Application of LTC5569 Dual Channel Mixer in MIMO RRU Design

High-performance wireless base stations are undergoing significant transformations to make the deployment of costly 4G networks more efficient and practical. As data transfer rates in 4G networks increase dramatically compared to 3G, performance demands have become even more rigorous. Equipment designers now face a number of complex challenges:

• Integrating multiple MIMO (multiple input, multiple output) channels into the RF unit.
• Reducing the size of the RF unit to fit into smaller enclosures.
• Enabling flexible configuration of RF units to support various frequency bands or communication standards.

As a result, the next generation of base stations may look quite different from traditional models. Instead of large equipment racks located in air-conditioned rooms at the base of towers, compact, weatherproof enclosures—commonly known as RRHs (Remote Radio Heads) or RRU (Remote Radio Units)—will be mounted on the top of towers. These units are about the size of desktop computers and are designed to withstand harsh environmental conditions. Each chassis contains numerous RF electronic channels but lacks baseband processing capabilities. The modulated signal is transmitted via high-speed fiber optic cables or microwave links to a central base unit that can serve multiple cellular base stations over long distances. This architecture is scalable and cost-effective for deployment.

Another emerging trend in next-generation systems is the ability of radios to operate across multiple frequency bands, often supporting multi-mode operations. Such systems can be easily configured through software to meet the specific requirements of any telecom carrier, regardless of the frequency band or standard used.

MIMO Receiver Increases Network Capacity

For any new generation of base stations, the primary goal is to provide higher data transfer rates and increased network capacity. Today’s networks are overwhelmed by the growing use of smartphones, laptops, and tablets. By operating two or more orthogonal receive channels simultaneously, MIMO transceivers help achieve higher data rates. These data streams are combined to boost the effective data rate.

In addition, multiple channels help reduce fading and multipath interference, which can degrade performance and cause data loss. The LTC5569 dual-channel mixer from Linear Technology is designed for simultaneous reception in two channels, with both mixers driven by a common local oscillator (LO) to maintain phase coherence. While using two separate mixers could also achieve this, integrating both into a single chip ensures better consistency and matching between devices. This allows for tighter coupling with two physically separated antennas or patch elements, enabling superior spatial diversity. The internal independent LO buffers of the two mixers offer excellent channel isolation, supporting the cascading of multiple data streams into a single, higher-rate stream.

By intelligently directing the signal in the same direction it receives, a smart antenna can be used in MIMO implementations. For this purpose, two or more receiving channels must measure the angle of the incoming signal, making LO phase coherence between the two channels essential.

Larger Bandwidth Enables Multi-Mode Operation

The 4G wireless network is expected not only to deliver much faster data rates than 3G but also to support a significantly wider bandwidth. This makes multi-mode operation possible. The wireless industry is pushing bandwidth requirements from 40MHz to 65MHz, and sometimes even up to 75MHz. This presents a major challenge for RF engineers due to the strict gain flatness requirements.

Figure 1 shows an application circuit using the LTC5569 dual-channel mixer, acting as an uplink receiver operating in the LTE TDD band from 2496MHz to 2690MHz. The circuit is relatively simple, requiring very few external components.

Figure 1: Example of a circuit operating on the MIMO TDD LTE band from 2496MHz to 2690MHz

In this application, the LTC5569's IF output needs to cover the 195MHz to 235MHz frequency range. The IF output is optimized for low return loss at higher frequencies to improve frequency response flatness. Measured IF output return loss is 20 dB at 235 MHz and 14 dB at 195 MHz, resulting in an IF output frequency response flatness of ±0.3 dB over a 40MHz IF bandwidth.

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