Microwave Backhaul: Definition, Advantages, and Applications

Backhaul refers to the network infrastructure that connects the access network (such as cellular towers or base stations) to the core network of a telecommunication system. For cellular tower connectivity with the core network, there are several types of backhaul technologies which include microwave backhaul, fiber optic backhaul, satellite, DSL, mmwave wireless point to point links, ethernet, hybrid backhaul etc.

Microwave backhaul utilizes a network of point-to-point microwave antennas to link cellular towers (e.g., base stations or eNBs) and the core network. Microwave backhaul offers numerous benefits in the deployment of 4G LTE and 5G New Radio (NR) networks by providing high capacity, low latency, and flexible connectivity.

It is an attractive alternative to fiber optic backhaul for cell sites in remote or inaccessible areas and in areas where permits are not available to lay optic fiber infrastructure. Microwave backhaul requires LOS (line of sight) and unbroken clearance between transmitting and receiving antennas. Multiple hops or a single hop are required to route or deliver traffic to the designated core network (CN).

Microwave backhaul uses different frequency bands to support different hop lengths (typically). For example, a 2.4 GHz microwave link can support a distance of 100 Km, 18 GHz supports 20 Km, and frequencies in the mmWave band support about 5 to 10 km.

There are two duplex techniques supported in microwave backhaul systems: FDD and TDD. FDD allocates two frequency channel pairs for two-way simultaneous communication. TDD allocates two time slots at the same channel frequency for communication. It uses a higher level of modulation schemes to provide high data rates with limited bandwidth but at the cost of a higher amount of transmit power. Adaptive modulation schemes are often used which changes modulation type based on traffic demand to achieve reliability. The other popular technique is to use different polarizations to increase the capacity of the microwave backhaul system.

Call Process Through Microwave Backhaul

Let’s understand the call process between two users via microwave backhaul links connected with their respective cellular towers. We will analyze the same when some user-A makes a call to user-B as shown in the block diagram.

• The cell phone of user-A connects with the nearest cellular tower (operating in say 800-900 MHz band for GSM). Up-conversion of frequency is carried out before connecting with microwave tower (if needed).
• This cell tower passes the call to the transmit part of the microwave tower (operating in say 6 GHz band).
• This microwave tower directs the call to the opposite receiving antenna as per the point-to-point link. Down conversion of frequency is carried out before transporting the call to the cell tower at the receiving end.
• The cellular tower directs the call to the core network of the mobile system or directly to the cell phone of the user-B.
• The above-mentioned process is carried out for traffic moving in the opposite direction, i.e., from the cell phone of user-B to user-A.

Microwave antennas are mounted at the top of high-rise buildings to achieve a clear line of sight (LOS) between two ends. The microwave antennas are mounted very close or adjacent to cellular base station antennas.

The figure depicts components of split mount radio system. As shown it consists of ODU (Outdoor Unit) and IDU (Indoor Unit). ODU devices include diplexer, Frequency transceivers (up and down converters) and antenna. The connections between antenna and radio units are made using coaxial cable or elliptical waveguide of suitable length. IDU contains modulator and demodulator and other baseband processing modules as per standard requirements. IDU is also known as Modem.

Advantages of Microwave Backhaul

Following are the benefits or advantages of microwave backhaul:

• It is more cost-effective compared to fiber optic cables, especially in remote or challenging terrains where physical infrastructure might be difficult to install or expensive.
• The microwave links can be set up relatively quickly compared to backhaul cable.
• It allows greater flexibility in network deployment which helps us to establish temporary microwave links during emergencies, events, etc.
• Modern microwave backhaul systems with redundancy and latest radio equipment offer high availability and reliability.
• The system is easily upgradable without any major changes to overall infrastructure.
• Microwave backhaul can cover longer distances efficiently while maintaining reasonable data transfer rates.
• Microwave signals travel at nearly the speed of light, which results in low latency connections between the two ends.

Limitations of Microwave Backhaul

Following are the limitations or disadvantages of microwave backhaul:

• It requires an unobstructed line of sight between antennas.
• Microwave signals are attenuated during extreme weather conditions such as heavy rain, snow, or fog, which lead to temporary disruptions in communications.
• Though modern microwave systems offer a good amount of bandwidth, it may not match the ultra-high capabilities offered by fiber and the latest mmWave 5G systems.
• May require regulatory approval to use microwave frequencies.
• Microwave frequencies can experience interference from other wireless devices operating in the same band.
• There are security concerns as it can be intercepted by unauthorized parties due to its travel through the air.

5G Microwave Backhaul Vendors

Following table mentions manufacturers or vendors of microwave backhaul used to provide connectivity for 5G NR (New Radio) base stations.

Conclusion

While fiber-optic networks remain essential for long-haul connections, microwave backhaul complements them by providing rapid deployment, greater flexibility, lower latency and cost-efficiency in various 4G LTE and 5G NR cellular network scenarios.