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- 1. 4. CDMA Air Interface Proprietary and Confidential 3G Basics Training • What is WCDMA? • CDMA comparison to TDMA and FDMA • CDMA benefits and Spread Spectrum Communications • WCDMA´s Key Radio system Features • Multipath advantage and Diversity • Power Control • Interference and maximum load • Handover types • TDD Basics • Radio Network Planning For internal use only 1 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 2. 4. CDMA Air Interface Proprietary and Confidential 3G Basics Training • Module objectives. The participant will be able to explain: • How CDMA is different compared to TDMA and FDMA? • What is the Spread Spectrum Communications principle? • WCDMA´s Key Radio system Features • What are the multipath propagation and diversity advantages • Power Control principles in WCDMA • How interference affects to maximum load • Handover types in WCDMA • TDD Basic features • Radio Network Planning principles For internal use only 2 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 3. Proprietary and Confidential What is WCDMA? Terms and Abbreviations • CDMA = Code Division Multiple Access • Multiple Access (MA) = A method of sharing a common transmission medium between multiple users (or terminals) communicating simultaneously • Code Division MA = Separating different users by unique signature codes What is CDMA? • CDMA = A Multiple Access method utilizing spread spectrum techniques What is WCDMA? • WCMDA = Wideband CDMA, one 3. Generation Standard (3GPP) . The access scheme is Direct-Sequence Code Division Multiple Access (DS-CDMA) with information spread over internal use only For approximately 5 MHz 3 bandwidth./ 10.02.00 © NOKIA Cdma05.ppt 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 4. Proprietary and Confidential CDMA comparison to TDMA and FDMA Frequency Division Multiple Access (FDMA) • Users transmit simultaneously using separate frequencies Frequency Mobile Station 1 Mobile Station 2 Mobile Station 3 Time Time Division Multiple Access (TDMA) • Users transmit at separate times Frequency Mobile Mobile Mobile Mobile Mobile Mobile Station 1 Station 2 Station 3 Station 1 Station 2 Station 3 Time For internal use only 4 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 5. Proprietary and Confidential CDMA comparison to TDMA and FDMA Code Division Multiple Access (CDMA) • User signals cover the whole frequency band all the time Frequency Mobile Station 1, Code 1 Mobile Station 2, Code 2 Mobile Station 3, Code 3 Time • The Base Station (BTS) separates the users by applying spread spectrum techniques - Code each users´ signal with a unique signature sequence - The receiver can separate the desired signal if it knows the sequence For internal use only 5 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 6. Proprietary and Confidential CDMA Benefits • Improved system capacity • Increase the amount of users (information) in a given bandwidth • Improved system performance in severe environments • Multipath propagation; Can benefit from multipath by using RAKE receiver • Interference from other users and from other systems • Simplified frequency planning • Universal frequency re- use is one • Enables seamless soft handover • Ongoing calls are handled in the boundary region between two cells by both Base Stations For internal use only 6 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 7. Definition of Spread Spectrum Proprietary and Confidential Communications • Signals are transmitted at much higher bandwidth than the information rate (typically at least 100 times higher) Conventional Spread Spectrum Power Power Frequency Frequency • The spreading is achieved with a code (spreading sequence) that is independent of theinternal use only information For transmitted 7 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 8. Proprietary and Confidential Spreading Symbol Symbol Spectrum +1 Data -1 Chip Chip +1 Code (pseudo -1 noise) +1 Data x Code -1 Despreading +1 Code -1 +1 Data -1 For internal use only 8 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 9. Proprietary and Confidential Detecting own signal. Correlator +1 Own signal -1 +1 Code -1 +1 Data after multiplication -1 +8 Data after Integration -8 +1 Other signal -1 +1 Code -1 +1 Data after -1 multiplication +8 Data after Integration -8 For internal use only 9 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 10. Proprietary and Confidential Basic Methods Spread Spectrum Concept Data Data Data Spreading De- De- modulation modulation spreading modulation Spreading Spreading Code Code Spreading codes must be synchronous ! Direct Sequence (DS) Frequency Hopping (FH) • • Modulate the signal directly with the Not used in WCDMA spreading sequence • Let the spreading sequence • The spreading sequence has a change the carrier frequency much higher bit rate than the data • For example, each bit is sent with • Comparable to data scrambling usea different carrier frequency For internal only 10 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 11. Proprietary and Confidential Direct Sequence (DS) Spread Spectrum Data Data Spreading De- modulation X X spreading Rb Rb Code Carrier Carrier Code generator generator Rc Rc • Code generator output is a pseudo- random periodic bit (chip) sequence of rate Rc • This chip rate Rc is typically > 100 times the data rate Rb Data Code Spread signal For internal use only 11 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 12. Proprietary and Confidential WCDMA Main Parameter Summary • Multiple Access (MS) scheme • Direct-Sequence Code Division Multiple Access (DS-CDMA) with information spread over approximately 5 MHz bandwidth. • Chip rate • 3, 84 Mcps • Spreading Factor • Uplink 4- 256 • Downlink 4- 512 • Frequency bands: • Uplink 1920 - 1980 MHz, • Downlink 2110 - 2170 MHz • Duplex scheme • Frequency Duplex Division (FDD) • TX - RX frequency separation is 190 MHz • Channel spacing • The nominal value is 5 MHz • Channel raster is 200 kHz For internal use only 12 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 13. Proprietary and Confidential WCDMA's Key Radio System Features • Wide 3.84 Mcps bandwidth ==> good frequency & interferer diversity ==> low Eb/No • coherent in both up- and downlink ==> low Eb/No • fast power control (PC) ==> minimizes interference ==> high spectral efficiency • robust RAKE diversity receiver ==> low complexity • dynamic variable rate multiplexing ==> flexibility = Codes with different spreading, giving 8-500 kbps .... P f 5 MHz t High rate multicode user Variable rate users 10 ms frame For internal use only 13 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 14. Proprietary and Confidential Multipath advantage in WCDMA • Multipath propagation causes several peaks in matched filter (MF) output • Allocate RAKE fingers to these peaks • Track and monitor the peaks For internal use only 14 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 15. Proprietary and Confidential UL Receiver diversity (space diversity) Amplitude = Antenna 1 Antenna RAKE = Antenna 2 combining (MRC) Fading Time RNC For internal use only 15 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 16. Proprietary and Confidential Power Control (PC) Loops in WCDMA MS BTS RNC Open Loop Power Control (Initial Access) Closed Loop Power Control Outer Loop Power Control For internal use only 16 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 17. Proprietary and Confidential Open and Closed Loop PC, Uplink TPC commands if SIR > (SIR)set then "down" MS adjusts else "up" power according to TPC commands MS1 P1 TPC commands • MS1 and MS2 are transmitting in the same frequency P2 => equalizing transmitter powers is critical ("near-far" problem) • Optimum situation: P1 = P2 at the BTS at all times BTS • Different path attenuations are compensated by using power control. MS2 • Open loop power control: MS adjusts it’s initial transmitter power according to received signal level • Closed loop power control: BTS commands MS to increase or decrease it’s transmission power at 1.5 kHz It is based on received signal to interference ratio (SIR) estimates in BTS. • Closed loop power control follows also the fast fading pattern at low and (lesser) medium speeds (< 50 km/h) For internal use only 17 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 18. Proprietary and Confidential Outer Loop PC, Uplink Required (SIR)set for 1 % FER • Outer loop power control maintains link quality • Optimizes capacity / range MS stands still time if SIR > (SIR)set then "down" else "up" if FER increase then (SIR) set "up" else (SIR) set "down" (SIR) set adjustment command RNC CN outer loop frame reliability info control For internal use only 18 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 19. Proprietary and Confidential Interference in CDMA • Interference is caused by Interference mobiles in the neighbouring cells as well as by mobiles in the same cell area (uplink) Signal • Total interference experienced by BTS is summary of all those • If interference increases the needed output power from MS needs to be For internal use only increased as 19 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 20. Proprietary and Confidential Maximum Load in CDMA Output Power "Safe area" 0% 50% 100% Load per carrier • CDMA System must be dimensioned according to estimated traffic. • Too much load makes the system unstable. • Dimensioning rule is that 50 % of the theoretical capacity can be used and still maintain good performance. For internal use only 20 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 21. Proprietary and Confidential WCDMA Handover types • Soft handovers • Softer handover between sectors in one BTS • Soft handover between different BTSs under one RNC • Soft handover between different BTSs in different RNCs • Hard handovers • Intra-frequency hard handovers between RNCs when soft handover is not possible. • Inter-frequency hard handovers • Intra-BTS between carriers • Intra-RNC between cell layers (macro to micro or vice versa) • Inter-RNC(and CN) between different core networks • Inter-System handovers • Handover WCDMA <--> GSM900/1800 • Handover WCDMA/FDD <--> UMTS/TDD For internal use only 21 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 22. Proprietary and Confidential Softer Handover • Softer handover is handled by BTS internally Sector/Antenna • Softer handover RAKE probability about 5 - 15 combining % • No extra transmissions across Iub • Basically same RAKE MRC processing as for multipath/antenna diversity (BTS / MS). More RAKE fingers needed. RNC • Provides additional diversity gain • Softer handover does For internal use only create067594AE, 1.0.0. additional 22 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X
- 23. Soft handover Proprietary and Confidential • Soft handover probability about 20- 40 % • Extra transmissions across Iub • UL / DL soft HO diversity processing very different •MS: MRC RAKE combining Except for the TPC symbol •RNC: frame selection exactly the same information (symbols) is sent via air. RNC y info Differential delay in order re liabilit of fraction of symbol duration CN frame frame selection / duplication fra m e re lia bi li ty i nf o • Soft handover does create additional interference and needs BTS LPA resources / 10.02.00 For internal use only 23 © NOKIA Cdma05.ppt 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 24. Proprietary and Confidential TDD (Time Division Duplex) Characteristics: Difficulties: • same frequency for up- and downlink, time division • risk for system f1 duplex uplink down interference • synchronization • utilises unpaired spectrum requirements between allocation base stations and • asymmetry between uplink between operators and downlink capacities M1->BS1 BS1->M1 • support for unlicensed CELL 1 operation M2->M1 BS1->BS2 M2->BS2 BS2->M2 CELL 2 For internal use only 24 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 25. UTRA TDD (Time Division Duplex) Proprietary and Confidential principles • Combination of TDMA Standardisation and CDMA • UTRA=UMTS Terrestrial • Users separated inside a Radio Access time slot by code • UTRA is both UTRA FDD • DS-CDMA used and UTRA TDD • user data rate granularity • TDD utilises unpaired spectrum • Basic parameters are harmonized, and the only few exceptions exist in physical layer Bundling of FDD and TDD frequency in licensing? TD/CDMA principle For internal use only 25 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 26. UTRA/TDD complements FDD where there is Proprietary and Confidential shortage of spectrum due to traffic or regulation • Additional capacity to network • Flexible support for asymmetric traffic • Seamless integration with UTRA/FDD using multimode terminals • Micro/picocell applications emphasizing high data rates • TDD technology is best suitable for indoor use where interference from base stations is manageable and lower range does not matter For internal use only 26 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 27. Proprietary and Confidential TDD utilizes Dynamic Channel Allocation For internal use only 27 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 28. Proprietary and Confidential TDD interference • To fight interference scheduling, coordination and synchronization is needed. • Adaptive system (DCA, RRM) 28 © NOKIA • Planning caninternal use only Cdma05.ppt / 10.02.00 For alleviate problems 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 29. Proprietary and Confidential TDD Conclusions • Key issues for TDD implementation by customers are • mature 3G TDD standard • seamless handover FDD/TDD/GSM • availability of multimode terminals • There are still high market and technology uncertainties • Competing technologies within the target market (TDD/FDD/WLAN) • Frequency allocation is the critical issue • Frequency bundling for UMTS • Warc2000 • Nokia provides a full end-to-end solution - TDD is needed to secure/complement Nokia FDD business For internal use only 29 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
- 30. Proprietary and Confidential WCDMA Radio Network Planning process • Network dimensioning • Site selection (cooperation with site acquisition and existing GSM sites) • Detailed network planning • coverage/capacity planning • propagation model tuning • parameter planning Propagation • soft/softer handover overhead analysis Model Tuning & optimization • Network testing and tuning Network Detailed Network Parameters Site Selection Download Dimensioning Planning OMC NETWORK REQUIREMENTS For internal use only 30 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.
Editor's Notes
- What is WCDMA? The available transmission medium for mobile communication is normally shared between multiple user. Co-ordination has to be done to prevent collision at multiple access to the same transmission medium. Three different principles for multiple access are currently used: Frequency Division Multiple Access FDMA, Time Division Multiple Access TDMA and Code Division Multiple Access CDMA. In FDMA different users are separated by frequency (“one frequency band for one user”), in TDMA different users are separated by time (“one Time Slot TS for one user”) and in CDMA different users use the same frequency range at the same time, but they are separated by unique signature codes (“one unique code for one user”). CDMA is a “Spread Spectrum Technology” SST. There exist several possibilities to implement SST systems. For mobile communication only Direct Sequence Spread Spectrum DSSS technology is used to spread the spectrum (Direct Spread CDMA). The carrier is modulated (spread) with a digital Code. The rate of the code (chip rate) is considerably higher than that of the information to be sent (bit rate). W-CDMA is an abbreviation for Wideband CDMA. Wideband CDMA utilises a wide band (5 MHz) for the CDMA multiple access. The bandwidth is wide compared to the so-called narrowband CDMA solutions of 2G (e.g. IS-95) and some CDMA proposals with narrow bandwidth for IMT-2000. Larger chip rate will be used. W-CDMA will be used for UMTS Terrestrial Radio Access UTRA.
- CDMA comparison to TDMA and FDMA Three different principles for multiple access of several user to the same frequency resource are currently used in mobile communication systems: Frequency Division Multiple Access FDMA, Time Division Multiple Access TDMA and Code Division Multiple Access CDMA. Frequency Division Multiple Access : FDMA is a multiple access scheme which is very common in 1G mobile communication systems. Users transmit simultaneously using separate frequencies. The available spectrum is divided into channels of equal band width (carrier). A physical channel, required for the duplex transmission of one user, is defined as one channel for Uplink UL and one channel for Downlink DL, UL and DL channel are separated by a duplex distance. Time Division Multiple Access : TDMA is a multiple access scheme which is very common in 2G mobile communication systems. Users might transmit at the same frequency, but they are separated by time. The transmission of a user takes place in Time Slots TS, which are given cyclically to the user. The cycle period is denominated as TDMA frame. In 2 G terrestrial cellular TDMA systems, which use Frequency Division Duplex FDD for duplex transmission, a physical channel is defined by one frequency for UL and DL and a certain Time Slot. In 2G systems using Time Division Duplex TDD for duplex transmission (e.g. Digital Enhanced Cordless Telecommunication DECT) a physical channel is defined by one frequency and two TS (one for UL, one for DL) in a TDMA frame.
- CDMA comparison to TDMA and FDMA Code Division Multiple Access : CDMA is a multiple access scheme which is partially used in 2G mobile communication systems and which will dominate in 3G. The signals of different user are transmitted simultaneously at the same frequency. Users are separated by codes. CDMA is a “Spread Spectrum Technology” SST. There exist several possibilities to implement SST systems. For mobile communication only Direct Sequence Spread Spectrum DSSS technology is used to spread the spectrum (Direct Spread CDMA). The carrier is modulated (spread) with a digital code. This digital code consists of unique signature sequences, which will be given by the Base (Transceiver) Station BTS to the Mobile Stations MS. The receiver can separate the desired signal from all other information transmitted simultaneously at the same frequency if it knows the (spreading) sequence. By cross-correlating the total received signal with synchronously generated replica of the spreading code only the desired information is re-transformed.
- CDMA Benefits CDMA offers several benefits compared to FDMA or TDMA systems: Improved system capacity : CDMA improves the system capacity, which means that the amount of user/information in a given bandwidth can be increased. In FDMA and TDMA cellular systems only a small fraction of the operators frequency spectrum is to be used in one single cell. This is necessary to prevent strong interference between user transmitting simultaneously at the same frequency in neighbouring cells. Between cells using the same frequencies a certain re-use distance must be kept. The use of only a small fraction of the available spectrum in each cell lowers the spectrum efficiency. In CDMA systems a frequency re-use of one is possible; each frequency can be used in every cell and the spectrum / system capacity is increased. Improved system performance in severe environments : CDMA improves the system performance in many ways. The transmission is protected against multipath interference. More-over, multipath propagation can be utilised using RAKE receiver. The RAKE receiver combines different resolved multipath signals, which are the result of reflections of the transmitted signal at obstacles. An other advantage is the interference rejection or anti-jamming capability. The interference from other users and from other systems is only background noise to the receiver and will be no problem when despreading the signal with the original code. Especially narrowband interfering signals might have several time the power of the the CDMA signal without being a problem for the information recreation. Simplified frequency planing, universal frequency re-use : CDMA system can use a re-use factor of one; each frequency band might be used in every cell. Seamless soft handover : CDMA enables seamless soft handover. Ongoing calls are handled in the boundary region between two cells by both Base Stations BTS. A secure handover without decreasing quality level and strong increase of MS power level is resulting on these soft handover.
- Spread Spectrum Communications CDMA is a Spread Spectrum Technology SST. Each user is assigned a unique code sequence, the so-called spreading code. The spreading code is used to encode the information bearing signal. The receiver decodes the received signal after reception with the same code and recovers the original data. The rate of the spreading code (chip rate) is considerably higher than that of the information to be sent (bit rate), resulting in a transmission of much higher bandwidth as the unspreaded signal. Why to spread? One reason is given in Information Theory: When information are transmitted at a given rate, it is possible to have a trade-off between bandwidth and Signal- to Noise (S/N) ratio. Respectively: the wider the bandwidth used for transmitting at a given information rate, the lower the S/N ratio is required. Spreaded information show larger tolerance to interference of other user, other systems or other technical or natural interference sources (anti-jamming effect). Furthermore, the spectral power density of the transmitted signal might be decreased several times, compared to an unspreaded signal. In CDMA as Spread Spectrum Technology (Direct Sequence CDMA = DS-CDMA) the information are spread by unique, digital codes (spreading sequences). The primary information is spreaded with “Chip” sequences of a much higher rate (chip rate) as the primary information rate. The ratio between the chip rate and the information rate is denominated as Spreading Factor SF. The spreading code or spreading sequence is independent of the transmitted information. Correlated with the spreading of the information is a higher bandwidth. The bandwidth after spreading is roughly SF times the unspreaded bandwidth. For DS-CDMA Spreading Factors between 10 and 1000, typically some 100, are used. In this way the narrow-band signal is spread up for a factor of some 100, creating a wide-band signal. In the same way the bandwidth increases with spreading, the spectral power density necessary for transmission decreases. For DS-CDMA only very small power densities, often below the level of natural background noise, are needed.
- Spreading / Despreading The information flow to be transmitted via the radio interface consists of digital information. The basic unit of the information flow, which will be spread up is a so-called symbol. A symbol consists of one or several bit, depending on the modulation principle used. The information rate might be given as bit/s or symb/s. Spreading : The data of the information flow are spread up with the spreading code. The spreading code consists of code sequences. The basic unit of a code sequence is one chip. The rate of the spreading code is denominated as chip rate Rc (chip/s or cp/s) The ratio between the chip rate Rc (cp/s) and the information rate Rb (symb/s) is denominated as Spreading Factor SF: SF = Rc/Rb. The bandwidth after spreading B (modulation bandwidth) is in rough terms SF times the bandwidth before spreading W: B SF W. The spreaded information is modulated onto the carrier frequency and sent via radio interface. Despreading : The receiver of the Mobile Station MS receives the total information of all participants using the same frequency and other background noise. The receiver, knowing the code sequence, decodes the received signal after reception and recovers the original data. This is possible since the cross-correlations between the code of the desired user and the codes of the other user are small (orthogonal or quasi-orthogonal codes are used).
- Detecting own signal / Correlator After reception, the receiver despreads the received signal (own signal) using the code sequence of the user. To be able to perform the despreading operation, the receiver must not only know the code sequence used to spread the signal, but the codes of the received signal and the generated code of the receiver must be synchronised. Correlator / Despreading: The received signal (own signal) is cross-correlated with the code sequence, resulting in a recovery of the original data. The data of the own signal after integration will be piecewise (over the duration of the original symbols) continuous functions. Correlating another signal (of other users with other codes) with the users code sequence will result in data with the length of a single chip. Data after integration will show randomly pattern.
- Basic Methods The figure shows the Spread Spectrum Concept : The data signal modulates an RF Carrier. The modulated carrier is then modulated by the Spreading Code. This code signal consists of a number of code “chips” that can be either +1 or -1. The chip rate of the code signal must be much higher than the chip rate of the information signal. After transmission of the signal via the radio interface, the receiver despreads the transmitted signal using a locally generated code sequence. To be able to perform the despreading operation, the receiver must not only know the code sequence used to spread the signal, but the codes of the received signal and the locally generated code must also be synchronised. After despreading, a data modulated signal results. After demodulation the original data can be recovered. There are a number of different techniques generating spread-spectrum signals. Direct Sequence Spread Spectrum DSSS: In Direct Sequence CDMA (DS-CDMA) the information bearing signal is modulated directly with the spreading sequence (spreading code). The spreading sequence has a much higher bit rate than the data. The principle of DS-CDMA is comparable to data scrambling. For civil mobile communication (like 3G WCDMA) only DS-CDMA is used. Frequency Hopping Spread Spectrum FHSS: In FH-CDMA the carrier frequency is not constant; it changes periodically. During a time interval the carrier frequency remains the same, but after each other time interval the carrier moves to another frequency. The hopping pattern is decided by the spreading code / sequence. Different to DS-CDMA, which occupies the whole frequency band when it transmits, FH-CDMA uses only a small part of the bandwidth when it transmits, but the location of the part differs in time. For example, each bit is sent with a different carrier frequency. Time Hopping Spread Spectrum THSS: The information bearing signal is not transmitted continuously. Instead the signal is transmitted in short bursts where the times of the bursts are decided by the spreading code.
- Direct Sequence (DS) Spread Spectrum In DS-CDMA the data signal can either be an analogue signal or a digital one. In the case of a digital signal, the data modulation is often omitted and the data signal is directly multiplied by the code signal. The resulting signal modulates the wideband carrier. From this direct multiplication the DS-CDMA gets it’s name. In the receiving direction the reverse the process is handled. The Code generator generates the code or spreading sequence, a pseudo-random periodic sequence. The basic unit of this sequence is one chip. The rate of the spreading sequence is the chip rate Rc (chip/s or cp/s). RC is typically some 100 times the data rate Rb. The ratio between the chip rate Rc (cp/s) and the data rate Rb (symb/s) is the Spreading Factor SF: SF = Rc/Rb.
- WCDMA Main Parameter Summary The UMTS FDD mode uses WCDMA as multiple access scheme: Multiple Access (MA) scheme is Wideband DS-CDMA (WCDMA). Frequency Division Duplex FDD is used for the duplex transmission. Channel spacing for UMTS is 5.0 MHz. The basic chip rate is 3.84 Mcp/s. The frame length of one continuous CDMA transmission (time in which the SF is fixed respectively the data rate is unchanged) is 10 ms or 20 ms (optional). The Base Stations BTS must not run synchronous (different to other CDMA solutions, e.g. 2G IS-95 or 3G cdma2000). Therefore, no external source of synchronisation, like GPS (Global Positioning System), is needed for the BTSs. Handover in UMTS FDD mode are normally Soft Handover, i.e. in at the border between different cells the MS corresponds to more than one BTS. Interfrequency Handover are needed for utilisation of Hierarchical Cell Structures HCS and Handover to 2G systems, like GSM. Spreading Codes: For Spreading variable length orthogonal short codes (Orthogonal Variable Spreading Factor OVSF codes) for channel separation and scrambling long codes for cell and user separation (DL: Gold sequences 2 18 for cell and user separation; UL: Gold sequences 2 41 for user separation; the codes are truncated for 10 ms cycles) are used. Physical data / control multiplexing: Multiplexing in the DL is done by time multiplexing; in the UL I/Q (QPSK modulation is used) or code multiplexing is used. Coherent detection: User dedicated time multiplexed Pilot symbols for UL and DL. Variation of data rate is possible via SF variation or using of multiple codes. Multi User Detection MUD and adaptive antennas are supported in UMTS FDD.
- WCDMA’s Key Radio System Features WCDMA uses 3.84 Mcp/s bandwidth resulting in good frequency and interferer diversity and therefore a low signal (emission) to noise ratio Eb/No. Carrier space of 5 MHz is used for UMTS. Coherent detection in both, UL and DL, results in low Eb/No. Use of fast Power Control PC (1500 PC cycles/s) minimises interference and results in high spectral density. Robust RAKE diversity receiver, combining resolved multipath signals (utilising multipath diversity), enable low complexity of the system and improve the performance. Flexible and dynamic data rates are very important for UMTS. WCDMA enable dynamic variable data rates. Data rates of one user might be changed every frame (10 ms) using spreading codes of different length (e.g. to vary the data rate for one application) or multiple different codes (for different applications of one user). Codes with different spreading factors SF enable variation of the user data rate from 8 - 512 kbit/s.
- Multipath advantage in WCDMA The original transmitted signal is reflected on its way between MS and BTS from obstacles such as buildings, mountains, etc. The receiver receives several copies via multiple propagation paths ( multipath propagation ). The different multipath signals arrive the receiver with different delays. If the signals arrive more than one chip apart from each other, the receiver can resolve them. Several peaks are caused in the Matched Filter MF output. A further benefit of WCDMA is obtained if the resolved multipath signals are combined using a RAKE receiver . In this way, the CDMA waveform facilitates utilisation of multipath diversity and enhances the system performance.
- UL Receiver diversity (space diversity) The influence of multipath fading / destructive interference can be reduced using different antenna with distances of (2n + 1) /2. Receiver antenna diversity can be used to average out receiver noise in addition to providing diversity against fading and interference. Antenna RAKE combining is done utilising Maximal Ratio Combining MRC. Receiver antenna diversity is best suited for Bases Stations BTS and for large sized Mobile Stations MS. At BTSs, receiver antenna diversity can be used to increase cell coverage by increasing the noise limited Uplink rage.
- Text from UMTS system training
- Uplink Power Control / Transmitter Power Control TPC Near-far problem : If several MS, being located in different distances to the BTS, transmit in the same frequency with equal power, the BTS receives signals of very different power level. The levels can vary over several magnitudes, which means that the weaker signals no longer can be detected. The optimum situation is that all signals, irrespective of distance, should arrive at the BTS with the same mean power. The power level of incoming signals may vary depending of several causes: local mean fading (path losses), medium scale fading (normal log fading), and small scale fast fading (short-term fading, multipath fading), such as Rayleigh fading. The different path attenuation's are compensated by using Power Control: Open loop power control is used by the MS to adjust it’s initial transmitter power according to the received signal level. The interference conditions from the channel are measured by the MS and the transmission power is adjusted accordingly to meet the desired Frame Error Rate FER. Closed loop power control is responsible to handle fast fading, where the is no correlation between UL and DL. The Signal-to-Interference Ratio (SIR) is measured in the BTS. The BTS commands the MS to increase or decrease it’s transmission power. Closed loop power control is realised in WCDMA all 0.667 ms (1.5 kHz). Closed loop power control procedure: The BTS measures the SIR. The SIR is compared to a set value (SIR)set. If SIR > (SIR)set, the a Transmitter Power Control TCP commands the MS to set the MS power “down”, else to set the power “up”. The closed loop power control follows also the fast fading pattern at low and (lesser) medium MS speeds up to 50 km/h. This is based on outer loop power control over Iubis interface (every 100ms up to several seconds) to get the desired FER/BER/BLER.
- Uplink Outer Loop Power Control The uplink outer loop power control task is the maintenance of the link quality. This is also of interest to optimise the capacity of the cells within WCDMA. The Frame Error Rate (FER) is hereby the main indicated. UL Outer Loop Power Control (Procedure): The FER is announced to the RNC (frame reliability info). The outer loop control, which is located in the RNC regards changes of the Frame Error Rate FER. If FER increases, the the Signal-to-Interference Ratio SIR must be set up. The value of the pre-defined (SIR)set is set up by the outer loop control when FER increases, otherwise (SIR)set is set down. The RNC commands the BTS every 100ms up to several seconds to adjust (SIR)set ((SIR)set adjustment command). Now the same procedure starts to the MS as in “normal” UL Power Control: The BTS compares the measured value SIR with the new (SIR)set. If SIR is larger as (SIR)set, then the BTS commands the MS to set the power “down”, else the MS sets the power “up”.
- Interference in CDMA The different Mobile Stations MS (UL band) and the different Base Stations BTS (DL band) are using common radio resources to transmit the user data. Hereby multiple access causes inter-cell disturbances : Mobiles which are linked to a specific base station, interfere each others transmission. This is also called own cell interference . intra-cell disturbances Mobiles, which are linked to neighbouring cells, are using the same radio resources (uplink and downlink). They also are interferers to mobiles of this cell (uplink). The total interference an MS faces is the combination of the own cell interference and the neighbouring cell interference. The MS not only has to filter out this “background noise”, it also has to increase its transmission power, so that its signals are adequately received at the BTS. The more active MS are within one region, the higher the power level used by the mobile, and - as a consequence - this limits the overall capacity of the system. Note: Using Multi User Detection MUD , also called joint detection and interference cancellation, is one possibility to reduce the effect of multiple access interference. Ioc = Spectral density of interference from users in other cells [W/Hz] Ii = Spectral density of interference from all other users in the cell [W/Hz] Io = total spectrum density (Ioc + Ii) plus noise density No W = WCDMA bandwidth [Hz] n = number of active connections S = wanted user signal received at the BTS [W]
- Maximum Load in CDMA CDMA systems must be dimensioned according to the estimated traffic. Too much load makes the system unstable. Dimensioning rule is that 50% of the theoretical capacity can be used maintaining still good performance. If the load increases beyond 50% of the theoretical capacity, the the output power of the different MS increases exponential, resulting in a loss of performance. The system becomes unstable, i.e. breakdown of the total cell communication cannot be excluded.
- WCDMA Handover Types The handover (hand-off) process can be divided into three phases: 1) measure-ment, 2) decision whether handover criteria are fulfilled, 3) realisation of the handover. The handover decision depends on the distance attenuation, UL interference and DL interference. There are different principles available to realise handover: hard handover : The MS traffic flow “jumps” from a connection with one BTS to a connection with another BTS when the handover is commanded by the network. The MS is always connected to only one BTS. Hard handover are used in GSM. soft handover : During a soft handover the MS is connected to more than one BTS simultaneously. softer handover : Soft handover between sectors in the same cell. Intra-system handover can be divided Intra-frequency and Inter-frequency handover. Different handover types will be used for UMTS Inter-system handover: UMTS Intra-frequency handover use – soft handover (FDD mode) – softer handover (FDD mode) – hard handover (TDD mode) Inter-frequency handovers use hard handover (TDD and FDD mode); for handover between different layer in the Hierarchical Cell Structure HCS or Hot Spot cells with several frequency bands. Inter-System handover between UMTS and GSM or the different UMTS modes (FDD = WCDMA and TDD) will be handled as hard handover.
- Softer Handover Softer handover are soft handover between two sectors of the same cell, i.e. are handled by the same BTS. Softer handover are handled internally by the BTS. No extra transmission is necessary across the Iub interface (BTS - RNC). Softer handover uses basically the same RAKE MRC processing as it is used for multipath diversity or antenna diversity. More RAKE fingers are needed. Softer handover have the same advantages as soft handover, i.e. it provides additional diversity gain. But softer handover creates additional interference and needs BTS LPA resources. The probability for softer handover is estimated about 5 to 15%.
- Soft Handover During a soft handover the MS is connected to more than one BTS simultaneously. An MS enters the soft handover state when the signal strength of a neighboring cell exceeds a certain threshold but is still below the current BTS’s signal strength. The information (symbols) transmitted via the radio interface (between the BTSs and the MS are exactly the same, except the Transmitter Power Control TPC symbols. Soft handover need extra transmission capacity across the Iubis interface. UL and DL soft handover diversity processing are very different: DL: The MS can coherently combine the signals from different BTSs since it sees them as just additional multipath components.This provides a benefit called macro diversity (i.e. the diversity gain provided by the reception of one or more additional signals. UL: Two or more BTS receive the MS signal. The information are transmitted via Iubis interface to the RNC. The RNC is responsible for the selection of the frames (received from the different BTS), depending of the frame reliability information. In soft handover BTS of different RNCs might be involved. Therefore, the Iur interface is necessary. Concerning one MS being in a soft handover with different RNCs following logical RNC types exist: 1) Serving RNC (SRNC) controlling the active MS and selecting the frames, 2) Drift RNC (DRNC) controlling only the radio resources of the involved additional cells, transmitting received information to the SRNC. SRNC can be relocated during the connection. The soft handover probability is estimated to 20 – 40%.
- WCDMA Radio Network Planning process The planning of the Radio Network will be started with the correct dimensioning of the network. This requires good forecasts /estimations about subscriber and traffic development in all parts of the area, which the network should cover. The network dimensioning is followed by a more detailed selection of the sites. Aspects of topography, physics and economy have to be regarded. An important economic aspect is the co-operation with site acquisition and existing GSM sites. The site selection is followed by a detailed network planning which incorporates multiple of different aspects as: coverage and capacity planning, propagation model tuning, cell parameter planning, soft and softer handover overhead analysis and optimisation,... A mutual dependency between site selection and detailed network planning starts to optimise the network planning. The network planning is controlled with propagation model tuning. After detailed network planning the parameters will be downloaded to the Operation & Maintenance Centre OMC. In the following network roll-out and even after network start with increasing user number and traffic load all the different process steps will interact with each other to develop and optimise the network permanently.