LTE Cat 1bis or 5G RedCap? RF design decisions behind an IoT modem refresh

Replacing an IoT modem is rarely a drop-in component change. The new module may fit the same product category and promise lower power or longer network support, but its band set, receive architecture, and certification path can force changes to the antenna, PCB, and enclosure. The choice among LTE Cat 1bis, LTE-M, NB-IoT, and 5G Reduced Capability (RedCap) should therefore be made as an RF system decision, not from a modem datasheet alone.

RedCap has now moved beyond specifications and demonstrations. A growing range of Release 17 chipsets, modules, and industrial devices is commercially available. So, operators are enabling the technology on 5G standalone networks, and Global Certification Forum (GCF) has established a certification route for RedCap products. The engineering question is therefore shifting from what RedCap is to what adopting it changes in the finished device.

Define the deployment requirements

Before asking whether 4G or 5G is the better fit, the design team needs to establish where the product must connect, which operators it must support, and how long it’s expected to remain in service. Cat 1bis uses ordinary LTE infrastructure and retains Cat 1’s nominal 10-Mbps downlink and 5-Mbps uplink while reducing the receiver to one-antenna path.

LTE-M and NB-IoT trade data rate for narrow bandwidth, coverage enhancement, and low-power operation. RedCap, introduced in 3GPP Release 17, occupies a different tier: it brings a reduced-complexity device onto 5G NR, with a maximum 20 MHz bandwidth in FR1 and without carrier aggregation.

Although they are often presented as generations on a roadmap, these technologies are not successive rungs on a migration ladder: LTE-M and NB-IoT remain appropriate for low-data, long-life endpoints, while RedCap addresses applications needing more throughput or lower latency than LPWA can provide. It also depends on 5G standalone network support. A global or roaming product may therefore obtain a more predictable footprint from Cat 1bis, whereas a product for a known private 5G network can make a much stronger case for RedCap.

Once the deployment footprint is clear, the next step is to define the bands required by the target countries, operators, and module variants. Low-band coverage around 600 to 960 MHz is often decisive for range and in-building service, but it also sets the most difficult physical requirement.

The wavelength is long, so a small antenna and short PCB ground plane pay an efficiency and bandwidth penalty. A RedCap design may also add NR bands above the range covered by an established LTE antenna, including mid-band. “Sub-6 GHz” on two datasheets does not make them interchangeable.

The above table provides an overview of RF design implications of LTE Cat 1bis, LTE-M, NB-IoT, and 5G RedCap. Source: Taoglas

Compare antenna configurations

Much of Cat 1bis’s appeal comes from its single-antenna architecture, which can eliminate a diversity antenna, its RF feed and the associated placement problem, although this simplicity comes at the cost of reception diversity. Comparisons put the resulting sensitivity or coupling-loss penalty at roughly 3 to 4 dB relative to Cat 1, which matters in basements, metal cabinets, and at the cell edge. A one-antenna product is simpler, but it can lead to mediocre efficiency.

At the lower-throughput end of the design spectrum, LTE-M and NB-IoT usually offer a similarly simple single-antenna configuration, with coverage-enhancement mechanisms that can add link margin through repetitions at the cost of rate, latency, and energy. A utility sensor may therefore favor NB-IoT, while a mobile tracker may favor LTE-M or Cat 1bis. Compare the complete traffic and coverage profile, not sleep current alone.

The antenna decision becomes less straightforward with RedCap because, in FR1, the standard allows either one or two receive branches. A one-Rx implementation supports one downlink MIMO layer; an implementation with two receive branches can support two. Not every RedCap design therefore has the same antenna configuration. The module’s supported bands, Rx branches, and fallback modes define the actual RF architecture.

Where the selected RedCap module uses two receive branches, the mechanical design must accommodate both antennas from the outset, including separation, isolation, and pattern diversity. Quarter-wavelength spacing at the lowest band is usually unrealistic in a compact IoT device. Orthogonal placement, opposite PCB edges, and careful cable routing can reduce coupling, but they do not replace measurement. Poor isolation and highly correlated patterns can erase the expected MIMO benefit.

RF design dependencies are shown in a cellular modem refresh. Source: Taoglas

Account for the complete RF design

Because an embedded cellular antenna does not perform independently of its host, the PCB forms part of the radiating structure, and changes to the ground-plane dimensions can shift resonance and reduce efficiency. The keep-out region must remain clear on every copper layer, while batteries, displays, shields, connectors, fasteners, and cables can detune the antenna even when they sit outside the nominal PCB clearance.

Reusing the existing mechanical design may appear to reduce the cost of a modem refresh, but it can conceal significant RF changes. An LTE antenna that was matched on the previous board may move when the module, power supply, or enclosure changes. Provide a feed matching network, maintain a controlled 50-ohm line, and tune the antenna on the production-intent PCB with the battery and housing installed. A return-loss plot from an open evaluation board is not evidence that the finished product will radiate efficiently.

Where a compact device must cover widely separated bands, a tunable antenna may offer another route; the Taoglas and Telit Cinterion Cat 1bis reference design, for example, pairs an ELS62-W module with a PCS.50.A tunable antenna and uses active switching to extend coverage in a small form factor. This recovers flexibility but adds control logic and every tuning state to the RF test plan.

Antenna efficiency affects more than the RF specification because, in weak coverage, an inefficient design can force the modem to transmit harder or for longer and can trigger repeated connection attempts. Peak gain is a poor selection metric: total efficiency, radiation pattern, feed loss, and the installed environment determine the useful link. For a battery endpoint, better radiated performance may deliver more life than a small difference in the modem’s headline sleep current.

Manage coexistence and interference

Since a refreshed modem rarely operates in isolation, its cellular antenna may have to share a small enclosure with GNSS, Wi-Fi, and Bluetooth antennas, as well as switching regulators, processors, displays, and high-speed buses that generate broadband noise. The problem is reciprocal: cellular transmission can desensitize GNSS, and digital noise can degrade cellular total isotropic sensitivity even with a good antenna match.

Coexistence testing should therefore cover every antenna pair across its operating bands, followed by active testing of the assembled device. Force cellular transmission on known troublesome bands while monitoring GNSS carrier-to-noise density, Wi-Fi throughput, and cellular sensitivity. Mitigation may require more spacing, a different orientation, rerouted cables, shielding, cleaner return paths, or targeted filtering. Reserve those options in the layout rather than discovering self-interference during certification.

Plan testing and certification

Although a pre-certified module reduces the amount of compliance work, it does not certify the host product’s antenna implementation, and the finished device may still require regulatory approval, PTCRB or GCF certification, and operator-specific acceptance. Over-the-air performance is assessed using total radiated power (TRP) and total isotropic sensitivity (TIS), which expose losses and desense hidden by conducted modem tests.

Certification planning is best started while the RF architecture can still change, with a matrix covering the radio technology, module variant, regions, operators, bands, antenna type, and number of RF paths. Schedule passive measurements and active OTA pre-scans on production-intent hardware before tooling is locked. Test low, middle, and high channels in each required band, and include realistic operating modes and accessories.

GCF introduced RedCap certification in 2023 and certified the first RedCap module in 2024, giving device makers an established compliance route. The finished product may still require regulatory approval and operator acceptance, however, so early coordination with the module vendor, test lab, and operator remains valuable.

Release 18 eRedCap is intended to reduce 5G device complexity further, but with commercial chipsets and devices expected in early 2027, it remains a roadmap consideration rather than a direct option for most modem refreshes starting today.

In practice, the choice depends on which constraints carry the most weight: Cat 1bis is a strong option where ordinary LTE coverage, mobility, moderate data rate, and a single antenna are the priorities. LTE-M or NB-IoT remains better suited to small, infrequent payloads where coverage enhancement and battery life dominate.

RedCap makes sense when 5G SA availability, higher throughput, lower latency, private-network integration, or product lifetime justify the new bands, possible second receive path, and new certification campaign. When commercial eRedCap hardware becomes widely available, it’s likely to be most relevant to designs seeking Cat 1bis-class throughput and complexity with a longer-term path onto 5G standalone networks.

The modem may be the line item that starts the refresh, but the antenna system determines whether it works outside the lab. Lock the deployment map and bands first; then make the modem, antenna, PCB, enclosure, and certification strategy an engineering decision.

Baha Badran is global head of engineering at Taoglas.

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