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LAB: 6 W-CDMA Downlink Signal Processing Introduction W-CDMA technology allows multiple users to share efficiently the same RF carrier frequency by dynamically adjusting the data rates and wireless link to meet the demands of each user. It is a technology based on code division multiple access (CDMA), thus all users transmit at the same time as opposed to time division multiple access (TDMA). Each user’s information signal uses a unique code for the communication purposes and appears to be noise to all with the exception to the correct receiver; hence, the need to correlate the appropriate channelization code. The following diagram illustrates the W-CDMA signal processing at the physical layer. Figure1 – Physical Signal Processing In W-CDMA, a scrambling code (SC) provides a unique identity to each UE and the Node B. The channelization code (OVSF) provides a unique identity to each communication channel established by a UE or Node B within a cell. Figure2 – W-CDMA Scrambling Codes Lab 6: W-CDMA DL W2020 Prof. Carl Hassanali and Imran Muhammad Page 1

The data channels in W-CDMA are segmented into time slots and frames. One W-CDMA frame is 10 ms long and is the basic unit of time supplementary with a channel coding and interleaving process. Figure3 – W-CDMA Frame Figures 3 shows the slot and frame structure for the W-CDMA Physical signaling and control channels. Each Node B is identified by a scrambling code. The signaling channel CPICH provides the identification to the Node B and as well, serve as a reference point for measurement purposes. The primary common control physical channel (P-CCPCH), see Figure 4, is another important channel used by the UE during the initial system acquisition. This channel is multiplexed with two sub-channels, Primary Synchronization (P-SCH) and Secondary Synchronization Channel (SSCH). These channels consist of two codes known as PSC and SSC. The codes are there to establish if the Node B is present and establish the slot boundary timing. Figure4 – W-CDMA Time Slot The user data and signaling information is carried over the Dedicated Physical Channel (DPCH). This channel is also used for the slot format and as well as for the UE power control. The user’s data and layer 3 (Network) signaling data are carried on DPDCH. The DPDCH is multiplexed with DPCCH, which contains the Transmit Power Control (TPC) bits and the Transport Format Lab 6: W-CDMA DL W2020 Prof. Carl Hassanali and Imran Muhammad Page 2

Combination Indicator (TFCI) bits (it indicates the slot format and data rate). The Pilot bits are used for short synchronization patterns embedded within each slot. Figure5 – Dedicated Physical Channel (DPCH) Format Equipment: Agilent 89600 installed on a computer . Procedure: 1. Preset the software Click File > Preset > Preset All 2. Recall the recording of a W-CDMA downlink signal Click File > Recall > Recall Recording 3. Navigate to the directory and load the signal: ( C:\Program Files\Keysight89600 Software \Help\Signals\WCDMA-HSPA) 3GPPDown.sdf 4. Start the measurement: Click the restart button from the toolbar. 5. Auto scale the Spectrum and Main Time traces. Auto scale Trace A Right click Trace A Select Y Auto Scale Auto scale Trace B Right click Trace B Select Y Auto Scale 6. Set the spectral and time measurement parameters such as range, scaling, center frequency and span if not already set in the signal analyzer Lab 6: W-CDMA DL W2020 Prof. Carl Hassanali and Imran Muhammad Page 3

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Measuring Occupied Bandwidth 1. Display OBW marker: Right click the Trace A and select Show OBW 2. Activate OBW Summary table: Right click the Trace B title (B: Ch1 Main Time) Select Marker from the Type menu on the left-hand side of the box Select Obw Summary Trace A from the menu 3. Pause the measurement to read the table values OBW 4.6716 MHz Power -0.792 dBm Total Power -0.792 dBm Power Ratio 00.999% Upper Freq 1002340156.19 Hz Lower Freq 997668584.69 Hz Table1 – OBW Measurements Basic Digital Demodulation: Once the signal is verified for spectral or time domain inconsistencies, configure the signal analyzer to demodulate the recorded signal. 1. Select the demodulator: Click MeasSetup > Measurement Type > Cellular > WCDMA(3GPP)/HSPA 2. Preset the demodulator parameters for downlink analysis: Click MeasSetup > WCDMA (3GPP)/HSPA Demod Properties > Format (tab) > Preset to Standard > Direction: Downlink Click Close 3. The digital demod Preset to Default feature automatically configure the analyzer for the WCDMA demodulation format and the physical layer properties. 4. Change display layout to Grid 3x2: Click Window > Trace Layout > Grid 3x2 (Alternatively, you can click on the drop down menu near the top of the menu toolbar. Select Grid 3x2 from the available options). 5. Restart the measurement: Click the Restart button 6. Auto scale Traces A, B and C: Right click on Trace A and Select Y Auto Scale. Repeat the same procedure for Trace B and C Lab 6: W-CDMA DL W2020 Prof. Carl Hassanali and Imran Muhammad Page 4

7. Pause the measurement using (||) button on the toolbar to read the table results and analyze the demodulated signal 8. Identify the physical layer parameters that are used to demodulate the WCDMA recorded signal. Click MeasSetup > WCDMA (3GPP)/HSPA Demod Properties Under the Format (tab), Record: Direction: Scrambling Code: 0 Chip Rate: 3840000 Hz Alpha: 0.22 Synchronization Type: Automatic Sync. Start Slot: Automatic Under the Channel/Layer (tab), Record: Despread Channel: Spread Code Length: 256 (15ksym/s) CDP Layer: Spread Code Length: 256 (15ksym/s) Under the Time (tab), Record: Result Length: 15 slots Measurement Offset: 0 slots Measurement Interval: 1Slots 9. Copy and Paste the composite CDP plot and place the markers indicating the signaling and data channels on your diagram. Lab 6: W-CDMA DL W2020 Prof. Carl Hassanali and Imran Muhammad Page 5

Record the x-axis units: Power Record the y-axis units: Code 10. Right click on the very first channel and add the marker and place it on top the first orange bar – This is a Common Pilot Channel (CPICH) in channel 0. . Record the value of Trace A marker: Symbol rate: _15ksym/s________________ Spreading Factor, SF = _256________ Code (value in the bracket): 0 Power level: -10.028 dB Why is the CPICH Channel the very first channel in the Composite CDP plot? The Primary Common Pilot Channel is used by the UEs to first complete identification of the Primary Scrambling Code used for scrambling Primary Common Control Physical Channel (P- CCPCH ) transmissions from the Node B . Lab 6: W-CDMA DL W2020 Prof. Carl Hassanali and Imran Muhammad Page 6

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What is the relationship between the CPICH Channel Power Level compared to the Total Transmit Power? the CPICH power is 10% of the typical total transmit power of 43 dBm . 11. Move the marker to code 1 which is the second yellow bar from the left side. This is a Primary Common Control Physical Channel (P-CCPCH) transmission from Node B. Record the values associated with this new marker. Symbol rate: 15kysm/s Spreading Factor, SF = 256 Code (value in the bracket): 1 Power level: -10.009 What are the purposes of the P-CCPCH? It is used to carry synchronization and broadcast information for users. 12. Now, move the marker to code 16 which is the third orange bar from the left side. This is a fixed rate SF = 256 Paging Indicator Channel (PICH) and carries paging indicators on the physical channel. Record the values associated with this new marker. Symbol rate: 15kysm/s Spreading Factor, SF = 256 Code (value in the bracket): 16 Power level: -15.014 dB 13. Finally, move the marker to any of the other channel with blue bars in the CDP plot. These are Dedicated Physical Data Channel (DPDCH) carrying data and voice load in the downlink direction. Record the values associated with this new marker. Symbol rate: 30kysm/s Spreading Factor, SF = 128 Code (value in the bracket): 11 Power level: -12.015 d B Lab 6: W-CDMA DL W2020 Prof. Carl Hassanali and Imran Muhammad Page 7

14. Pause the running signal and Record the additional information of demodulated downlink signal. i) What type of modulation scheme is used for the downlink? QPSK ii) How many possible states are there for each RF symbol? 128 iii) How many bits are encoded per symbol? 4 iv) What are the total numbers of Dedicated Data Channels in the downlink direction? 134 v) What is the composite EVM value and what does it represent? 270.56 M%RMS vi) What is the Peak Active CDE and what does it represent? -50.34 dB vii) How many slots are there in W-CDMA frame? 0 15. Move the marker on the Dedicated Physical Data Channel (DPDCH) to have a more in-depth investigation. a) Place markers on the IQ Meas Time traces. b) Click on the composite CDP to make it active. c) Couple all the markers: Markers  Coupled Markers d) Now despread the marker: Markers  Copy Marker to  Copy Marker to Despread Channel What is the Mod. Format? QPSK List the despread bits? 0 10010011 00100110 00011001 01100101 10001101 Using the despread bits, explain what you see on the IQ constellation diagram. A Qpsk IQ Diagram Lab 6: W-CDMA DL W2020 Prof. Carl Hassanali and Imran Muhammad Page 8

16. Now, let’s have a look CPICH channel in more depth. a) Using the marker to select code zero and place marker on the Composite CDP plot b) Place a marker on the IQ Meas Time trace c) Click on the Composite CDP to make it active d) Couple all the markers: Markers  Couple Markers e) Now despread the marker: Markers  Copy Marker to  Copy Marker to Despread Channel Describe your observations about the CPICH channel? It shows the marker on the orange bar standing first coupled to that of the composite CDP How many symbols are there in CPICH channel? 256 Why are all the bits for the CPICH channel zeros? Start Code = 0 Why is there no change in the state on the IQ diagram for the CPICH Channel? Coz of the same state Reference: http://literature.agilent. com/litweb/pdf/5980 - 1239E.pdf www.sharetechnote.com Lab 6: W-CDMA DL W2020 Prof. Carl Hassanali and Imran Muhammad Page 9

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