In the telecom industry there is a tendency to reuse previous technology, designs, hardware and software that reduces overall development costs and time to market for new products. In part this is usually driven by the need for backward compatibility but it can and often does result in a sub-optimal design that is a compromise between what is already available with the need to meet new requirements.
In the specific case of higher RF power Metrocells, there are three approaches one can take:
1) Scale down an existing Macrocell design to meet the Metrocell form factor.
2) Attempt to scale up a (residential) femtocell to meet the power and technology requirements of a Metrocell.
3) Start afresh from the ground up making best use of the latest components and technology
The 'scale down' approach can introduce Macrocell technology in a smaller form factor, but the overall design can result in a bulky and expensive product. Conversely, the 'scale up' approach may meet the size and cost criteria but may not satisfy all of the technical requirements. As with many technological challenges often the best approach is to design the product starting from the ground up and optimizing the design to meet specific requirements. This way the design can benefit from the most advanced technologies, inventions and engineering "know-how" while satisfying all of the needed requirements in the most optimal way. The increasing availability of advanced systems on a chip (SoCs) and 3rd party wireless protocol stacks makes it possible to design Metrocells with features and complexity that are on par with a Macrocell but at a fraction of the cost and optimized to the required parameters.
Three challenges of high RF transmit power
To illustrate such a design let's assume a target product optimized for low cost, small size and performance that supports 2Tx by 2Rx 7Watts Tx power for 4G single band or 2Tx by 2Rx 3Watts Tx power for 3G and 4G, dual band. To keep the product cost low, among other things, the product size and weight must be low.
For natural convection products the size and weight are proportional to the power consumption and heat dissipation. From the engineering point ofview, power consumption and heat dissipation are two critical parameters that have to be minimized.
There are a number of challenges that need to be addressed:
First, given that 3G and 4G radio signals have a high crest factor (peak-to-average ratio), the radio of a small cell needs to support advanced features such as Doherty PA topologies, Digital Pre-Distortion (DPD) and Crest Factor Reduction (CFR) to keep its power efficiency high. The radio must also provide a very wide operating band to support the full bands of operation for 3G and 4G technologies. These kinds of requirements are difficult to meet with the existing RFICs. While RFICs require little power, their RF performance is limited up to 1W transmit power with a narrow operational bandwidth. This may not be an issue for low power Enterprise or Femtocells, but it is a major challenge for higher power Metrocells.
A second challenge for higher power RF output is heat dissipation. Even with DPD, CFR and other advanced features, the radios will dissipate significant amounts of heat that must be managed. These products typically have to operate over -40C to +50C temperature range, plus solar loading, while being naturally convection cooled (no forced air). To solve this thermal issue the radio is generally mounted directly onto the heatsink synergistically reducing size and cost.
The third issue is that unlike the indoor Enterprise or Femtocells, the outdoor Metrocells support Tx powers of 1W per antenna and above, meaning one needs to incorporate low insertion loss Tx and Rx filters in order to keep the overall efficiency high and size low. This implies use of cavity filters with high Q metallic or ceramic resonators. Cavity filters are large and occupy significant volume. To reduce their impact on the overall system size, with certain technologies and experience, they can be built into the RF chassis and used as the heatsink. Below is a typical view of the 2x7W radio mounted over the duplexer that is built in the RF Chassis that supports 3G-4G technology over wide BW and operating conditions.
Six important Metrocell system design attributes
With the introduction and evolution of 3G and 4G SoCs the system baseband designs became much simpler and easier to implement at the hardware level. Conversely, the system complexity increased and as such needs to be managed to achieve the right cost and size of the product. Let's assume the Metrocell design is based on a SoC that supports the PHY air interface and complete protocol stacks for LTE and HSPA.
First of all, the Metrocell needs to support very accurate network level synchronization and depending on the deployment conditions this might be SyncE, IEEE 1588, or GPS with Hold Over. In real systems all three are required to support the varied real world deployment conditions. The Metrocell design must select the most suitable and accurate option at any given time.
Second, the interface to the radio is highly dependent on the radio design itself and the corresponding available SoC interface. Although it might be possible to interface a SoC directly with a radio, as is commonly done for low power Femtocells, for Metrocells the interface between the SoC and the radio needs to have higher dynamic range and support other advanced features to support high efficiency radio technologies.
Next, the availability and flexible support of numerous backhaul options is crucial for Metrocell adoption and low total cost of ownership. The Metrocell architecture and design must support copper, optical and in some cases even wireless NLOS or LOS backhaul options. In addition, some Metrocells also need to support WiFi. Although at first it might appear there is little complexity to integrate a WiFi sub-system in a Metrocell design, managing the WiFi sub-system, supporting its data, interfacing with the network element manager, as well as adding one or two extra radio frequency bands with 2+ antennas for each band while minimizing the size and cost of the Metrocell is not a trivial task.
Additionally, the Metrocell typically needs to support wide range of power options, (e.g. 90-260V AC or -48V DC), protect itself from Brown-outs (short power outages), and all interfaces must be protected against lightning strikes. Below is a typical view of the Metrocell digital subsystem that supports 3G-4G technology plus WiFi, SyncE, IEEE 1588, and GPS with Hold Over and both copper and optical backhaul options.
To be applicable worldwide and operate in a wide range of environmental conditions, the Metrocell must be fully functional from -40C to 50C, cope with an extra 10C-15C of solar loading, and must be IP-67 weatherproofed from dust and moisture (withstanding up to 1m submersion). This implies that every device that consumes 1W or more needs to be integrated with the heatsink of the chassis.
Last but not least, the design complexity does not stop here as the system must be easy to deploy, configure, operate and manage. The Build in Self-Test (BIST), Auto Configuration & Provisioning, Remote Test and Flight Recorder assist the operators with the deployment and operation.
For more information on this design approach, visit the Tektelic website