Multicarrier OpenBTS (public version)¶
- Instructions and Configuration
- Wideband Distortion
- Other Effects
Multicarrier BTS (MCBTS) is an approach to expand GSM capacity without using additional transceivers, antennas, or power amplifiers. Instead, a single wideband front end is used along with digital signal processing to extract and combine individual GSM channels. As a flexible, software-defined radio approach to cellular, OpenBTS and products such as the USRP and UmTRX are well suited for implementing MCBTS functionality.
Generally speaking, the same requirements that apply to the single carrier BTS case apply to the multicarrier case as well. There are relaxed specifications, however, for spurious emissions, intermodulation, and receive blocking. The multicarrer BTS class was introduced in Release 8 in 3GPP TR 45.050, which covers the unique RF requirements of the configuration. The requirements were later added to the core specification 3GPP TS 45.005. Conformance test specifications for MCBTS are found in 3GPP TS 51.021
3GPP TR 45.050, Annex ZB: Introduction of multicarrier BTS class
Multicarrier transceiver architectures applied to GSM BTSs would allow several GSM carriers to be processed by a single transmitter and power amplifier in the downlink and by a single wideband receiver in the uplink.
The public version of MCBTS support uses a polyphase channelizer, or transmultiplexer, to efficiently extract a combination of channels from a single wideband sample stream. A similar synthesis filter is used on the downlink to aggregate multiple channels into a single output. The approach takes advantage of the fixed 200 kHz channel spacing of GSM ARFCN carriers in order alias channels to baseband without using direct downconversion.
Filtering operations are SIMD optimized for Intel SSE3 through the use of intrinsic functions. ARM NEON support using hand coded assembly is implemented in a separate branch and will be merged at a later time.
Multicarrier ARM support is currently experimental. At minimum, a Cortex-A9 or faster processor is recommended.
Instructions and Configuration¶
To be added...
The wideband multicarrier GSM signal is no longer constant envelope and therefore suffers from similar effects as other modern wireless schemes such as WCDMA or OFDM. The high peak-to-average ratio of the signal places higher demands on power amplifiers and the efficiency in which they can operate. Digital predistortion (DPD) along with crest factor reduction (CFR) are methods to linearize the PA and reduce these detrimental effects. There are currently no known open source efforts to implement these features at this time.