CDMA truy cập và chuyển mạch P4

CDMA: Access and Switching: For Terrestrial and Satellite Networks Diakoumis Gerakoulis, Evaggelos Geraniotis Copyright © 2001 John Wiley & Sons Ltd ISBNs: 0-471-49184-5 (Hardback); 0-470-84169-9 (Electronic) 4 Code Division Switching 4.1 Overview In this chapter we present and analyze the switchingarchitecture of the exchange node for switched CDMA networks. As we have discussed in the previous chapter, in such a network CDMA traffic channels will be routed by the exchange node from any input to any output link. If we consider a traditional switchingapproach, the exchaneg node can be implemented as shown in Figure 4.1. In this case we assume that Time Multiplexed Switching(TMS) is used to provide the switch functions. As shown, after the despreadingoperation, all CDMA user channels are time multiplexed, then routed to the destination output port by the TMS, demultiplexed, spread again, and then combined for the output CDMA channel. The TMS approach, however, introduces additional complexities, because the switch input and output ports require time multiplexing, while the incoming and outgoing signal is based on code multiplexing. (In traditional switching methods such as time slot interchangers or space switching, traffic channels are time multiplexed in each input or output port.) Also, the complexity for a strictly nonblockingTMS switch fabric is singificant. This means that in applications such as SS/CDMA where the available power and mass at the satellite are limited, TMS may not be an efficient switching approach. Therefore, we propose an alternative switchingmethod which is based on code division. That is, the signals in the switch are distinguished and routed according to their spreadingcodes. This method is directly applicable in all switched CDMA networks such as SS/CDMA, BS/CDMA or CS/CDMA. In this chapter we provide illustrative Code Division Switch (CDS) architectures, performance and complexity evaluation analysis and comparisons with traditional switchingmethods. As shown, the proposed CDS architecture is nonblockingand its hardware complexity and speed is proportional to the size of the switch. Also, the CDS routes the CDMA user channels without introducinginterference. The switch performance evaluation includes the amplitude distribution of the combined signal in the CDS bus and the interference evaluation of the end-to-end link in the proposed network applications. The code division switch performance evaluation will utilize the satellite switching (SS/CDMA) as a basis for study. This work was originally presented in references [1] and [2]. 84 CDMA: ACCESS AND SWITCHING NxN Receiver Switch Transmitter Input Link 1 RF/BB DESPR : M U X DESPR D SPREAD E M U X SPREAD Outpu Σ BB/RF Link 1 TMS : DESPR N RF/BB : DESPR M U X D SPREAD E M U X SPREAD Σ BB/RF N Figure 4.1 The exchange node in a SW/CDMA using TMS. 4.2 Switched CDMA (SW/CDMA) Architectures In this section we examine the network and switch architectures in SS/CDMA and SW/CDMA for terrestrial wireless and cable applications. We also examine traditional switch architectures (such as the TMS) for routingCDMA channels, and present a CDS method for routingtime multiplexed channels. 4.2.1 Satellite Switched CDMA (SS/CDMA)System As we have described in the previous chapter, the on-board design of a SS/CDMA system provides the CDS modules, the switch control unit and the transceivers of the control channels (Access and Broadcast). The switchingand control architecture at the exchange node on board the satellite is illustrated in Figure 4.2. Traffic channels are routed from uplink to downlink beams via the switch modules without data decodingon board the satellite. The Traffic channel modulation and spreadingprocesses are based on the Spectrally Efficient CDMA (SE-CDMA) which are illustrated in Figures 3.27 and 3.28 of Chapter 3. The SE-CDMA spreading process requires the followingcodes: (1) a set of orthogonal codes wk havinga chip rate Rc1 assigned to satellite users k = 1,2,..,Lu within each beam; (2) pseudo-random (PN) codes ci with a chip rate Rc1 assigned to satellite beams i = 1,2,...N; and (3) a set of orthogonal codes wi with a chip rate Rc2 for orthogonal isolation of Lb satellite beams, i = 1,2,... ,Lb. The PN-codes spreadingrate Rc1 is the same as the rate of the user orthogonal codes wk. The orthogonal codes wi, however, require a higher spreading rate Rc2 = LbRc1. The process of spreading a previously spread signal at a higher rate is called CODE DIVISION SWITCHING Access Control Channels Beam (1) 85 Broadcast Control Channels Beam (1) R Beam (i) C V T CONTROL R UNIT N Beam (j) Beam (N) Uplink Traffic Channels Beam (1) Beam (N) Downlink Traffic Channels Beam (1) NXN Beam (i) CODE DIVISION SWITCH (CDS) MODULES Beam (j) Beam (N) Beam (N) Figure 4.2 The CDS control system. overspreading (see Chapter 1, Section 1.4.2). When Lb = 4 the system is called a Fully Orthogonal (FO), when Lb = 2, a Mostly Orthogonal (MO), and when Lb = 1 (i.e. Rc1 = Rc2 = Rc) is called it Semi-Orthogonal (SO) SE-CDMA. Hence, the SE-CDMA will eliminate the interference between users within each beam, as well as between the Lb beams in the cluster, while it allows a frequency reuse of one. In a particular implementation, presented in Appendix 4A, Rc2 = 9.8304 Mc/s and Lu = 60. Also, the orthogonal codes can be either Quadratic Residue (QR) codes or Walsh codes when the length L = 2k. The Code Division Switch (CDS) The proposed CDS architecture is shown in Figure 4.3. Each uplink CDMA channel is first converted into an Intermediate Frequency (IF) and then into baseband (BB) without demodulatingthe incomingsignal (switchingat IF has also been considered). After that, the signal is despread by the uplink orthogonal beam code wi and the PN beam code ci (see Figure 4.4-A). Each particular user signal is then recovered by the Traffic Channel Recovery Circuit (TCRC) shown in Figure 4.5. This is achieved by despreadingwith the user’s uplink orthogonal code wk. The signal will then be respread with the user (wm) and beam (cj, wj) downlink codes. Finally, the signal will be overspread again by an orthogonal (switch) code wn (n = 1,2,..,Ls), havinga chip rate Rc3 = LsRc2. This step of overspreadingwill achieve orthogonal separation of all user Traffic channels in the system, and thus can be combined (summed up) into a common bus. The number of wn codes, Ls, is equal to the number N of switch ports (Ls = N), if no prior orthogonal separation between uplink beams exists. In such a case the rate is Rc3 = N ·Rc1. The SE-CDMA scheme, however (shown in Figures 3.27 and 3.28), has the Lb beams already orthogonalized. Hence, Ls = N/Lb and Rc3 = (N/Lb) · Rc2. Each uplink beam in the cluster will 86 CDMA: ACCESS AND SWITCHING From the CU UPLINK BEAM-1 RF/BB TCRC-1 Beam Despread TCRC-L CDB De-overspreading DOWNLINK BEAM-1 BB/RF on From the CU Σ BEAM- N RF/BB TCRC-1 Beam Despread TCRC-L BEAM-N De-overspreading BB/RF CDB: Code Division Bus Figure 4.3 The Code Division Switch (CDS) module. then be overspread by the same wn orthogonal code (n = 1,2... ,N/Lb). For Lb = 4 (FO/SE-CDMA), N = 32 and Ls = 8, the chip rate is Rc3 = 78.6432 Mc/s. (See the example presented in Appendix 4A.) The I and Q components are combined (summed-up) in parallel by two separate adders (in the case where both I and Q are summed, the rate will be Rc3 = 2N ·Rc1). The steps of overspreading, the codes involved, and the correspondingchip rates for this application are shown in Figure 4.6. After overspreading, all incoming (I or Q) signals are combined (summed up) into a (I or Q) bit stream called a Code Division Bus (CDB). The CDB then contains all Traffic channels spread by their correspondingdownlink user and beam destination codes. Hence, each downlink beam may be recovered by the de-overspreadingcircuit shown in Figure 4.4-B, and routed to its destination port. The signal will then be converted into an IF, and subsequently into an RF frequency for downlink transmission. The set of all codes in the TCRCs for routingthe Traffic channels to their destinations are supplied by a Control Unit (CU). The number of TCRCs required in each beam is Lu, and is equal to the number of Traffic channels per beam (beam capacity), so that no blockingoccurs in the switch. Also, uplink orthogonal codes, wk and wi, require synchronization in order to maintain orthogonality. This is achieved by a synchronization mechanism which adjusts the transmission time of each user so that all codes are perfectly aligned upon reception at the TCRC despreaders. An equivalent functional arrangement of the code division switch is shown in Figure 4.7. The corresponding circuits for Traffic channel recovery and respreading are shown in Figure 4.8. In this architecture the incoming signal, after conversion to baseband, is despread by the uplink beam orthogonal code (beam recovery), and CODE DIVISION SWITCHING A The Beam Despreader Rc2 87 òLbTc2 LbTc2 Rc1 0 Rc2 òLbTc2 Rc1 0 Wi Ci B The De-overspreading circuit Rc3 Rc3 òLsTc3 LsTc3 Rc2 0 òLsTc3 LsTc3 Rc2 0 Wn Figure 4.4 The beam-despreading and the de-overspreading circuits. then overspread so that it can be combined (summed up) into the Code Division Bus (CDB). Overspreadingby the switch codes wn allows orthogonal separation in the CDB between all uplink beams or incomingswitch inputs. The beam recovery and overspreading(BR&OS) operation is illustrated in Figure 4.8-A. A Traffic channel recovery and respreading(TCR&RS) circuit recovers the desired Traffic channel from the CDB by de-overspreading its signal with the corresponding switch orthogonal code (wn, n = 1,..,n), and then despreadingit with the uplink user code wk. After recovery, Traffic channels are routed to the desired downlink beam (output port) by respreading them with the correspondingdestination user ( wm) and beam (cj,wj) codes. The TCR&RS circuit is shown in Figure 4.8-B. At the output, all TCR&RS circuits having the same destination beam will be combined (summed up) and converted into the RF carrier for downlink transmission. Each output beam requires Lu TCR&RS circuits equal to the maximum number of Traffic channels per beam. Comparingthe two architectures presented above (Figures 4.3 and 4.7), we observe that both of them perform the same functions, but in a different order. In the first configuration (Figure 4.3), Traffic Channel Recovery (TCR) takes place before channels are combined into the CDB, while in the alternative configuration (Figure 4.7), TCR takes place after the CDB. In the alternative configuration, only beam recovery takes place before the CDB to the rate Rc1 = LuRs. In both cases, the CDB has the same rate which is Rc3 (Rc3 = NRc1 = LsRc2 and Ls = N/Lb). The relation between chip rates is shown in Figure 4.6. Performance comparisons between the above CDS configurations are provided in Section 4.3. In the above CDS architectures, the baseband signal (i.e. the output of the RF to baseband converter for any M-ary PSK scheme, M ≥ 4), has two components, I ... - tailieumienphi.vn