CPRI and OBSAI

The rapid spread of smartphones and tablets together with many new Cloud services in the last decade have led to explosive growth in mobile data traffic. Operators are supporting mobile data traffic growth by increasing the bandwidth of mobile communications networks. This has been an important driver for a complete change in mobile communications systems with the adoption of the Centralized-Radio Access Networks (C-RAN), sometimes called Cloud-Radio Access Networks. Another important driver for operators has been reducing network running costs.

Using C-RAN, the mobile fronthaul is configured with centralized Base Band Units (BBU) controlling multiple, distributed Remote Radio Head (RRH) units at antenna sites. BBUs and RRHs are connected via general-purpose interfaces, most commonly the Common Public Radio Interface (CPRI), or in some cases the Open Base Station Architecture Initiative (OBSAI).

Many cellular networks have begun to deploy a distributed architecture where the traditional radio has been split into a baseband unit (BBU) and remote radio head (RRH). This provides the ability to replace the coaxial RF cable from the radio unit at the base of a tower to the antenna array at the top of a tower with a fiber optic cable. RF cables have limitations in that they suffer power loss and are subject to performance degradation over time due to damage or corrosion.

Standalone Base Station Configuration

Conventional standalone Base Stations suffer from high running costs and restricted installation locations due to the need for power supply and antenna infrastructure. The problems of conventional Base Station installations are solved by separating the Remote Radio Head (RRH) and the Baseband Unit (BBU).

The CPRI communications rate can be expanded to 1.2288 ~ 24.3302 Gbit/s depending on the antenna configuration and LTE bandwidth.

Antenna Configuration	CPRI Communication Rate for LTE Bandwidth
Antenna Configuration	5 MHz	10 MHz	20 MHz	20 MHzx2 (3.5G-band TD-LTE)
2x2 MIMO	614.4 Mbit/s (IP Rate 37.5 Mbit/s)	1.2288 Gbit/s (IP Rate 75 Mbit/s	2.4576 Gbit/s IP Rate 150 Mbit/s)	4.9152 Gbit/s IP Rate 300 Mbit/s)
4x2 (4x4) MIMO	1.2288 Gbit/s IP Rate 75 Mbits)	2.4576 Gbit/s (IP Rate 150 Mbit/s)	4.9152 Gbit/s (IP Rate 300 Mbit/s)	9.8303 Gbit/s (IP Rate 600 Mbit/s)

CPRI and OBSAI Bit Rates

CPRI bit rates are referred to as "option#". There are now 10 options, according to CPRI Specification V7.0:

Option	Bite Rate (mbps)	Line Code
1	614.4	8B/10B
2	1,228.8	8B10B
3	2,457.6	8B10B
4	3,072.0	8B10B
5	4.915.2	8B10B
6	6,144.0	8B10B
7	9,830.4	8B10B
8	10,137.6	64B/66B
9	12.1651	64B/66B
10	24.3302	64B/66B

4 OBSAI bit rates are defined:

Bite Rate (mbps)	Line Code
768	8B10B
1536	8B10B
3072	8B10B
6144	8B10B

CPRI IQ Bit Mapping

A CPRI link transports digitized RF signals (antenna-carriers) in a complex baseband format. Each sample has an in-phase (I) and quadrature (Q) component. The CPRI specification was specifically created to handle the signal data in this format. The I and Q samples are interleaved together to create a single word. Those words are put together in a pattern to satisfy the target sampling rate and bit width of the signal.

CPRI allows different bit widths to be used depending on the requirements of the air interface and whether the signal is in the uplink or downlink. In the downlink, the number of bits can range from 8 to 20. To allow for higher sampling rates in the uplink, it can use between 4 and 20 bits. Also, smaller width IQ samples can be sign- extended or padded with reserve bits to fill the space of larger bit widths. This is useful if the IQ sample processing in the downlink and uplink have different widths, but the user prefers to have the same size for both when mapping the samples onto the CPRI link.

When dividing spectrum, RF carriers are given a specific frequency band in which to operate. Similarly, on the CPRI link, a carrier is given a specific block in each basic frame, called an antenna container or AxC, to populate with its IQ sample data. The size of the antenna container (NAxC) is based on the bit width and sample rate of the IQ data. The size and structure of the AxCs is defined by the mapping method.

CPRI over Optical Transport Network (OTN)

Several vendors are working on CPRI over Optical Transport Network (OTN) Solutions supporting transport of the raw radio (CPRI) data from the RE over optical fiber to a centralized location for baseband processing.

A single location can serve multiple REs.
This level of consolidation has huge power and cost savings over the distributed approach without impacting network scalability.

OTN supports transport of several protocols over the same fiber, offering OTN operators fault management, performance monitoring, and protection mechanisms coupled with low cost-of-entry and the ability to support current, future, and legacy infrastructure technologies.
OTN operators also enjoy the advantage of using the same network-wide management system.

Baseband Unit (BBU)

As the need for more throughput at a lower cost increases, wireless network providers have moved to using a remote radio head (RRH) where the radio equipment is connected to the baseband unit (BBU) by a fiber optic cable. This provides a new level of flexibility in how the cell site is deployed, including siting the RRH at the masthead (for low RF losses) or locating the BBU at a remote location (for improved operational efficiencies).

Development Inspection

In the field, the REC to RE connection uses optical fiber with various connection methods, such as mechanical splices, connectors, etc., where the optical signal suffers reflections.

At 1.2288 Gbit/s, these reflections can cause problems in problem-free circuits as the data rate increases. The effect of these problems becomes bigger as the data rate increases and the width of 1 bit becomes smaller. The need for reflection attenuation, BER, and delay measurements, as well as Link evaluations is critical at deployment of optical fiber networks.

Installation Measurements	Bit Rate (Gbit/s)
Installation Measurements	0.6144	1.2288	2.4576	4.9152	9.8304
Optical Signal Loss	✔✔	✔✔	✔✔	✔✔	✔✔
Optical Return Loss	✔✔	✔✔	✔✔	✔✔	✔✔
BER	✔	✔	✔	✔✔	✔✔
Delay	✔	✔	✔	✔✔	✔✔
Link Evaluation	✔	✔	✔	✔✔	✔✔

Key: ✔✔ Essential | ✔ Required

Field Measurement Solution

Mobile networks are being converted to optical fiber to cover the increase in traffic caused by the spread of LTE services. Anritsu has a full range of effective onsite field measurement solutions, starting with mobile network installation and maintenance (I&M) and extending to long-distance and FTTH fiber I&M. These solutions meet all needs in every field of work from the exchange to the user, including RRH, mobile backhaul, FTTH, etc.

CPRI/OBSAI Options Available

CPRI and OBSAI options have been added to the Network Master Flex MT1100A and Network Master Pro MT1000A to support the following functions. They are used at installation, development, and manufacturing inspection of CPRI/OBSAI transmission equipment.

CPRI Option 1 (614.4 Mbit/s) to Option 8 (10.1376 Gbit/s)
OBSAI bit rates of 768 Mbit/s to 6.144 Gbit/s
Frame BER and Unframed BER (Pure PRBS pattern) measurements
Delay measurements
REC, RE, and Link inspection evaluations
Up/Down Alarm monitoring using pass-through connection
CPRI evaluation at CPRI over OTN

Anritsu’s Base Station Analyzer has an option to enable CPRI RF measurements to be made at ground level. Specifically, the uplink LTE spectrum can be viewed in real-time on a live network to monitor for interferers. This provides a powerful test capability without the need to call a tower climbing crew. Many common causes of poor KPI indicators can be diagnosed, especially those resulting from accidental or illegal transmitters interfering with the uplink.

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