Boosting Reliability with Antenna Diversity: Implementing the CC1120’s RX Diversity & TX/RX Switching

In the real world, radio waves rarely travel a single, clean path from transmitter to receiver. They reflect off buildings, terrain, and moving objects, creating multiple signal paths that can constructively or destructively interfere at the antenna—a phenomenon known as multipath fading. This can cause deep, periodic drops in signal strength (fades), leading to packet loss and unreliable links. While increasing transmit power or improving sensitivity helps, a more elegant and effective solution is antenna diversity, a powerful technique natively supported by the Texas Instruments CC1120.

Antenna diversity operates on a simple principle: if one antenna is in a fade, another placed a quarter or half wavelength away (physically or with different polarization) is statistically likely to be receiving a stronger signal. The CC1120 facilitates this by providing dedicated pins (RFC and RFD) for controlling an external RF switch that toggles between two receive antennas. Crucially, the CC1120’s packet handling hardware can be configured to make this switching intelligent and automatic.

The most common implementation is Receive (RX) Diversity with Selection. In this mode, the system uses one antenna as the default. When a packet reception starts (preamble detected), the CC1120 can be programmed to momentarily switch to the second antenna, measure the Received Signal Strength Indicator (RSSI), and then immediately switch back to the first antenna to measure its RSSI at the same instant. Based on a simple comparison, it locks onto the antenna with the stronger signal for the remainder of that packet’s reception. This “listen-before-commit” strategy happens rapidly during the preamble, with no impact on the data payload.

For even greater robustness, systems can implement per-packet antenna selection, where the antenna used for the last successfully received packet is stored and used first for the next reception attempt. The CC1120’s flexible GPIOs and internal status registers allow an external MCU to implement sophisticated diversity algorithms, potentially using metrics like Packet Error Rate (PER) over time to decide which antenna to favor.

The performance gain from diversity can be substantial, often providing 5 dB to 10 dB or more of “diversity gain” against fading. This effectively deepens the receiver’s sensitivity in a dynamic environment, translating directly to fewer dropped packets, higher network throughput, and extended coverage range—especially valuable in urban, industrial, or indoor settings where multipath is severe.

The CC1120 also simplifies Transmit/Receive (TR) switching for systems using a single antenna for both functions. A dedicated GPIO can be configured to automatically go high during transmit and low during receive, directly controlling the bias voltage line of a GaAs FET TR switch. This tight integration eliminates the need for additional logic or careful MCU timing.

Implementing antenna diversity with the CC1120 is a cost-effective strategy to add a significant margin of reliability to any wireless system. By leveraging spatial or polarization diversity to mitigate the unpredictable effects of multipath fading, designers can deliver a user experience marked by consistent, unwavering connectivity, making the CC1120 an ideal choice for mission-critical industrial controls, high-performance security systems, and robust data backhaul networks.