December 20, 2018

Sub-Band Free Radios – Do they really help?

By Stuart Little, Director of International Product Line Marketing

In the past years, a few microwave vendors have introduced ‘sub-band free’ RF outdoor units into the market. The main claim of these radios is that a single hardware variant can be deployed in any frequency sub-band, simplifying and lower costs involved with ordering, deployment and sparing of microwave networks.

However, these new radios are not available in all bands and come with a number of limitations, including lower RF performance, larger size, and weight, higher cost, limitations in modulation and channel sizes, amongst others.

Overall sub-band free radios can be useful, but they also introduce operational and planning considerations that could actually increase overall network TCO. Operators should as always check the fine print before considering them for wide deployment.

What is a Sub-Band Free Radio?

Modern digital radios require bandpass radio frequency filters to prevent leakage of the transmitted signal into the receiver chain. These filters typically referred to as diplexers, have a passband that can accommodate multiple radio channels, the number of which is dependent on the frequency band, the transmit to receive separation, and the channel bandwidth. Higher frequency bands have much wider diplexers in the order of 400 MHz or more, while lower bands may have diplexers only a few channels wide, which limits the ability to tune the radios via software control. This means that the operator must order the specific diplexer option required, and also hold spare units for each diplexer variant deployed in the network.

Sub-band free radios enable tunability across the full range of the transceiver, usually the full half-band (low or high). This involves diplexers that can be remotely tuned via software control to the required range. Currently, available solutions involve mechanical methods to physically adjust a moving dielectric plate inside each branch of the diplexer assembly using a drive mechanism (stepping motor). As a result, the tunable radio is larger, heavier and more expensive than the standard, non-tunable ODU.

Another way to achieve a sub-band free ODU is to remove the diplexers from the transceiver unit altogether and package them in a separate diplexer unit that fits between the ODU and the antenna. But diplexer unit would have the same tuning limitations as the regular diplexer in a standard ODU, so it does not solve the supply chain issue, as the diplexer unit still needs to be forecasted, ordered and managed from vendor to site. Removable diplexers also increase the size, weight, cost and mechanical complexity.

Other Implications and Limitations of Sub-Band Free ODUs

Apart from increased size and weight, and likely higher prices, sub-band free ODUs that use tunable diplexers come with a number of other limitations that users should be aware of:

• Limited supply. Sub-band free ODUs are only supported by two vendors today, greatly limiting the available choice in the market, and locking in operators to those particular vendors.
• Limited frequency coverage. Only a limited set of frequency bands and TR spacings are supported.
• Limited RF performance. Sub-band free ODUs only support ‘standard’ RF performance levels, particularly in bands below 13 GHz, and include additional Tx Power and Rx Threshold losses, so will not be suitable for longer hops since larger antennas will be required, wiping out any operational cost benefit.
• Lower Reliability and MTBF. Sub-band free ODUs using mechanically tunable mechanisms will clearly have a lower MTBF since these mechanisms now include actual moving parts driven by two stepper motors that are installed inside the ODU. Any mechanical device must be inherently less reliable than an all-electronic ODU. This could mean that once the unit is deployed on site it cannot be tuned to the correct frequency, or during sparing where the spare unit cannot be re-tuned to the correct configuration.

In addition to these issues, there can also be various other vendor-specific limitations involved in using these new ODUs, especially for an operator who has an existing installed base of radios from that vendor.

Summary and conclusions

Sub-band free ODUs have been available from a very limited number of vendors for several years but have not been a widely adopted feature in the microwave market. There are many good reasons for this, including limited and reduced RF performance, that can lead to shorter paths and larger antennas. Limited frequency band availability means that these new ODUs cannot be adopted across the board by operators, leading to a mix of ODUs and installation and maintenance practices. Other issues, such as limitations in modulation and channel bandwidths supported, IF cable length conditions and other factors, may complicate the introduction of these new ODUs even further, requiring higher expenses and overall driving up TCO, which is the exact opposite of what these new ODUs are meant to achieve.