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Advanced baseband technology in third-generationradio base stationsVZhongping Zhang, Franz Heiser, Jtirgen Lerzerand Helmut LeuschnerWCDMA, one of the technologies selected for the air interface of the3GPP standard, is widely used in emerging third-generation mobile communicationsystems. This interface supports data rates of up to 2 Mbit/son a common 5 MHz frequency carrier. Moreover, with the introduction ofHSDPA, the peak service rate for packet access in the downlink can beincreased to more than 10 Mbit/s.Ericsson's radio base station has been designed to comply with the3GPP standard. The kernel part of WCDMA technology has been implementedin the baseband of the radio base station. Compared to previousgenerations, the baseband signals in WCDMA are spread with a highchip-rate code at 3.84 megachips per second on a 5 MHz frequency band.This is much wider than the frequency band used in GSM, cdmaOne andCDMA2000, or PDC. Therefore, to process the signals, more advancedtechnology is deployed in WCDMA baseband. Ericsson's baseband technologyuses the very latest ASIC, DSP, and FPGA technologies.Numerous requirements are being channeled toward the basebandplatform, both to support a technical implementation of WCDMA and tosatisfy operator and radio network management points of view. Being thekernel in WCDMA, the baseband platform must be able to efficiently handlethe entire life cycle of an RBS, from initial deployment, with a lowcost,low-content focus, to subsequent scaling for newly developed servicesand traffic growth. Moreover, it must do so while networks areevolving and expanding with more users and new mixes of end-user services.New radio network functions and features will also be addedthrough base station hardware and software to perfect the WCDMA system.The authors describe the implementation of Ericsson's WCDMA baseband.They also show how it has been prepared to grow with and meetthe needs of future developments by facilitating small, incrementalupgrades and thanks to a flexible architecture that supports the expansionof the uplink and downlink together with critical functionality thatresides in loadable hardware.Figure 1Indoor RBS and baseband subrack.Architecture of the radiobase stationThe functionality of a radio base station(RBS) is divided into two main parts: userplanefunctions and control-plane functions.The user-plane functions are associated withtransport, baseband, radio and the antenna.The control-plane functions pertain to thetransmission of user data and operation andmaintenance (O&M) data. Ericsson's RBS isbased on the connectivity packet platform(CPP, formerly called Cello packet platform)—thatis, the RBS employs the infrastructureof hardware and software modulesprovided in CPP. 1Figure 1 shows a typical indoor RBS withpower subrack, baseband subrack, radio frequencysubrack and power amplifier subrack.2 User-plane signals from the radio networkcontroller (RNC) via the lub interfaceare input directly via CPP boards to thebaseband parts, whereas control-plane signalsare input to the baseband parts via thetraffic and O&M control parts of the mainprocessor. Figure 2 shows the architectureof the Ericsson RBS3000. 1 Please note thatfor simplicity's sake the CPP parts and mainprocessor are not shown.The archirecture can be broken down intoa cell-specific part and a non-cell-specificpart. The cell-specific part contains transceiver(TRX) boards, multicarrier poweramplifier (MCPA) boards and antenna interfaceunit (AIU) boards, whereas the commonpart contains boards for baseband processing.In Figure 2, rhe baseband processinghas been split between the transmitter(TX) and random access and receiver (RAX)boards. The TX board handles downlinkprocessing and enables coding, spreadingand modulation. The RAX board handlesuplink processing and enables demodulation,de-spreading and decoding.Baseband functionsThe physical layer functions on the basebandboards have been implemented to include• the mapping and de-mapping of physicalchannels and transport channels;• multiplexing and demultiplexing;• channel coding and decoding;• spreading and de-spreading;• modulation and demodulation;• physical layer procedures; and• physical layer measurements.In addition, the baseband boards in a radiobase station perform the following functions:32 Ericsson Review No. 1, 2003