhaul technology, users must address the challenge of overcoming unexpected interference. Meeting this challenge requires a robust wireless system with built-in mechanisms that address interference, especially when the systems are transmitting TDM traffic, since TDM is relatively intolerant of network performance degradation.

While native Ethernet is the ideal transmission environment, legacy systems dictate that microwave links be able to handle TDM traffic over Ethernet or IP links. Service providers are definitely en route to an all-Ethernet infrastructure, but the need remains for the foreseeable future to provide a gap-bridging solution that accommodates existing TDM traffic.

As a result, achieving carrier-grade transport requires five-nines availability, a low bit error rate, short latency, and clock accuracy, with low values of jitter and wander. These can be challenging. On the upside, unlicensed bands offer free usage of hundreds of MHz of unregulated spectrum. Yet at the same time, the wide-open, uncoordinated nature of the transport can create interference issues that affect service availability and bit error rate.

Additionally, the Time Division Duplex (TDD) nature of unlicensed bands may cause some delay, which can result in degraded throughput at greater distances. TDD also affects TDM clock accuracy, as the TDD radio clock is not synchronized with the TDM clock, so the TDM clock must be reproduced at the remote side of the link. Interference complicates this process.

Alternative solutions such as TDM over packet (a.k.a. Pseudowire) rely on the IP/Ethernet protocol to overcome network congestion caused by jitter distribution. Pseudowire, which emulates the native TDM and carries it via a tunnel through a packet-based network, is a viable alternative in situations where a service provider needs to bridge a gap, wireless or otherwise. It isn’t always a perfect solution, due to occasional issues with the constancy of jitter and delay. But where the network architecture dictates this type of transport, it is an effective approach, particularly in a point-to-point connection in a ring topology.

Ten Ways to Mitigate Interference
Ten mechanisms work together to mitigate interference and deliver the level of performance that carriers require. All of these pertain to both Ethernet and TDM, with one exception, Clock Recovery, which is for TDM traffic only. They are:

Mechanism 1: Automatic Channel Selection (ACS). This ensures that transmission always occurs in the best possible channel. ACS responds to interference by monitoring the available radio channels, then dynamically selecting the channel most suitable for transmission at that time. ACS provides robustness in unlicensed bands, and its “always on” nature mitigates the variable and non-deterministic interference common to these bands.

Mechanism 2: Automatic Adaptive Rate. This dynamically adapts the transmitted rate by changing both the signal modulation and coding. It optimizes data throughput according to interference conditions in order to maintain service quality. When increased interference that could affect the quality of the link is detected, the air interface rate is instantaneously modified to the most suitable rate.

In the case of TDM traffic, this decreases the Ethernet throughput temporarily while ensuring that TDM and Ethernet traffic is maintained and that the link stays up. This, of course, also has a direct impact on both link reliability and service availability.

For Ethernet-based systems, robustness is achieved by the transport layer protocol (TCP/IP). The radio protocol then needs only to deal with spectrum efficiency. In contrast, a native TDM system has to maintain balance between high availability and high efficiency, since the TDM is more vulnerable to link degradation caused by interference.

Transmission at a lower modulation scheme such as BPSK 1/2 yields robustness, while higher modulation such as 64 QAM enables higher capacity and shorter transmission times. Optimal performance is a matter of finding the right combination.

Mechanism 3: Configurable Channel Bandwidth. This lets the user choose between higher bandwidth with a relatively large spectrum footprint and lower channel bandwidth with narrow spectrum usage. In crowded environments, where interference-free spectrum is rare, the ability to specify channel bandwidth is important for optimization of the unlicensed frequency band.

Mechanism 4: Clock Recovery. For TDM, an algorithm reproduces the input TDM clock at the remote end of the link. This requirement is unique to TDM and does not apply to Ethernet-based systems. The quality of the recovery is defined by standards such as G.823 and G.824. Typically, TDM over IP applications use algorithms designed to recover the clock over standard networks, but are not optimized for unlicensed TDD wireless links.

TDD makes the recovery more complex since the output packet rate is determined not only by the input TDM packet rate, but also by radio frame duration. Clock recovery mechanisms for systems operating in unlicensed bands must tune their parameters dynamically as a function of the radio channel quality, level of interference, and system/link delays, to ensure clock synchronization and recovery at the remote end.

Mechanism 5: Advanced Forward Error Correction (FEC). In this mechanism of error control for data transmission, the sender adds redundant data to messages. The receiver can thus detect and correct errors upon reception of the transmitted data. The advantage of forward error correction is that retransmission of data can often be avoided, reducing average bandwidth requirements. It is a very useful approach in situations where retransmissions are relatively costly or impossible.

Mechanism 6: Automatic Repeat reQuest (ARQ). RF interference can damage transmissions, resulting in corrupted data at the destination site. Without an intelligent method for detecting and resending corrupted or missing data, service can be significantly degraded, and, in some extreme cases, be halted entirely. ARQ is a common protocol for error control in data transmission. When the receiver detects an error in a packet, it automatically requests the transmitter to resend the packet. This process is repeated until the transmission is error-free or the error continues beyond a predetermined number of transmissions. The ARQ algorithm ensures data integrity and bit error rate by using acknowledgments and retransmissions after certain timeouts. Continuous measurement of received data quality and retransmissions of data preserve data integrity, but at the price of delays, delay variation (jitter), and degraded performance. Ethernet-based radios and systems using TDM over IP commonly use standard ARQ, since they are designed to handle the distributed jitter that may be caused by retransmissions. Some systems avoid ARQ because of the negative effects; they gain lower delay and a more accurate clock, but at a cost of greater susceptibility to interference.

Mechanism 7: Hub Site Synchronization (HSS). Radios using the TDD method can experience interference from other radios located at the same site if they are transmitting and receiving according to different time patterns. HSS synchronizes the transmission pulses of all co-located systems. Using an external cable connected to all affected radios, a pulse is sent to each radio that synchronizes its transmission with the others. This pulse synchronization ensures that the transmission and receipt of packets occurs at the same time for all co-located units, eliminating the possibility of interference from some units transmitting while other units are receiving.

Mechanism 8: Orthogonal Frequency Division Multiplexing (OFDM). This is a modulation technique for effective transmission of large amounts of digital data over a radio link, thanks to its inherent characteristics such as low overhead, low latency, and high resiliency to interference. Based on the concept of redundant transmission, OFDM works by splitting the radio signal into multiple, smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver. By replicating the content signal using multiple narrowband sub-carriers to repeat transmissions over time, OFDM ensures that complete content arrives at the transmission destination. This technique is especially effective for protecting against the effects of multipath fading derived from the cancellation of carriers under heavy interference conditions. When a system employing OFDM encounters RF interference, it recovers the affected signal from duplicate carriers that were not affected.

Mechanism 9: Multiple In, Multiple Out (MIMO). This involves the use of multiple antennas at both the source (transmitter) and destination (receiver). They combine to minimize errors and optimize data speed. This eliminates the multipath effects that can occur in certain environments (such as hilly terrain, congested urban environments, or proximity to utility wires) with single antenna transmissions. From a single antenna, the wavefronts can get scattered in these environments, and the signals take more than one path to the destination. The late arrival of portions of the signal causes the multipath fading effect.

Mechanism 10: Directional Antenna Design. The design of the antennas used at each end of a wireless link affects link budget and performance in conditions of RF interference. Directional antennas focus signal transmission and reduce interference effects.

Microwave to the Rescue
Microwave radio can do things today that it could not do just a few years ago, and that has made it a valuable alternative for mobile and wireless backhaul and other point-to-point and point-to-multipoint needs. At the same time that it gives providers the ability to handle rapid increases in bandwidth demand, it can also provide a cost-effective traffic over Ethernet and other packet-based networks and provide high reliability and high capacity that assures providers they will meet their SLAs cost effectively.

While link interference is a concern, there are many ways to address this issue. Utilizing systems that have inherent quality-enhancing mechanisms, planning the network architecture effectively, and quickly solving problems as they arise will guarantee top performance and smooth network operation.

 Carmen Folz is a Solutions Engineer for RAD Data Communications, a global provider of microwave radio systems and other network access equipment. She has more than 15 years of experience in datacom/telecom and broadband wireless technologies. For more information, please email carmen_f@rad.com or visit www.radusa.com.

Getting to Know Your Microwave: Definitions
Fading: Conditions such as slow or fast, flat and frequency-selective fading, which may decrease the radio signal's reception quality.

Multipath: A phenomenon that results in radio signals reaching the receiving antenna by two or more paths. It can occur in near Line of Sight (nLOS) and Non Line of Sight (NLOS) scenarios. In both cases, the signal reflections are received along the main path signal, causing a "multipath" effect that reduces signal quality.

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