W-CDMA and RNC

1. W-CDMA and RNC

2. W-CDMA • W-CDMA stands for Wideband Code Division Multiple Access. • It is based on IMT-2000 specification and this specification is developed by 3GPP. • It is the main third generation air interface in the world. • 3G mobile communication systems are referred as UMTS (Universal Mobile Telecommunication System). • W-CDMA is targeted at 2GHz. • Within 3GPP, W-CDMA is called UTRA (Universal Terrestrial Radio Access) FDD(Frequency Division Duplex) and TDD(Time Division Duplex). • W-CDMA is completely a new air interface without any constraints from the past. • Data Rate • Circuit Switched : 384 kbps at hot spots and 144 kbps in wide coverage. • Packet Switched: 2Mbps

3. W-CDMA cont… • WCDMA uses a digital wideband spread spectrum technology to transmit multiple independent conversations across single or multiple 5MHz segments of radio spectrum. • All of signal’s power are spread simultaneously over a wide frequency band. • The spread signal would then appear in a noise like signal scattered over the same wide transmission band with a much lower power per bandwidth. • Each user is tagged with a distinctive Pseudo-random Noise (PN) code that only the user’s receiver can recognize. • Receiver knows in advance how the transmitter will spread the spectrum. Based on this info, it compresses and reconstitutes the desired signal. • In nutshell, • In WCDMA, multiple users share the same frequency band at the same time. Each conversation is specially encoded and decoded for each particular user. • It uses spread-spectrum. The core principle of spread spectrum is the use of noise-like carrier waves.

4. W-CDMA cont… • Each carrier has a bandwidth 5MHz. Four times than that of CDMA. • The large bandwidth is needed to support higher bit rates. Concept of Bandwidth on demand (BoD) is well supported. • Chip-rate is 3.84 Mcps. A chip is a bit in the code word, which is used to modulate the information signal. • Spreading factor : ratio b/w chip rate and data rate.

5. W-CDMA cont… • UTRA FDD mode • the uplink and downlink use separate freq bands. • in Europe and most of Asia, bands of 2 x 60 MHz ( 1920-1980MHz & 2110-2170 MHz) are used • each carrier is divided into 10ms radio frames • UTRA TDD mode • both uplink and downlink use the same frequency carrier but different time slots. • in Europe and most of Asia, 25 MHz is available for TDD ( 1900-1920MHz & 2020-2025 MHz) are used • each carrier is divided into 10ms radio frames and each frame further into 15 time slots.

6. High level 2G and 3G Architectures

7. Power Control • Near-far problem during uplink. • Optimum strategy is to equalize the received power per bit of all the MS at all the times. • Solution: Fast closed-loop power control. • BS performs frequent estimates of received Signal-to-interference Ratio (SIR) and compares it to a target SIR. • If measured SIR is higher than the target SIR, base station commands the MS to lower the power and vice versa. • This measure-command-react cycle is executed at 1.5KHz for each MS. • Same technique is also used on downlink. • Additional power to MS at the cell edge, as they suffer from increased other-cell interference. • Outer loop power control • RNC adjusts the target SIR setpoint in the base station according to the needs of the individual radio link and aims at a constant quality. • Target SIR depends on MS speed and multipath profile.

8. Rake receiver Maximal ratio combining using Rake receiver Rake Receiver • In a multipath channel, the original transmitted signal reflects off obstacles in its way, and the receiver receives several copies of the original signal with different delays. • These multipath signals can be received and combined using a RAKE receiver. • It consists of correlators, also known as RAKE fingers, each receiving a multipath signal. After despreading by correlators with a local copy of the appropriately delayed version of the transmitter's spreading code, the signals are combined. • Since the received multipath signals are fading independently, combined composite signals in a rake receiver improves the overall combined signal quality and performance. • Both MS and BS have rake receiver.

9. Soft Handover Softer Handover Handoffs • Handoff : The process whereby a mobile station moves to a new traffic channel is called handoff • Soft hand-off: Soft Handoff is a process in which the mobile is directed to hand off to the same frequency assigned to an adjacent cell without dropping the original RF link.The mobile keeps two RF links during the soft-handoff process. • Softer hand-off: During softer handover, a MS is in the overlapping cell coverage area of two adjacent sectors of a base station. • Hard handoff : occurs when two base stations are not synchronized or are not on the same frequency resulting in interruption in voice or data. • Inter-frequency hard hand-off • Inter-system hard hand-off

10. RNC • RNC stands for Radio Network Controller. • In 2G it corresponds to called Base Station Controller. • RNC is a key network element in emerging 3G wireless networks. • Provides the control functions and physical links • Between the SGSN/MSC and Node B • RNC interfaces the core network via lu interface and uses lub interface to control Node B. • Provides functions such as • call set-up and tear down • voice and data traffic processing • call handover • cell configuration data and control of RF power levels in base transceiver stations • Resource management of radio network and optimization of radio resources

11. sasharma: Diff between RNC and BSC • In RNC, lur --a new interface is added for performance in WCDMA networks and to make mobility transparent to the core network • RNCs has more radio functions as WCDMA requires more soft handovers between cells, base stations and RNCs. • RNC provides bridging functions for connecting to • IP packet-switched networks, • traditional ATM AAL2(voice) and AAL5 (data) functions, • IP over ATM (IPoATM) and • packet over SONET(POS). • Support for both IPv4 and IPv6. • RNC can be viewed as a heterogeneous networking device. • RNC handles more signaling per call. • More processing is required in RNC and thereby more processing power.

12. Diff between BSC and RNC cont… • Most significant difference lies in the nature of processing • BSC : transcoders with signal processors account for the majority of processing resources. • RNC : the brunt of processing entails shuffling data using general-purpose processors. • Internal data paths • BSC works with strict multipexing architecture • RNC requires full switching capabilities.

13. Interface Reference Points of RNC • lub • Connects Node B transceivers and the RNC usually via an OC-3 link • lur • RNC to RNC connection for call handover, usually via an OC-3 link • lu-cs • Core network interface between the RNC and the circuit switched networks. Usually implemented as an OC-12 rate link. • lu-ps • Core network interface between the RNC and packet-switched data networks. Usually implemented as an OC-12 rate link.

14. Radio Access Network • Two roles of RNC • Serving RNC • It has the overall control of the MS that is connected to RAN. It controls the connection on the lu interface for the handset • Drift RNC • When a MS must use the resources in a cell not controlled by its Serving RNC, the Serving RNC must ask the controlling RNC for those resources. • This request is made via lur interface. • In this case, Controlling RNC is called Drift RNC for this particular handset.

15. Radio Access Bearer • The main service offered by WCDMA RAN is the Radio Access Bearer (RAB). • RAB carriers the subscriber data between the MS and the core network. • 3GPP has defined four different classes of RABs. • Conversational (used for e.g. voice telephony) • low delay, strict ordering • Streaming (used for e.g. watching a video clip) • moderate delay, strict ordering • Interactive (used for e.g. web surfing) • moderate delay • Background (used for e.g. file transfer) • -no delay requirement • Conversational and Streaming RABs require a reservation of resources in network and are primarily meant for real-time services. • Interactive and Background RABs are called “best effort” as they require no reservation of resources . • Core network asks the RNC to provide a RAB based on the service request.

16. Thanks