Wireless OEMs face constant pressure to reduce costs. This pressure causes large OEMs to create standards initiatives to rapidly "commoditize" radio network components. In 2002, Hyundai, LGE, Nokia, Samsung, and ZTE began the Open Base Station Standard Initiative (OBSAI). OBSAI's objective: “...create an open market for cellular base stations (BTS).”
BTS Structure
A BTS has four main modules: radio frequency (RF), baseband, control, and transport. Figure 1 depicts a typical BTS. The radio frequency module (RFM) receives signals via portable devices and converts them to digital data. The baseband module processes the encoded signal and brings it back to baseband before transmitting it to the terrestrial network via the transport module. Coordination between these three functions is maintained by a control module.
OBSAI seeks to meet its objective for open market BTS components by defining standard interfaces for the four modules. In the OBSAI specification, interfaces between modules are known as reference points (RP). Figure 1 shows RP1 as the interface that allows communication between a control module and the other three modules. RP2 provides a link between the transport and baseband modules, while RP3 connects the baseband and RF modules.
Most of the industry focus today revolves around achieving lower cost RF modules and power amplifiers (PA), as these two components usually account nearly 50 percent of the BTS cost. Some OEMs and PA vendors are now working on integrating those two functions into a single lower-cost module resulting in initiatives such as Common Public Radio Interface (CPRI). CPRI works to devise an RF module standard interface to encourage alternative, competitive sources for RF modules and PAs. Similar to CPRI, OBSAI works to define reference point 3 (RP3) prior to the other reference points to promote more competitive sources in the RF module and PA market.
Figure 1. Base Station Overview
Notes:
- BB = baseband
RP3
The OBSAI RP3 specification defines the interface between the baseband module and the RF module. The specification allows for a maximum of nine pairs of unidirectional links for every baseband and RF module. For a typical BTS, those links can be connected with a mesh or a centralized combiner and distributor (C/D) topology. The C/D topology is more suitable for a large BTS, as you can manage it more easily than the mesh topology. A switch card acting as the C/D with a high performance FPGA and serializer/deserializer (SERDES) solution, such as Stratix® IV GX FPGAs, is a very desirable solution in this type of a case. Stratix IV GX devices are Altera's latest generation of FPGAs with embedded transceivers. Stratix IV GX FPGAs integrate up to 32 SERDES-based transceivers, designed for optimum jitter performance across the entire operating range of 600 Mbps to 8.5 Gbps. For very large configurations, an inter-cabinet connection can be required. You can use a mesh, bridge, or a C/D topology. OBSAI also includes a special specification (RP3-01) for remote RF heads. Figure 2 illustrates how you can implement the RP3 connection for different topologies.
Figure 2. BTS RP3 Topology
RP3 uses a four-layer protocol stack (see Figure 3), namely the physical, data link, transport, and application layers. The application layer provides mapping of packets to the payload. Supported packet types include W-CDMA, CDMA2000, and GSM/EDGE, and it can be expanded to accommodate future packet types. The transport layer provides end-to-end routing of the messages. The data link layer provides framing and synchronization of the messages. The physical layer is responsible for sending out messages on the electrical interfaces as well as serializing and coding the data.
Figure 3. RP3 Protocol Stack
Key characteristics of the RP3 physical layer requirements:
|
Physical Layer Specification |
XAUI-like |
|
Data Rate |
768 Mbps or 1,536 Mbps |
|
Bit Error Rate (BER) |
10-15 |
|
8B/10B |
Required |
The transceiver implementation in Stratix IV GX FPGAs provides clock data recovery (CDR), SERDES, 8B/10B encoder/decoder, pattern detector, and word aligner, all of which are used to provide the physical layer implementation of the OBSAI specification. These dedicated blocks both simplify system design and leave the FPGA available to support the upper layers of the specification and custom system architecture.
RP3-01
The latest work by OBSAI on RP3 is a RP3-01 extension. It specifies the RP3 interface protocol for remote RF head use. Figure 4 shows the reference architecture of a BTS with remote RF units.
Figure 4. RP3-01 Reference Architecture
Notes:
- DL = downlink
- UL = uplink
- LU = local unit
- RRU = remote radio unit
- CCM = clock and control module
Line rates that are integer multiples of 768 Mbps up to 3,840 Mbps are considered OBSAI-compatible line rates. Due to the number of line rates available, auto-negotiation between the remote RF units and the local units are defined. The specification also defines Ethernet transmission between two RP3-01 nodes.
Due to the lack of a physical RP1 link on a remote unit, the RP1 information has to be mapped into the RP3 link. Other items in the specification include delay measurement, synchronization between RP3-01 units, and data multiplexing across the RP3-01 link.
