Common Public Radio Interface (CPRI) has been around for quite some time. But now, enhanced CPRI (eCPRI) is becoming an important technology to understand for 5G.
Before looking in detail at eCPRI, it’s helpful to understand some of the basic topology of the cellular network, which currently uses CPRI.
CPRI, Remote Radio Heads (RRH) in 4G and 5G
At the outer circle of a cellular network topology, remote radio units (RRUs) are distributed every few miles in cities and suburban areas. These RRUs comprise antennas and also some compute functionality.
Fiber runs from a cluster of these RRUs to connect to a more centralized baseband unit. The baseband unit is also sometimes called the “central processing unit.” Baseband units are typically distributed within approximately 10-mile circles for good coverage in populated areas. The connection between the RRUs and the base station is often referred to as “fronthaul.”
CPRI is an interface that sends data from the RRUs to the baseband unit: CPRI is a serial interface, which is a very high-speed connection, a way to translate all those radio signals back to the computing function.
As we go to 5G, the fiber between the RRUs and the baseband unit is going to carry much more traffic, and that makes it more difficult to do a serial interface. Extreme 5G requirements are stretching the limits of fiber bandwidth.
Enter eCPRI, which is a way of splitting up the baseband functions and putting some of that functionality in the RRU to reduce the burden on the fiber.
AT&T is among many carriers that are working on eCPRI. AT&T has made “the world’s first” eCPRI connection for mmWave at its 5G Labs in Redmond, Washington. AT&T made calls testing eCPRI, using systems from both Nokia and Samsung Electronics America.
This opens the door for higher network throughput with less fiber, which will create more efficient mmWave deployments, among other benefits, This is also a significant step in creating an open architecture within the radio access network (RAN).
Another problem with the CPRI interface today is that it has become a proprietary technology.
In addition to supporting more bandwidth across fewer fibers, the enhanced CPRI also addresses the proprietary concerns. eCPRI will be an open interface, making it easier for carriers to mix and match vendor equipment for their RRUs and their baseband units.
Historically, because each vendor would have its own implementation of CPRI, it became proprietary. This forces carriers to buy both their RRUs and their baseband unit from the same vendor in order for the interface to work. With O-RAN this interface will be open. Carriers such as AT&T who implement eCPRI will be able to run equipment from different vendors, or even generic off-the-shelf equipment.
However, existing networks with CPRI installed will most likely remain in place for years to come.
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This article explains details of the 5G gNodeB (gNB) architecture and choices for Central Unit (CU) and Distributed Unit (DU) Split (CU DU Split)
How is the gNodeB Comprised?
The logical architecture of gNB is shown in figure below with Central Unit (CU) and Distributed Unit (DU). Fs-C and Fs-U provide control plane and user plane connectivity over Fs interface.
In this architecture, Central Unit (CU) and Distribution Unit (DU) can be defined as follows:
Central Unit (CU): It is a logical node that includes the gNB functions like Transfer of user data, Mobility control, Radio access network sharing, Positioning, Session Management etc., except those functions allocated exclusively to the DU. CU controls the operation of DUs over front-haul (Fs) interface. A central unit (CU) may also be known as BBU/REC/RCC/C-RAN/V-RAN
Distributed Unit (DU): This logical node includes a subset of the gNB functions, depending on the functional split option. Its operation is controlled by the CU. Distributed Unit (DU) also known with other names like RRH/RRU/RE/RU.
The CU DU Split options:
Central Unit (CU) and Distributed Unit (DU) Functional Split Options
As a part of study item for New Radio (NR), 3GPP started studying different functional splits between central and distributed units. For the initial phase, 3GPP has taken LTE protocol stack as a basis for the discussion, until RAN2 defines and freezes the protocol stack for New Radio (NR). They have proposed about 8 possible options shown in below figure.
Option 1 (RRC/PCDP 1A-like split)
Option 2 (PDCP/RLC Split 3C-like split)
Option 3 (High RLC/Low RLC split, Intra RLC split)
Option 4 (RLC-MAC split)
Option 5 (Intra MAC split)
Option 6 (MAC-PHY split)
Option 7 (Intra PHY split)
Option 8 (PHY-RF split)
Option 1 (RRC/PDCP, 1A-like split): In this split option, RRC is in the central unit while PDCP, RLC, MAC, physical layer and RF are kept in the distributed unit. Thus the entire user plane is in the distributed unit.
Option 2 (PDCP/RLC split): Option 2 may be a base for an X2-like design due to similarity on U-plane but some functionality may be different e.g. C-plane since some new procedures may be needed. There are two possible variants available in this option.
Option 2-1 Split U-plane only (3C like split): In this split option, RRC, PDCP are in the central unit. RLC, MAC, physical layer and RF are in the distributed unit.
Option 2-2: In this split option, RRC, PDCP are in the central unit. RLC, MAC, physical layer and RF are in the distributed unit. In addition, this option can be achieved by separating the RRC and PDCP for the CP stack and the PDCP for the UP stack into different central entities.
Option 3 (High RLC/Low RLC Split): In this option, two approaches are taken based on Real time/Non-Real time functions split which are as follows:
Option 3-1 Split based on ARQ
Option 3-2 Split based on TX RLC and RX RLC
Option 3-1 Split based on ARQ
Low RLC may be composed of segmentation functions;
High RLC may be composed of ARQ and other RLC functions;
This option splits the RLC sublayer into High RLC and Low RLC sublayers such that for RLC Acknowledge Mode operation, all RLC functions may be performed at the High RLC sublayer residing in the central unit, while the segmentation may be performed at the Low RLC sublayer residing in the distributed unit. Here, High RLC segments RLC PDU based on the status reports while Low RLC segments RLC PDU into the available MAC PDU resources.
Option 3-2 Split based on TX RLC and RX RLC
Low RLC may be composed of transmitting TM RLC entity, transmitting UM RLC entity, a transmitting side of AM and the routing function of a receiving side of AM, which are related to downlink transmission.
High RLC may be composed of receiving TM RLC entity, receiving UM RLC entity and a receiving side of AM except for the routing function and reception of RLC status reports, which are related to uplink transmission.
Option 4 (RLC-MAC split): In this split option, RRC, PDCP, and RLC are in the central unit. MAC, physical layer, and RF are in the distributed unit.
Option 5 (Intra MAC split)
Option 5 assumes the following distribution:
RF, physical layer and lower part of the MAC layer (Low-MAC) are in the Distributed Unit
Higher part of the MAC layer (High-MAC), RLC and PDCP are in the Central Unit
Therefore, by splitting the MAC layer into 2 entities (e.g. High-MAC and Low-MAC), the services and functions provided by the MAC layer will be located in the Central Unit (CU), in the Distributed Unit (DU), or in both. An example of this kind distribution given below.
In High-MAC sublayer the centralized scheduling in the High-MAC sublayer will be in charge of the control of multiple Low-MAC sublayers. It takes high-level centralized scheduling decision. The inter-cell interference coordination in the High-MAC sublayer will be in charge of interference coordination methods such as JP/CS CoMP.
In Low-MAC sublayer the time-critical functions in the Low-MAC sublayer include the functions with stringent delay requirements (e.g. HARQ) or the functions where performance is proportional to latency (e.g. radio channel and signal measurements from PHY, random access control). It reduces the delay requirements on the fronthaul interface. Radio specific functions in the Low-MAC sublayer can for perform scheduling-related information processing and be reporting. It can also measure/estimate the activities on the configured operations or the served UE’s statistics and report periodically or as requested to the High-MAC sublayer.
Option 6 (MAC-PHY split): The MAC and upper layers are in the central unit (CU). PHY layer and RF are in the DU. The interface between the CU and DUs carries data, configuration, and scheduling-related information (e.g. MCS, Layer Mapping, Beamforming, Antenna Configuration, resource block allocation, etc.) and measurements.
Option 7 (Intra PHY split): Multiple realizations of this option are possible, including asymmetrical options which allow obtaining benefits of different sub-options for UL and DL independently.
This option requires some kind of compression technique to reduce transport bandwidth requirements between the DU and CU.
In the UL, FFT, and CP removal reside in the DU and for the two sub-variants, 7-1 and 7-2 are described below. Remaining functions reside in the CU.
In the downlink, iFFT and CP addition reside in the DU and the rest of the PHY resides in the CU.
Considering above there are three sub-variant available for this option described as below
Option 7-1 In this option the UL, FFT, CP removal and possibly PRACH filtering functions reside in the DU, the rest of PHY functions reside in the CU. In the DL, iFFT and CP addition functions reside in the DU, the rest of PHY functions reside in the CU.
Option 7-2 In this option the UL, FFT, CP removal, resource de-mapping and possibly pre-filtering functions reside in the DU, the rest of PHY functions reside in the CU. In the DL, iFFT, CP addition, resource mapping and precoding functions reside in the DU, the rest of PHY functions reside in the CU.
Option 7-3 (Only for DL): Only the encoder resides in the CU, and the rest of PHY functions reside in the DU.
Option 8 (PHY-RF split): This option allows to separate the RF and the PHY layer. This split permit centralization of processes at all protocol layer levels, resulting in very tight coordination of the RAN. This allows efficient support of functions such as CoMP, MIMO, load balancing, mobility.
Benefits of RAN Spilt Architecture
Some of the benefits of an architecture with the deployment flexibility to split and move New Radio (NR) functions between central and distributed units are below:
A split architecture (between central and distributed units) allows for coordination for performance features, load management, real-time performance optimization, and enables NFV/SDN
Configurable functional splits enables adaptation to various use cases, such as variable latency on transport
Which CU DU split function to use where?
The choice of how to split New Radio (NR) functions in the architecture depends on some factors related to radio network deployment scenarios, constraints and intended supported services. Some examples of such factors are:
Support of specific QoS per offered services (e.g. low latency, high throughput)
Support of specific user density and load demand per given geographical area (which may influence the level of RAN coordination)
Availability transport networks with different performance levels, from ideal to non-ideal
Application type e.g. Real-time or Non- Real Time
Features requirement at Radio Network level e.g. CA, eICIC, CoMP etc.
Reference: 3GPP TR 38.801 Radio Access Architecture and Interfaces Release 14
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RadioMobile: Popular software for 5G Network Coverage planning
RadioMobile is a widely-available software package which can be used for 5G Network Coverage planning, including path profiling and clearance criteria, power budgets, choosing antenna sizes and tower heights.
For website for RadioMobile, please see this link here:
For 5G Coverage Planning, the software package can be configured with the characteristics of your required radio network.
CPE types and locations
Link Budget & Fade Margins
The software enables quick and rapid calculation of link budget and fade margins for any frequency band.
The software uses the freely available SRTM terrain data which can download “on demand” for calculation of terrain heights. Combined with LandCover, this enables estimation of trees/forests also.
Line of Sight
The software uses the terrain database to allows quick establishment of available Line of Sight and “what if” adjustment of antenna/tower heights in a 5G radio network design
Radio Fresnel Zone
RadioMobile automatically calculates the Fresnel Zone for any required link, with graphical display enabling quick feasibility and identification of any obstacles to be noted.
Radio Parameters & Network Properties
Any new user to Radio Mobile will have to enter link parameters for the chosen equipment. This includes transmit power, receive sensitivity and antenna gains. Some vendors such as CableFree include this data as a planning service with their products
Radio Mobile: Free to Use
The Radio Mobile software is free to use including for commercial use. Radio Mobile software is a copyright of Roger Coudé. The author notes:
Although commercial use is not prohibited, the author cannot be held responsible for its usage. The outputs resulting from the program are under the entire responsibility of the user, and the user should conform to restrictions from external data sources.
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In order to understand the importance of low-PIM antennas, cable and connectors in a modern 5G network, it is critical to understand what PIM refers to. PIM stands for Passive Intermodulation. It is a problem that happens largely in passive devices such as antennas, cables and connectors where interfering signals are generated by nonlinearities within a wireless system’s mechanism. Amplitude modulation or the mixing of two signals occurs, producing difference in signals within a specific band leaving behind significant interloping.
PIM: Nonlinearity Effects
Passive intermodulation product is also explained as resulting from the mixing of two high power natures at the nonlinearities of devices, such as in corroded connectors, metal oxide intersections and divergent metals. As the signal amplitude goes up, the nonlinearities effect will be very significant. Higher intermodulation result as lots of products on first inspection would show a system that is not just linear but seemingly incapable of producing intermodulation.
PIM Distortion & Telecommunication
A single internet provider such as a broadband carrier can also produce PIM in case it goes through a fault or a surface that generates PIM. Such distortions in telecommunication signals would look like side lobes interfering with end-to-end channels and hinder coverage. In most communication systems today, PIM is a serious problem. Where there is a sharing of received highly powerful and transmitted signal, PIM interference is a serious vulnerability. After interference of PIM in the path of reception, filtering or unscrambling it is very difficult.
The Real Cause Of Passive Intermodulation
When mechanical components interact, particularly where two distinct metals connect, the result is nonlinear elements. A major cause is mostly unlike material’s junctions leading to Passive Intermodulation manifestation in coax cables, antennas and coax connectors among others. This is brought about by oxidation, loosely done connectors, corrosion, dirt, and rust among others, including any kind of adulteration on the passive devices. PIM is also caused by close metal objects from anchors, pipes, guy wires to roof flashings resulting in a nonlinearity that allows a mix. As the nonlinearity goes up, the PIM reception amplitude also surge.
Other PIM sources to avoid include a number of ferromagnetic materials such as a number of metals like steel, nickel and ferrites. Such elements display hysteresis once they come into contact with backing magnetic fields. Mechanical contacts done shoddily, cracked and cold solder joints, workmanship and manufacturing faults, among others, cause serious levels of PIM.
The Conditions That Bring About PIM
Normally, to bring about Passive Intermodulation, two fairly powerful RF signals rather close in terms of frequency are required. High power transmitters of about 20 watts and above will see the outputs creating PIM. As the power goes up, more severe the PIM effects produced.
Another important aspect to note about Passive Intermodulation is that as the components soar in age, PIM increases. An older system is very susceptible, including surroundings afflicted by high temperatures, lots of air pollution, extreme vibrations or salted air.
Other PIM Negative Effects
Apart from interfering with signals, Passive Intermodulation has other negative effects, especially on a receiver that usually has a high sensitivity. As interference of the signals takes place, the desired signals will be blocked as the noise floor goes up. Interference of the signals has also been deemed to lower the sensitivity of the receiver. In a typical cell signal setting anywhere, the negative end effects would be manifested in serious scenarios such as dropped calls, low system capacity among others.
Which Wireless Networks are Most Affected?
Most PIM problems are mostly susceptible to internet cell networks such as 4G & 5G LTE, HSPA and CDMA. For all of these, Passive Intermodulation is a significant challenge.
FDD-LTE has been cited as being highly sensitive to Passive Intermodulation effects. Where the PIM levels of interference are elevated, the performance and efficiency of LTE networks is greatly hampered. As this occurs, it is possible for the base station to mistake distorted signal as a channel in use thereby refusing to have the channel assigned. The result in the system is the loss of revenue, critical channel capacity and airtime.
Since modern equipment is largely sensitive in its nature, the lowest Passive Intermodulation effect levels are capable of brutally degrading the performance. For instance, a single one decibel loss in uplink sensitivity following PIM interference has been found to lower reception by over 10 percent.
PIM problems have been cited in lots of macro sites in external environments, an issue that had to be sorted out. In DAS (distributed antenna systems), the reliability and high data amount values are critical; lots of components appear in the path of the radio frequencies leading to PIM effects. These systems require passive components from directional to hybrid couplers, coax, and splitters among others to be placed near the source of the signal. As a result, passive components PIM specification has to be of the utmost level.
Modern Passive Components And PIM
Modern low-PIM passive components such as connector series are designed with installation ease in mind to provide the most desired mechanical and electrical reliability. They come in handy where low-Passive Intermodulation effect connection is required such as in installations in need of coax lengthy cable jumpers for the provision of low Passive Intermodulation levels and superb RF performance.
What Low-PIM Means to the Network Operator
To any consumer, integrator, or installer, high PIM levels mean the cellular reception will be very poor and the bandwidth will be limited. For a carrier, it means customers will not trust their cellular services and will probably move on to competing service provider. Low-PIM passive components mean that the signal will be strong offering extra and reliable bandwidth for customers and a perfect scenario for all involved. As a result, the modern low Passive Intermodulation antennas, cables and connectors among other passive components are designed for everyday use in a design perfectly harnessed to lower Passive Intermodulation. All these components arrive ready for use having been tested for seamless installation.
Today users expect their cell phones to have a consistent signal no matter the mobile device they are using. All network operators work tirelessly to ensure the signal is highly improved. Cell phone signal boosters offering perfect signal amplifiers for use in residential and commercial setups where the signal is poor and unreliable, including every single passive component maker is alive to the PIM effect. Therefore PIM testing today is a highly critical process that will probably continue being so.
A typical 4G LTE network sees a high capacity 100 Mbps to 1Gbps data rate and increasing each year. Such a high rate of transmission keeps on exposing the susceptibility of networks to PIM interference on a large scale. The fidelity of a 4G LTE is hedged on network superiority than 2G or 3G networks.
The carrier, consumer, low-Passive Intermodulation components manufacturer, cell phone signal boosters and antennas manufacturers as well have to always consider Passive Intermodulation interference in their assembly processes. Passive Intermodulation testing and assuring peak low-PIM performance is thus imperative.
Low-PIM connectors for instance are designed with contact connectors on the outside and center using a single construction piece as much as possible. N-type connectors among others are being over-molded in a way that eliminates thermal stresses, vibration and wind effects.
The design incorporates lots of aspects and forces of contact assuring sufficient pressure for the prevention of Passive Intermodulation, including gold or silver-plated mid contacts while the connector bodies are white bronze or silver plated. All passive components from connectors, cables to antennas are being manufactured in controlled environments by highly trained professionals who understand the desirability of low PIM.
Passive Intermodulation DAS Specifications
Low-PIM specification for DAS solutions is very important. To know how low you should aim for, you can stick to the vendors mostly aimed -110 dBc or -153 dBc for top of the range solutions. Note that the specification need to be constantly spread in all the bands from 4G LTE, 3G to 2G right from the 700 MHz frequency to as high as 2.7 GHz. This assures a consistent superb performance for every device and satisfaction for every customer.
Passive Intermodulation Effects During Installation
Even with the proper top quality low-PIM passive components and DAS solutions you can still suffer Passive Intermodulation effects. This can happen as you install or after installing the low-Passive Intermodulation passive device. If you have suspended ceiling grids or you rebar ceiling grids, you have metallic objects on the path of the cell signal, identify this with a view to minimizing Passive Intermodulation. At times, these metal objects are hard to avoid. Directional antennas can help mitigate their effects to a greater extent.
Seek to ensure connectors, cables and other low-Passive Intermodulation passive components installed are periodically inspected to avoid such effects as corrosion. As the component deteriorates without proper inspection Passive Intermodulation will be elevated.
The most important thing is to stick to low-PIM interior antennas, low-PIM RF Splitters, as well as cables, connectors, couplers and other passive components. Low-PIM solutions will eliminate the costly requirement of upgrading these components, ensuring your cell phone signal is never interrupted by high-PIM effects.
5G NR (New Radio) is a new radio access technology (RAT) developed by 3GPP for the 5G (fifth generation) mobile network. It was designed to be the global standard for the air interface of 5G networks.
The 3GPP specification 38 series provides the technical details behind NR, the RAT beyond LTE.
The study on NR within 3GPP started in 2015, and the first specification release was made available by the end of 2017. While the 3GPP standardization process was ongoing, industry had already begun efforts to implement infrastructure compliant with the draft standard, with the expectation that the first large scale commercial launch of 5G New Radio would occur in 2019.
The Frequency bands for 5G New Radio are being separated into two frequency ranges:
Frequency Range 1 (FR1), including sub-6 GHz frequency bands Frequency Range 2 (FR2), including frequency bands in the mmWave range (20-60GHz)
5G NR Development
In 2018, 3GPP published Release 15, which include what is described as “Phase 1” standardization for the 5G NR standard. 3GPP is expected to publish Release 16, which include the “Phase 2” of 5G NR, by the end of year 2019
5G NR Deployment modes
Initial launches will depend on existing LTE 4G infrastructure in non-standalone (NSA) mode, before maturation of the standalone (SA) mode with the 5G core network.
Non-Standalone (NSA) mode of 5G New Radio refers to an option of 5G NR deployment that depends on the control plane of existing LTE network for control functions, while 5G NR exclusively focused on user plane. The advantage of doing so is reported to speed up 5G adoption, however some operators and vendors have criticized prioritizing the introduction of 5G NR NSA on the grounds that it could hinder the implementation of the standalone mode of the network.
Standalone (SA) mode of 5G New Radio refers to using 5G cells for both signalling and information transfer. It includes the new 5G Packet Core architecture instead of relying on the 4G Evolved Packet Core. It would allow the deployment of 5G without the LTE network. It is expected to have lower cost, better efficiency, and assist development of new use cases.
Frequency bands for 5G are divided into “sub 6GHz” and “mmWave” bands. “Sub 6” is generally for longer range coverage, with in-building penetration. “mmWave” bands use higher frequencies which offer short range, and no or little penetration of building structures.
This list of 5G frequency bands is taken from the latest published version of the 3GPP TS 38.101,the following tables list the specified frequency bands and the channel bandwidths of the 5G NR standard.
Note that the NR bands are defined with prefix of “n”. When the NR band is overlapping with the 4G LTE band, they share the same band number.