Open WiMAX Base Station: Exploring the union of MicroTCA and OBSAI

by Rana Pratap Sircar and Syed Najeeb Rizv – Wipro Technologies

Today, WiMAX  Base Stations with proprietary Hardware, Home-Grown OS & Middleware solutions are falling short on the front of inter-operability and modularity. This gave birth of an idea to Open up the Base stations with the adoption of Open Standards and increasingly accepted hardware form factors like MicroTCA.

 

“How about assembling a Base Station quick & fast with RF, Baseband & Transport AMCs available across various chipset Vendors with well defined standard interfaces?”

 

This article gives an insight into an Open Base Station Architecture with a case study based on PICMG Micro Telecommunications Computing Architecture (MicroTCA) & Open Base Station Architecture Initiative (OBSAI) specifications.

 

 

I. Typical MicroTCA considerations

 

The MicroTCA specification outlines the System Requirements for equipments with small form factors. The driving force for the standard was the idea to plug the existing Advanced Mezzanine Cards( AMCs) directly into the backplane, without any modifiation.

 

Where Advanced Telecommunication Computing Architecture (ATCA) is designed for high capacity and high performance applications, MicroTCA can be used for cost effective and physically smaller applications with relatively low performance and capacity targets.

 

A typical single carrier MicroTCA system shall have a collection of interconnected elements consisting of 1-12 AMCs, 1-2 MiocroTCA Carrier Hubs (MCHs), 1 backplane, 1-4 Power Modules(PMs), 1-2 Cooling Units (CUs) and a shelf.

[Source: MicroTCA]

 

MicroTCA supports AMCs in different sizes(by virtue of half height, full height, single width  and double width combinations), hence providing enough flexibility for realizing modular Base stations of various form factors (pico shelf, cube shelf, back-to-back shelf, single tier shelf etc.).

 

MicroTCA Carrier

 

Leveraging the idea of Carrier board from ATCA (defined in AMC.0), MicroTCA is based on a concept of MicroTCA carrier which is a virtual combination of carrier functions required to host 1-12 AMCs.

 

The carrier function requirements include system management based on IPMI, power delivery, switching fabrics, clocks&synchronization, backplane interconnects, and others as specified in AMC.0 specification (Up to sixteen MicroTCA carriers can be grouped together to form a single MicroTCA shelf).

 

Advanced Mezzanine Cards(AMCs)

 

The primary responsibility of a MicroTCA Carrier is to support various types of AMCs that perform diverse functions like CPUs for control & feature processing, Network Processing Units for packet processing and I/O, DSP AMCs for signal processing, storage AMCs and I/O for subscriber lines and Ethernet. Depending on the complexity of the system there can be a diverse mix of AMCs or it may host only a single type of AMC.

 

MicroTCA Carrier Hub(MCH)

 

The core component of any MicroTCA system is the MCH which provides shelf management, switching fabrics and clock distribution needed to support 1-12 AMCs. An MCH provides the management, clock and interconnect fabric signals to each AMC position over the backplane.

 

Interconnect fabric: An interconnect fabric provides the connectivity between the various AMCs in a MicroTCA shelf. It consists of a central switch and several high speed lanes (SerDes interconnects with 1-10 Gbps capacity) to each of the AMC positions.

 

As per PICMG, currently MCH can support any of the following protocols for interconnenct fabric:

 

• PCI Express and advanced switching (AMC.1)
• Gigabit Ethernet(AMC.2)
• Fiber Channel for storage(AMC.3)
• Serial Rapid IO (AMC.4)

 

However, other proprietary switch fabrics are possible.

 

MicroTCA interconnect fabric supports star, dual star and full mesh topologies.The fabric implemented in an MCH is the hub of a star network. If it is intended to implement dual star network for redundancy support, two MCH modules are required.

 

Typically, seven fabrics are required to be implemented in the MCH to provide connectivity to maximum of 12 AMCs.

 

Clock and Synchronization: For a typical MicroTCA based Base station, the key requirement is support for 1 PPS clock typically sourced from a GPS antenna. The MCH generates a 30.72 MHz closk that needs to be synchronized with the 1PPS external clock source and distributes the clocks to WiMAX AMCs. The MCH can use the GbE / SRIO fabrics for clock synchronization.

 

JTAG testing: JTAG interface is optional but recommended for testing of MicroTCA shelves and the respective components. JTAG Switch Module (JSM) can find a special slot in the backplane or integrated into other modules.

 

MCH shall provide the Test Access Port( TAP) or there shall be an external tester interface to enable the System level testing support provided by JSM. This interface provides a start JTAG architecture to enabl eteh MCH or an external teser to test and debug individual modules.

 

Backplane and Connectors

 

A typical MicroTCA system has a backplane into which the AMCs and other components are directly plugged. MicroTCA specifies several types of high performance connectors, which enable high bit rates across AMC positions and the backplane. A special connector type called MicroTCA Carrier Hub connector is used to connect an MCH with the backplane.

 

An AMC connector provides 21 ports for fabric interconnect  connectivity, whereas an MCH connector provides upto 84 ports of fabric interconnect connectivity to maximum of 12 AMCs.

MicroTCA supports two interconnect models or a mix of both, namely centralized MCH switch model (point-to-point connectivity from one module to every other module) and/or Module-to-Module direct connectivity module.

 

Redundancy support

 

Several fault tolerance and redundancy options are available in MicroTCA as dependency of the redundancy required the components of a MicroTCA shelf can be duplicated including AMCs, MCHs, PMs and CUs. Availability of “five nines” is supported in fully redundant MicroTCA shelves.

 

Hot Swap support is another important function of the MicroTCA carrier. Individual AMCs can be removed or installed in the shelf without disrupting the operation of the System. Given that all AMCs share the same power infrastructure, it's the responsibility of the Power modules and the Carrier manager functions in the MCH to ensure that the System behaviour is not disrupted as the AMCs are inserted or withdrawn.

 

2. Typical OBSAI considerations

 

The OBSAI Architecture specifies four basic functional blocks and three internal interfaces ( RP1, RP2 and RP3). Each block represents a logical segregation of BTS functionalities and can be realized by one or more than one modules as shown in the figure below.

 

[Source: OBSAI]

 

Each module represents a physical implementation of a subset of the block functions.

 

Functional blocks:

 

RF Block (RFB): This block is responsible for transmitting/receiving the RF signals and processing them to/from the BaseBand block. The primary responsibility of this block includes Antenna interface, D/A & A/D conversion, Up/Down conversion, Power Amplification, RF combining and filtering, Tx/Rx diversity, Carrier Selection, Clock & Synchronization, OAM&P functions, Low noise amplification, Peak power reduction and calibration of parallel Tx/Rx channels in case of smart antenna technologies.

 

 

BaseBand Block (BB):This block processes the frames received on the air interface with respect to the respecticve carrier technology. In case of WiMAX the BaseBand Block performs the PHY and MAC layer processing in accordance with IEEE 802.16d/e specifications.

 

Transport Block (TB):This block takes care of External Network Interface functions, internal networking functions to BaseBand block & CCB, QoS functions with respect to transport layer, clock & synchronization, OAM&P functions and security aspects with respect to the transport layer.

 

Control and Clock Block (CCB):This block is primarily responsible for congestion control in other blocks, admission control for new radio access links/users, BTS level OAM&P functions, FCAPS management, BTS level RF scheduling and Radio Resource management, Network Interface signaling termination and System Clock generation & distribution.

 

Interfaces

 

OBSAI defines external & internal interfaces for each one these blocks. Let's consider the external interfaces first.

 

External Interfaces:

 

Network Interface = R6 to ASN GW & R3 to CSN as defined by WiMAX Forum NWG specs

Radio Interface = R1 to SS as defined by IEEE 802.16d/e specs

 

Internal Interfaces:

 

Reference Point Function Reference Point 1 (RP1): Interchanges control, performance, status, alarm and provisioning data between the Control and Clock block and other BTS blocks.

 

Reference Point 2 (RP2): Interchanges user data packets between the Transport Block and the Baseband Block.

 

Reference Point 3 (RP3): Interchanges formatted air interface user and signaling data user between the Baseband Block and the RF Block.

 

 

3. Open WiMAX Base Station Architecture

 

Having analysed the key considerations of MicroTCA & OBSAI, let's analyse how a Base Station based on MicroTCA form factor and based on OBSAI specification would look like.

Here are some of the key AMCs that would be present in the system connected over the Backplane.

 

RF AMC

 

The following figure explains the key components required for RF including Band Pass Filters, Power Amplifiers, Up & Down Digital converters. But the key point is the interfaces exposed to the Baseband and Control & Clock module. As you can see the RF card provides the interfaces based on RP3 & RP1 Reference points specified by OBASI. It exposes an External radio interface based on R1 interface specified by IEEE 802.16d/e specification.

 

 

BaseBand AMC

 

The following figure explains a typical Baseband card with WiMAX MAC & PHY functioanlity implemented either on a SoC or a NP (depending on the reference design) based on IEEE 802.16d/e specification. Again as you can see the Baseband card provides three interfaces based on RP1, RP2 & RP3 Reference points as specified by OBSAI.

 

 

Network Interface AMC

 

The following figure exoplains the Network Interface Card which terminates the Network traffic from R6 or R3 External interface as specified in WiMAX forum specs. As far as internal interfaces are concerned the card exposes interfaces to Baseband and Control & Clock cards based on RP1 & RP2 Reference points.

 

Control & Clock Card

 

As depicted in the following figure the Control & Clock Card interfaces with other Base Station modules over RP1 reference points.

 

 

Backplane topology

 

The following figure explains one of the possible backplane configurations based on the guidelines that Gigabit Ethernet (GbE) is well understood and carries low risk but has lot of overhead. PCI express has problems connecting more than few devices, whereas Serial Rapid IO (SRIO) is the most efficient but expensive options ( generally used across DSP farms).

 

 

Just to highlight, OBSAI specifies that for CCM and TM, 1+1 redundancy is recommended whereas for BB cards & RF cards N+1 can be used. Also, As specified in the OBSAI RP2 Specification, the RP1 and RP2 Ethernet network shall be implemented as a dual star with a TM at the center of each.

 

As is is evident from the figure I/O & CCB cards are connected over PCI Express lanes whereas SRIO provides connectivity between Baseband and RF cards. GbE is used as a general purpose switch fabric.

 

Conclusion

 

The Architecture discussed above proposes an Open Base Station Architecture and suggests possible permutations for backplane configurations and switch fabrics. The key idea is to achieve interoperability through standard interfaces exposed by the AMCs, recommended switch fabric configurations and standardized backplane topologies.

 

http://www.wipro.com

 

 

Author's Biographies

Rana Pratap Sircar: Senior Consultant for Broadband Wireless at Wipro Technologies. Rana has more than 13 years experience in Telecom Space. He has done his Masters from Indian Institute of Technology (IIT) Delhi in Optical communication and Physics. He has spoken in multiple national and international forums. Rana's areas of interests include Seamless Mobility, broadband wireless technologies etc.

 

Syed Najeeb Rizvi: Technical Consultant for wiMAX at Wipro Technologies. Syed has six years of  experience in Telecom & Embedded Space. He has done his Masters from Indian Institute of Technology (IIT) Delhi in Process Engg. & Design. His areas of interests include Wireless & Embedded technologies.