Application of LTC5569 Dual Channel Mixer in MIMO RRU Design

Crystal oscillator

Photocoupler

High-performance wireless base stations are undergoing fundamental changes to make the deployment of high-cost 4G (fourth generation) networks more acceptable and efficient. At the same time, as the data transfer rate of 4G networks increases to many times higher than current 3G networks, performance requirements become more and more demanding. Equipment designers face many challenges:

- Plug in many MIMO (multiple input, multiple output) channels in the RF unit

- Plug the RF unit into a smaller footprint and smaller enclosure

- Easily configure RF units to support any frequency band or communication standard

Therefore, the appearance of the new generation base station may be different from the past. A small, weather-proof sealed enclosure, commonly referred to as an RRH (remote radio head) or RRU (remote radio unit), will replace the large equipment rack placed in the air-conditioned room at the bottom of the tower. These chassis are as large as a desktop computer and are designed to be mounted on the top of the tower to withstand wind and rain. Each chassis has a large number of RF electronic unit channels, but no baseband modulation or demodulation processor. The modulated signal is sent and sent through multiple 100Gbps fiber optic cables or through point-to-point microwave links. These signals are sent to a base unit that may be tens of kilometers away and fed to multiple cellular base stations at a time. This type of base station architecture is easy to scale and may be more economical to deploy.

Another trend in next-generation systems is the ability to operate radios in multiple frequency bands (in many cases capable of performing multi-mode operation). Such systems can be easily configured with software to suit the service requirements of any telecommunications carrier, regardless of operating frequency bands or usage standards.

MIMO receiver increases network capacity

For any new generation of base stations, the most important goal is of course to provide higher data transfer rates to increase capacity. Today's networks are overloaded due to the proliferation of smartphones and laptops/tablets. By operating two or more orthogonal receive channels in parallel, the MIMO transceiver helps achieve higher data rates. Their data bit streams are combined to increase the effective data rate.

In addition, multiple channels help to mitigate fading and multipath interference experienced by wireless receivers, which can result in performance degradation and data loss. Linear Technology's LTC5569 dual-channel mixer is designed to provide dual-channel simultaneous reception (configured so that each mixer's LO is driven by a common input), maintaining phase coherence for both channels Sex. Although this can also be achieved by using two discrete mixers, having both mixers built into a single chip allows for a much better and consistent match between devices. Such a two-channel mixer provides a higher level of signal integrity by allowing tight mating with two physically separate antennas or patching elements. Thus, superior spatial diversity can be achieved. The internal independent LO buffers of the two mixers provide excellent isolation between the two channels to support cascading two or more data bit streams into a single, much higher rate data bit stream.

By intelligently beaming the signal in the same direction as it receives, a smart antenna can be used in the MIMO implementation. To this end, two or more receiving channels must measure the angle of the incident signal. This makes it necessary to maintain the LO phase coherence between the two channels.

Larger bandwidth enables multi-mode operation

It is expected that the 4G wireless network will not only have a much higher data transmission rate than the current 3G, but also a much wider bandwidth. This makes multi-mode operation possible. The wireless industry is pushing bandwidth requirements from 40MHz to 65MHz, and in some cases even up to 75MHz. This is not a simple task for RF engineers because the gain flatness conditions are very demanding.

Figure 1 shows an application circuit using the LTC5569 dual-channel mixer, which acts as an uplink receiver operating in the LTE TDD band from 2496MHz to 2690MHz. Please note that this dual channel mixer is simple overall and uses 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 is required to cover the 195MHz to 235MHz frequency range. The IF output is optimized for optimum return loss at higher IF output frequencies to improve IF output frequency response flatness. The measured IF output return loss is 20 dB at 235 MHz and 14 dB at 195 MHz. This actually achieves an IF output frequency response flatness of ±0.3 dB over a 40MHz IF output bandwidth.

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