Emergence of C-RAN (Separation of Baseband and Radio, and Baseband Centralization)

Today, global LTE operators are competing with one another more fiercely than ever, under the pressure to provide better service quality and higher speed. Conventional Internet traffic has been increasing steadily. What's more, mobile traffic resulting from OTT video services, such as YouTube, Mobile IPTV, etc., is growing fast. Real fast. With this fast growth, it has become inevitable for all the global LTE operators to competitively install more base stations such as Macro, Small cell, etc. within their service coverage.

What's fortunate for the operators is that at least their LTE base stations have architecture that allows for easier cell site installation and maintenance. Previously, they had some restrictions because their base stations were:

1. standalone base stations where both baseband unit (BBU) and remote radio head (RRH) are co-located in a cell site.
2. mostly installed indoor (e.g. inside medium-large buildings) for stable power supply and A/C.

Due to these restrictions, each cell site has to have not only a base station, but also its own power supply (UPS) and A/C facilities. This of course costs more money and time. Larger footprints are required for these additional equipments and facilities (for example, higher rent for leasing 175 sf instead of 350 sf). More costs (e.g. civil works, labor charges, etc.), and time for installing equipments and facilities (and network). And last but not least, higher monthly electricity bills.
 
As a result, the operators will have to face higher CAPEX/OPEX for building and operating a nationwide network. Besides, cell sizes are getting smaller as the networks evolve from 3G, 4G to 5G. This means more cell sites to build and operate, and hence more troubles and hassles, to the operators.

To address the foregoing issues, a new RAN architecture, C-RAN (Centralized/Cloud RAN), was introduced. With C-RAN, operators can remove the BBU from each cell site, and keep all of them cool in a single location (Centralized) for easier maintenance. Then, they can leave the RRHs unmoved for better radio signal reception. BBUs and RRHs, located in different places separated from each other, are connected using fiber cables (Dedicated Fiber per RRH or Dedicated λ per RRH).

RRHs, designed for outdoor use, are simple but very hardened devices that run well without A/C facilities, which means no indoor space to rent for them. So, operators can minimize their rental costs as they only need rooftop spaces for RRHs. They can also lower their electricity bills for they only need to supply power for RRHs.

Carrier Aggregation in LTE-A (LTE Advanced)

With the separation of BBU and RRH, interface specification for their communication was needed, and thus established accordingly: 1) CPRI (Common Public Radio Interface), 2) OBSAI (Open Baseband Remote Radiohead Interface), and 3) ORI (Open Radio Interface). Both CPRI and OBSAI are designed mainly for base station vendors, but most of those vendors and telecom operators have adopted CPRI. Let’s find out about CPRI, then.

CPRI (Interface Specification for Separated DUs and RUs)

With the establishment of CPRI Specification in 2003, leading base station vendors, like Ericsson, NSN, ALU, NEC, Huawei, etc., have standardized interface specifications for interoperation between base station devices (BBU and RRH). Although the five vendors are not currently claiming their copyright on the specification, they have the exclusive right to revise it. Its latest version is CPRI version 6.0 released in August, 2013, and it is expected to be further updated once CPRI compression features are defined.

 

REC (on BBU side) and RE (on RRH side) exchange 

1) User Data, 

2) CPRI Control & Management data, and 

3) CPRI frame synchronization information through the CPRI interface as seen below.

• User data is delivered as Baseband Digital IQ Stream, as conveyed in an IQ Data Block of a CPRI Basic Frame. RRH, upon receiving the data, converts it into analog, amplifies and then radiates over the air, to UEs eventually.
• Control & Management data and Synchronization information are delivered through CPRI Subchannels (specifically, through Control Words in the CPRI Basic Frames). This information is only used by REC (on BBU side) and RE (on RRH side), and has nothing to do with the LTE Layer.

 

CPRI subchannels are created per CPRI Hyper frame (66.67us), and a hyper frame consists of 256 basic frames (260.42ns). Each basic frame has 1 Byte of a Control Word and 15 Byte of Payload. 256 control words in one hyper frame collectively constitute 64 subchannels. The figure below shows where in a CPRI subframe the Control & Management data and Synchronization information are mapped and delivered. For instance, the Synchronization information is conveyed as included in the Control Word of the first basic frame of a hyper frame. This corresponds to the first Byte of Subchannel #0.

As such, the CPRI specification defines the structure of a CPRI frame to be used for delivering user data, and exchanging the control & management information between REC and RE. It also defines i) the location in the frame where user data is conveyed, and ii) the significance and locations of overhead bytes (as SONET/SDH does).

We have so far the background and concept of CPRI in this post. In the next post, we will find more about the detailed features of CPRI.

• OBSAI (Open Baseband Remote Radiohead Interface)

OBSAI, quite similar to CPRI, was established in late 2002 before CPRI. It has been adopted by many base station vendors like Samsung, NSN, etc. Aimed to build an open base station structure, OBSAI divides key features of a base station into modules, and interconnects the modules using an interface named RP (Reference Point). However, most global vendors have chosen CPRI over OBSAI. Besides, RP3 – sub-specification under OBSAI that corresponds to CPRI – has never been updated since its v4.2 document release in 2010.

• ORI (Open Radio Interface)

CPRI obviously has restricted compatibility. To address this issue and provide better compatibility among vendors, ORI was introduced by a standardization organization, ETSI. ORI standardization group, a member of ETSI ISG (Industry Specification Group), is currently participated by operators like SK Telecom, KDDI, etc. as well as vendors like Samsung, Huawei, ZTE, etc. As mentioned earlier, the ORI specification is based on the CPRI specification. But, it is designed to specifically clarify the previously-defined but still unclear options, and define undefined matters, while still keeping main focus on Control & Management data specification of its underlying CPRI specification. Through these efforts, it is aimed to provide more functionality (compression) and compatibility among vendors than the CPRI specification.