Channelisation Codes (2)

Transcription

1 Channelisation Codes (2) Scram. #0 +Chan. #1 Scram. #0 +Chan. #0 Scram. #1 +Chan. #0 R.Scram. #1 +Chan. #0 R.Scram. #1 +Chan. #1 R.Scram. #0 +Chan. #0 R.Scram. #0 +Chan. #1 p. 51

2 Channelisation Codes (3) Usage Length Number of Codes Code Family Spreading Channelisation Code Uplink: Separation of DPDCH and DPCCH from the same terminal Downlink: Separation of downlink connections to different users in one cell chips ( µs) Downlink also 512 chips Number of codes under one scrambling code=spreading factor OVSF Yes, increases transmission bandwidth Scrambling Code Uplink: Separation of terminal Downlink: Separation of sectors or cells Uplink: (1) 10ms=38,400chips or (2) 66.7µs=256 chips Option (2) can be used with advanced base station receivers Downlink: 10ms=38400 chips Uplink: several millions Downlink: 512 Long 10ms code: Gold code Short code: Extended S(2) code family No, Does not affect transmission bandwidth p. 52

3 Transmit Diversity (1) WCDMA system: performance degraded by multipath channels use several transmit antennas at the BS (transmit diversity) to improve the downlink transmission performance using multiple antennas at a MS: increase the complexity, not preferred Two categories of transmit diversity open loop mode: space time transmit diversity (STTD) (space time code: Alamouti code) p. 53

4 Transmit Diversity (2) Transmit diversity via space time coding Alamouti scheme: simple but ingenious channel information is not required at transmitter side!!! two transmit antennas systems * s 12 h 1 y = h s + h s + n y h s h s n * * 2 = Tx s 1 * s 2 h 2 Rx { 1 } { 2 } { } 1 1 E s = E s = E s = P 2 2 sig Y y1 h1 h2 s1 n1 = * = * * * y h 2 2 h + 1 s 2 n2 H eff p. 54

5 Alamouti scheme SNR at receive side Transmit Diversity (3) Matched Filter Y y1 h1 h2 s1 n1 = * = * * * y h 2 2 h + 1 s 2 n2 H eff H * * H h1 h2 y1 h1 h 2 h1 h2 s1 h1 h n1 2 Z = Heff Y = * * * h2 h = * * * * * 1 y2 h2 h 1 s + h2 h1 2 h2 h1 n2 noise 2 2 h * * * * 1 + h2 0 s ( 2 2 ) 1 hn hn 2 2 s 1 hn hn 2 2 = + = h 2 2 * * 1 + h2 + * * 0 h s 2 hn s 2 1 hn hn 1 + h hn 1 2 useful signal noise ( + ) { } * * ( ) ( h1 + h2 ) E 2 h 2 h 2 s h 1 + h 2 sig ( P ) SNR at receiver: SNR= h + h P = = E h n h n σ σ sig p. 55

6 Transmit Diversity (4) Two categories of transmit diversity (con't) close loop mode: based on the feedback information (FBI) sent via uplink Ant 1 Spread/scramble w 1 CPICH 1 Σ Tx DPCCH DPDCH DPCH Σ Tx Ant 2 CPICH 2 w 2 w 1 w 2 Weight generation Determine FBI message from Uplink DPCCH Rx Rx p. 56

7 Transmit diversity: close-loop Transmit Diversity (5) * hs 1 H h 1 p 2 2 ( ) r = h s+ h s + n 1 2 H Tx * hs 2 H h 2 p h 1 h 2 Rx channel information: h 1 ~h H = h + h 1 2 SNR at receiver: SNR= = 2 2 ( ) 2 E{ n } 2 2 ( h1 + h2 ) Psig E h h H s 2 σ 2 p. 57

8 Transmit Diversity (6) Performance comparison two transmit antennas, BPSK modulation identical independent distributed (i.i.d.) Rayleigh fading channel SNR = 2 2 ( 1 2 ) h + h P sig alamouti 2 = 2σ 2 2 ( 1 2 ) h + h P sig SNR tran_mrc 2 σ p. 58

9 Intra-mode handover Handover (1) FDD mode relies on the Ec/N0 measurement performed on the common pilot channel (CPICH) can be soft handover, softer handover or hard handover soft handover needs relative timing information between the cells. BSs in WCDMA is asynchronous; timing adjustment is needed to carry out coherent combining in the Rake receiver in soft handover Inter-mode handover Dual-mode FDD-TDD terminals operating in FDD handover to the TDD mode Inter-system handover Handover between UTRA (WCDMA) and GSM systems, UTRA (WCDMA) and Multi-carrier CDMA systems p. 59

10 Handover (2) SSDT (site selection diversity transmit power control) is a macro diversity method in soft handover mode Soft Handoff Old BS New BS New BS 1. The MS selects one of the cells from its active set to be primary and others non-primary 2. Each cell is assigned a temporary identification (ID). MS periodically informs a primary cell ID to the connecting cells 3. The non-primary cells switch off the transmission power Benefits of SSDT only one BS is transmitting signals to the MS=>reduce the interference caused by multiple transmissions in a soft handover mode when multiple BSs transmit to the same signals to the MS, a number of Rake fingers are needed; if only one BS is considered, less fingers are needed no power imbalance problem due to power control command reception error p. 60

11 Power Control Fast closed loop power control (inner loop power control) one command per slot (1500Hz) (IS-95: 800Hz) basic power adjustment step size: 1dB applied to both uplink and downlink Open loop power control applied only prior to initiating the transmission on the RACH or CPCH (uplink) adjust the transmit power on uplink based on the measurement on downlink not very accurate: variation in the component properties, impact of environment, different frequencies in IS-95, being active in parallel with close loop power control, allow corner effects or other sudden environmental changes to be recovered in UTRA (WCDMA), not needed to be operated simultaneously with fast closed loop power control p. 61

12 Link Parameters for WCDMA (1) Copied from T. Ojanpera and R. Prasad, WCDMA: Towards IP Mobility... p. 62

13 Link Parameters for WCDMA (2) Copied from T. Ojanpera and R. Prasad, WCDMA: Towards IP Mobility... (downlink) (1.5kHz) p. 63

14 Summary - WCDMA vs. GSM Air Interface p. 64

15 Summary - WCDMA vs. IS-95 Air Interface p. 65

16 Summary of WCDMA Main differences between 2G and 3G. Key system parameters of WCDMA: 5MHz bandwidth, 3.84Mcps, data rate: 144Kbps to 2Mbps; two duplex modes: FDD and TDD. System architecture of WCDMA: compared to that of GSM and GPRS Uplink and downlink: spreading factor, modulation, detection, variable data rates Design criteria of UE in WCDMA. In uplink, why are DPCCH and DPDCH complex scrambled? Uplink and downlink: the multiplexing methods of DPCCH and DPDCH and the reason Spreading and scrambling. In uplink, when to use long and short scrambling codes, respectively? Diff. between channelisation codes and scrambling codes Transmit diversity: open loop (Space Time Coding) and close loop Handoff and power control: similar to IS95. p. 66

17 CDMA2000 p. 67

18 CDMA2000 (standardized by 3GPP2) Introduction based on IS-95 (or cdmaone), backward compatible with IS-95 Two phases CDMA2000 1XRTT: using 1.25MHz, provide higher data rates CDMA2000 3XRTT: using multiples of 1.25MHz, options of multicarrier transmission or direct sequence spreading Muticarrier CDMA 1.25MHz 1.25MHz f 5MHz Single carrier DS-CDMA G BW G f 5MHz p. 68

19 System Architecture New functional elements for packet data service p. 69

20 Mobile Station (MS) Network Elements (1) all functionalities of IS-95 terminals + additional features and capabilities to support new packet data services + enhanced signaling messages Base Station (BS) base transceiver station (BTS) + base station controller (BSC) compared to IS-95: similar basic functionalities, significant hardware and software changes to provide multimedia services Packet Control Function (PCF) belongs to radio access network manages the buffering and relay of packet between the BS and the PDSN Packet Data Service Node (PDSN) acting as a foreign agent (FA), providing routing services managing the radio-packet (R-P) interface and PPP sessions for mobile users Initiating authentication, authorization and accounting for the mobile user to the AAA server and receiving service parameters for the mobile user from the AAA server p. 70

21 Network Elements (2) Home Agent (HA) mobile IP registration packet forwarding Authentication, Authorization and Accounting (AAA) authentication: user and device identity verification for network access and userbased QoS requests, authentication to establish dynamic security associations between network entities authorization: has access to subscribers profiles, the device register and the operator's policy repository; decides whether a user or device is authorized for a particular service with a specific QoS accounting: collecting and storing the billing-related data concerning the offered services, their associated QoS and the multimedia resources requested and used y individual subscribers p. 71

22 Forward Channels p. 72 Common channels Dedicated channels Shared channel Radio Interface - Physical Layer Pilot channels: Pilot Channel (F-PICH) Transmit Diversity Pilot Channel (F-TDPICH) Auxiliary Pilot Channel (F-APICH) Auxiliary Transmit Diversity Channel (F-ATDPICH) Synchronization Channel (F-SYNCH) Broadcast channel: Broadcast Control Channel (F-BCCH) Paging channels: Paging Channel (F-PCH) Quick Paging Channel (F-QPCH) Control channels: Packet Data Control Channel (F-PDCCH) Common Assignment Channel (F-CACH) Common Power Control Channel (F-CPCCH) Common Control Channel (F-CCCH) Fundamental Channel (F-FCH) Supplemental Channel (F-SCH) Dedicated Control Channel (F-DCCH) Auxiliary Pilot Channel (F-APICH) Packet Data Channel (F-PDCH) Traffic channel Traffic channels

23 Forward Channels (1) Common Channels: carry information directed to one or more MS Dedicated Channels: assigned to one and only one MS p. 73

24 Traffic CH Forward Channels (2) F-FCH and F-SCH: user data F-DCCH: control + user data F-PDCH: bursty data, high speed, non-real time F-PDCCH: signaling support for F-PDCH F-CACH and F-CPCCH: support a form of reverse link access procedure (reservation access mode) F-PDCH Control CH the only shared channel similar to a dedicated channel in many physical characteristics, only a single MS decodes this channel at a time assigned to a MS by Layer 2 signaling, short assignment: 1.25, 2.5 or 5ms p. 74

25 Functional Construction Forward Channels (3) Modulation, coding and spreading (MCS) characteristics of physical channels vary greatly as the standard evolved In case of traffic channels, the fundamental functional building blocks are common to all channels Functional Block Diagram for Forward Traffic Channels CRC: Error detection Rate matching source bits Block encoder Conv. or Turbo encoder Symbol repetition or puncturing Interleaver long code cos(2πf c t) Scrambling Modulator Orthogonal spreading Quadrature spreading Filter s(t) p. 75 PN I PN Q sin(2πf c t)

26 Forward link data scrambling Functional Blocks (1) provides privacy of communication: scrambling sequences unique among users uses a long PN sequence, maximum length linear shift sequences (MLLSRS), sequence period applies a user-specific (for dedicated channels) or code channel-specific (for common channels) mask to get a unique sequence the masking process effectively shifts the MLLSRS, resulting in a unique long PN sequence p. 76 user-specific or code channel-specific

27 Forward link modulation Functional Blocks (2) forward link: strong common pilot signal, higher order modulation Forward link channel Modulation scheme F-PICH un-modulated known pilot symbols F-SYNCH F-PCH F-PDCH BPSK BPSK adaptive modulation with QPSK, 8PSK, 16QAM important, low BER to provide higher data rate when channel is good All Others QPSK p. 77

28 QPSK vs. BPSK Functional Blocks (3) Advantages of QPSK: higher bandwidth efficiency, increase coding gain p. 78

29 QPSK vs. BPSK (cont'd) Functional Blocks (4) Disadvantages of QPSK: more sensitive to inaccurate carrier-phase recovery, larger SINR degradation Advantages of BPSK: less sensitive to inaccurate carrier-phase recovery, smaller SINR degradation decision error correct decision carrier phase error carrier phase error QPSK BPSK Forward link: carrier phase estimated by higher powered F-PICH, resulting in small phase error => QPSK is better than BPSK p. 79

30 Forward link orthogonal spreading Functional Blocks (5) orthogonal code: Walsh code spreading factor: 4~128, exceptions: F-APICH and F-ATDPICH: 512 achieves bandwidth expansion Source: V. K. Garg, IS-95 CDMA and cdma2000 at most 61 length-64 Walsh codes for traffic transmission p. 80

31 Functional Blocks (6) Forward link quadrature spreading (complex spreading) Purpose: separate different BSs and reduce multipath interference a pair of short PN sequences (2 15 ) are used: one for I branch and one for Q branch PN I (x)=x 15 +x 13 +x 9 +x 8 +x 7 +x 5 +1 PN Q (x)=x 15 +x 12 +x 11 +x 10 +x 6 +x 5 +x 4 +x 3 +1 all BSs use identical short PN sequences but distinct transmission timing offsets p. 81

32 Peak-to-average ratio Why is complex spreading necessary? time Peak Average The power amplifier can be used more efficiently by reducing the peakto-average ratio, resulting in reducing the power consumption of power amplifier Output power Back-off p. 82 Operation point Peak Input power 82

33 Transmitter Model of Complex Spreading g k C P, k ( n) S ( n) js ( n) I + Q Re( ) h(t) E k cos ( ω t + φ ) 0 k S k ( t) jd k ( m) C, ( n) D k complex spreading Im( ) h(t) Ek sin 0 ( ω t + φ ) k It can reduce the signal envelope variation, due to the correlation of Re(.) and Im(.) p

34 Transmitter Model of Dual-Channel Spreading g k C P, k ( n) h(t) S I (n) E cos( ω t + φ ) ( t) k 0 k S k D k ( m) C, ( n) D k S Q (n) h(t) Ek sin 0 ( ω t + φ ) k I and Q channels are uncorrelated. p

35 Normalized Deviation of Envelope Complex spreading Dual-channel spreading 0.35 ND I/Q unbalanced power ratio -- g Normalized deviation of complex spreading and dual-channel spreading signals p

36 Transmit diversity Forward Link Transmit Diversity two antennas are used to transmit the same code channel achieve increased diversity gain, improve forward-link performance Two modes Orthogonal Transmit Diversity (OTD): take advantage of the decoding process, achieve diversity in the Viterbi decoder path metrics, code dependent, the more powerful the code, the better the performance Space Time Spreading (STS): based on Alamouti code 2T d k (n+1) T d k (n) 0 OTD encoder 2T d k (n) T d k (n) -d k (n+1) d k (n+1) 0 Source: A. Dabak, S. Hosur, T. Schmidl, and C. Sengupta, A comparison of open loop transmit diversity schemes for third generation wireless systems, in Proc. WCNC, vol. 1, T d k (n+1) T d k (n) 0 STS encoder 2T d k (n) -[d k (n+1) ] * d k (n+1) -[d k (n) ] * T d k (n) + [d k (n+1) ] * d k (n+1) + [d k (n) ] * 0 p. 86

37 Reverse Channels (1) Common channels Access Channel (R-ACH) Enhanced Access Channel (R-EACH) Common Control Channel (R-CCCH) Reverse Channels Dedicated channels Fundamental Channel (R-FCH) Traffic Supplemental Channel (R-SCH) channels Dedicated Control Channel (R-DCCH) Pilot Channel (R-PICH) Channel Quality Indicator Channel (R-CQICH) Acknowledgment Channel (R-ACKCH) p. 87

38 Reverse Channels (2) Common Channels employ a base station-specific PN sequence, simultaneous transmissions by multiple MSs on the same common channel cannot be discriminated, possibility for collisions contention-based channels, use specialized random access protocols carry signaling in support of registration, authentication, call origination, or small amounts of user data Dedicated Channels spread by mobile-specific PN sequences, can be discriminated at BS R-FCH, R-DCCH: carry signaling or user traffic R-SCH: carry only user traffic R-CQICH and R-ACKCH: in support of F-PDCH (Forward-Packet Data CHannels) p. 88

39 Reverse Channels (3) Functional Block Diagram for Reverse Traffic Channels CRC: Error detection Rate matching source bits Block encoder Conv. or Turbo encoder Symbol repetition or puncturing Interleaver long code cos(2πf c t) Orthogonal spreading Quadrature spreading Filter s(t) Other channels' modulation symbols PN I PN Q sin(2πf c t) p. 89

40 Reverse link modulation Functional Blocks (1) reverse link: dedicated pilot channel, code-multiplexed with data symbols, limited transmit power, carrier-phase estimation error is large => BPSK modulation Reverse link orthogonal spreading orthogonal code: Walsh code, spreading factor: 2~64 p. 90

41 Functional Blocks (2) R-SCH 2 W 1,0 R-SCH 1 W 2,0 W 2,1 W 4,0 W 4,2 W 4,1 W 4,3 W 8,0 W 8,4 W 8,2 W 8,6 W 8,1 W 8,5 W 8,3 W 8,7 W 16,0 W 16,8 W 16,4 W 16,12 W 16,2 W 16,10 W 16,6 W 16,14 W 16,1 W 16,9 W 16,5 W 16,13 W 16,3 W 16,11 W 16,7 W 16,15 W 32,0 W 32,16 R-PICH R-DCCH R-FCH R-CQICH W 64,0 W 64,32 W 64,16 W R-ACKCH 64,48 R-EACH or R-CCCH Tree Structure of Walsh functions for reverse physical channels p. 91

42 Functional Blocks (3) Reverse link quadrature spreading Purpose: separate different MSs and code channels a pair of short PN sequences (2 15 ) are used: one for I branch and one for Q branch a long code mask is needed to discriminate users' signals at the BS PN I ' PN Q ' Same as those used in forward link PN I ' PN Q ' p. 92

43 Hard handoff Handoff (1) interfrequency handoff CDMA2000: operate on multiple carriers, sometimes require a MS's handoff to a different carrier Soft handoff forward link: multiple BSs transmit identical traffic channels data symbols, the MS combines the demodulated signals prior to frame decoding, spatial diversity reverse link: multiple BSs receive same data symbols from the MS, decode independently and send to BSC, BSC chooses the one with the highest quality, selection combining, weaker spatial diversity Softer handoff forward link: same as that in soft handoff reverse link: some BSs are with the same BTS, the BTS can combine the demodulated signals prior to frame decoding p. 93

44 Softer handoff Handoff (2) when MS has n+m BSs in the active set, n of them corresponds to different BTSs, the MS is in n-way soft, m-way softer handoff two way soft handoff BTS1 BTS2 p. 94 one way soft two way softer handoff two way softer handoff

45 Review Power Control (1) Power control is especially important in CDMA systems. Reverse link power control consists of open loop and close loop power controls, two control loops operate concurrently (different to WCDMA) Open loop: long-term channel variation (distance, shadowing) Close loop: short-term channel variation (fast fading) and open loop inaccuracy Reverse link open loop power control Principle: the larger the received signal strength, the smaller the transmission loss between MS and BS is expected to be, and vice versa MS sets its transmit power inversely proportional to the total received power p. 95

46 Reverse link close loop power control Power Control (2) cooperate with BS: inner loop and outer loop MS receives power control commands on the F-PCSCH power control step size: 1dB, frequency: 800Hz (same as IS-95) close loop power control - inner loop BS estimates the received SNR on R-PICH, compares it against that allocated to the MS (target), and sets the power control bit close loop power control - outer loop tracks QoS and adjust the R-PICH target SNR used by the inner loop A measure of QoS: R-FCH FER close loop imperfections power control commands received in error: power control commands are transmitted uncoded to minimize the turnaround time incorrect SNR estimate: the received R-PICH SNR is only on average equal to the target but exhibits fluctuations that are more pronounced when the MS moves at high speed (fast fading) p. 96

47 Power Control (3) Reverse link power control in soft handoff Soft handoff: multiple BSs receive the identical signals from the MS, each BS sends a power control command based on the received R-PICH SNR, MS receives different commands Rule: or-of-the-downs, the MS increase the transmit power if and only if all received power control commands are up commands; the MS is power controlled by the BS with the best reverse link maximizes reverse-link capacity (low transmit power, low interference to the system), guarantees that the MS can close the reverse link at all times with at least one BS Reverse link power control in softer handoff Softer handoff: the active set includes two or more BSs with the same BTS, the BTS soft-combines the received signals of these BSs inner loop power control: based on the combined received SNR same power control command is sent from these BSs to the MS; MS combines the power control subchannels to obtain a single power-control command p. 97

48 Forward link power control Power Control (4) consists of open loop and close loop power controls (different to WCDMA) Forward link open-loop power control used when the forward traffic channel is initialized (e.g., call setup), the BS selects its initial transmit power level and initializes the code channel gain, no reference, implementation dependent simplest solution: initialize the transmit power to the maximum allowable level to maximize call setup reliability done once and only once at call set up (different to reverse link) Forward link close-loop power control activated when the MS receives two consecutive good frames at call set up and starts reverse link transmission BS receives power control commands on the R-PCSCH and adjusts the transmit power accordingly p. 98

49 Power Control (5) Forward link close-loop power control: inner loop MS estimates the SNR of received F-FCH, compares it against the target value, and sets the power control command power control rate: 800Hz Forward link close-loop power control: outer loop BS sends the target F-FCH FER to MS; MS measures the FER, compares it against the target FER, and decides the target SNR value of F-FCH implementation dependent Forward link power control in soft handoff MS sends the power control command to multiple BSs in the handoff active set due to transmission error, some BSs may receive an up command while others may get a down command => a drift of the F-FCH transmit power of one BS relative to the other degrades MS receiver performance => researches on reducing the transmit power drift p. 99

50 Summary - Link Parameters for cdma2000 Copied from T. Ojanpera and R. Prasad, WCDMA: Towards IP Mobility... p. 100

51 Summary - Link Parameters for cdma2000 (Con't) cdma2000 1x cdma2000 3x p. 101

52 Comparison between WCDMA & cdma2000 (1) 5MHz WCDMA 1.25, 5MHz CDMA Mchips/s Mchips/s for direct spreading Mchips/s for multicarrier spreading p. 102 Adapted from T. Ojanpera and R. Prasad, An overview of air interface multiple access for IMT-2000 / UMTS, IEEE Communication Magazine, vol. 36, p. 85, Sep

53 Comparison between WCDMA & cdma2000 (2) WCDMA CDMA2000 ( 3.84Mchips/s) ( 1.5KHz) Adapted from T. Ojanpera and R. Prasad, An overview of air interface multiple access for IMT-2000 / UMTS, IEEE Communication Magazine, vol. 36, p. 85, Sep p. 103

54 Summary of cdma2000 cdma2000 1xRTT and 3xRTT: bandwidth: 1.25MHz and 5MHz, respectively. cdma2000 system architecture: compare to that of IS-95 and WCDMA Understand the concepts of common channel, dedicated channel and shared channel. Detailed names and functions of specific channels: not requested. Why do different channels employ different modulation schemes? Comparison between QPSK and BPSK. Understand complex spreading and dual channel spreading: diagrams, comparisons. Transmit diversity: compared to that of WCDMA. Handoff and power control: similar to IS95 with some new operations. Measures or techniques employed in physical layer (uplink/downlink) of cdma2000 to increase the transmission rate compared to IS95: spreading factors, channel coding, modulation, multi-code transmission. p. 104