Remote Radio Head

A remote radio head is an operator radio control panel that connects to a remote radio transceiver via electrical or wireless interface. When used to describe aircraft radio cockpit radio systems, this control panel is often called the radio head.

Current and future generations of wireless cellular systems feature heavy use of Remote Radio Heads (RRHs) in the base stations. Instead of hosting a bulky base station controller close to the top of antenna towers, new wireless networks connect the base station controller and remote radio heads through lossless optical fibers. The interface protocol that enables such a distributed architecture is called Common Publish Radio Interface (CPRI). With this new architecture, RRHs offload intermediate frequency (IF) and radio frequency (RF) processing from the base station. Furthermore, the base station and RF antennas can be physically separated by a considerable distance, providing much needed system deployment flexibility.

Typical advanced processing algorithms on RRHs include digital up-conversion and digital down-conversion (DUC and DDC), crest factor reduction (CFR), and digital pre-distortion (DPD). DUC interpolates base band data to a much higher sample rate via a cascade of interpolation filters. It further mixes the complex data channels with IF carrier signals so that RF modulation can be simplified. CFR reduces the peak-to-average power ratio of the data so it does not enter the non-linear region of the RF power amplifier. DPD estimates the distortion caused by the non-linear effect of the power amplifier and pre-compensates the data.

More importantly, many wireless standards demand re-configurability in both the base station and the RRH. For example, the 3GPP Long Term Evolution (LTE) and WiMax systems both feature scalable bandwidth. The RRH should be able to adjust – at run time – the bandwidth selection, the number of channels, the incoming data rate, among many other things.

RRH system model

Typically, a base station connects to a RRH via optical cables. On the downlink direction, base band data is transported to the RRH via CPRI links. The data is then up-converted to IF sample rates, preprocessed by CFR or DPD to mitigate non-linear effects of broadband power amplifiers, and eventually sent for radio transmission. A typical system is shown in Figure 1.