To Cancel or to Synchronize – That’s the 5G Question

The frequency spectrum has been divided by the regulators into bands. Each band, in turn, is divided into channels. In general, radios are designed to operate in any channel within a particular band. In many parts of the world, the 3.5 GHz band has been allocated to future 5G networks. Within this band, multiple operators are expected to be allocated channels as wide as 100 MHz, possibly without any guard-band in between them.

This is not a particularly unique scenario – already today operators commonly share the same band, although each one of the operators is assigned a different channel within the band. For example, AT&T, Verizon Wireless, and T-Mobile in the United States all have channel allocations within LTE Band-2 (1900 MHz). Similarly, many LTE operators in geographies around the world share the popular LTE bands, such as Band -3 (1800 MHz) and Band -7 (2600 MHz).

All the examples above are of LTE FDD bands (Frequency Division Duplexing). Operators using FDD bands are allocated two separate frequencies: downlink frequency to transmit from the base station to the UEs (the phones) and uplink frequency to transmit from the phones to the base station. To avoid interference between neighboring radios within the same band, regulators have solved the problem by allocating all uplink channels to a different sub-band than the downlink channels.

This way, all transmitters on a shared tower (or in the phone) are aggregated to one sub-band, while all receivers are aggregated to a different sub-band. With sufficient guard band, known as duplexer gap, between the uplink and downlink sub-bands and a filter known as duplexer between them, interference has been sufficiently suppressed. The duplexer, by the way, accounts for the majority of the size and weight on the base station.

Unlike most current LTE networks, 5G networks will mostly use Time Division Duplexing (TDD), just like WiFi radios. As every consumer knows, neighboring WiFi radios interfere with each other, even if they operate on different channels. The closer the radios are to each other (both in terms of physical proximity and frequency), the bigger the problem. Channel filters to separate between radios operating on different channels within the same TDD band do not usually exist, and they are generally not desired since the radio is designed to operate on any channel within the band. In many TDD environments such as unlicensed spectrum, the specific channel of operation is often dynamically selected based on geographic allocation and the desire to mitigate interference in the environment. In the United States, the only TDD LTE operator is Sprint. Unlike the FDD bands that are shared between operators, Sprint pretty much own the entire LTE Band-41. 

The only common filters available for TDD bands are band-pass filters. Band pass filters are available to protect a radio operating in one band from radios operating in neighboring bands. These filters ensure that Sprint does not suffer interference originated from other operators’ base stations, nor other operators suffer from Band-41 transmissions.

These band-pass filters are dependent on some frequency separation between the bands. For example, Sprint’s Band-41 (2.5-2.7 GHz) is adjacent to the unlicensed 2.4 GHz ISM band where many WiFi access points operate. Avoiding interference between the WiFi radio and Band -41 (or Band -7) LTE radio in mobile phones is still one of the biggest challenges facing RF engineers designing consumer devices. Without a good solution in place and unknown to users, devices simply avoid the use of neighboring channels to artificially create a guard-band, on the expense of extremely expensive licensed frequency allocation.

Neighboring 5G radio heads operating on the same band (3.5 GHz or millimeter wave frequencies, for example) and installed on the same tower, even if they operate on different channels, will also interfere with each other. Specifically, each time one of the radios transmits, it would interfere with the ability of the other radio to receive.

As of today, there are no solutions for the interference caused by two neighboring TDD radios – only workarounds. The most effective workaround is to synchronize all radios in the band such that they all transmit at the same time and all receive at the same time. This, for example, is how all cellular TDD networks in China operate. Logistically, this is only practical in a much regulated environment. Technologically, this eliminates one of the key benefits of TDD to independently set the uplink and downlink ratio. Note that synchronization would have to be extended to all base stations, small cells, private networks, etc operating in the same TDD band.

Other workarounds include physical separation of antennas, reduction of output power, and the selection of frequency channels that are not immediately adjacent. These workarounds are not only insufficient to prevent interference, but they are wasteful, costly, and degrade overall network performance.

Kumu Networks’ Self-Interference Cancellation technology allows co-located radios to co-exist in the same band without interfering with each other, even if they are using literally adjacent channels (zero guard-band between them). It operates like a tunable channel filter. But unlike traditional filters, it is software programmable, and it does not need any guard band, thus can protect a radio from transmissions in the immediately adjacent channel.

Kumu Networks’ Cancellation Modules are designed for the RF Front End. They are placed between the two interfering radios using couplers, not directly in the Transmit/Receive path. The module uses a copy of the transmit signal and an algorithmic model of the channel between the two radios to generate a cancellation signal that is basically the inverse of the interfering signal at the receiver.

So 5G operators and regulators are facing the question “to cancel or to synchronize?”. Unlike a technological cancellation solution, synchronization requires a broad agreement between everyone regarding the exact TDD frame to be used. Cancellation can easily be bolted on the interfering transmitter and the affected receiver. Doing nothing would result in meaningfully degraded performance of the radio network, which would directly result in reduced coverage and throughput to consumers.