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ltewp

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Renshou Dai

This document provides an overview of how Sage Instrument's 8901A UCTT tool analyzes and measures metrics from 4G LTE signals. It can analyze signals from 1.4MHz to 20MHz bandwidth at up to 10 measurements per second. The tool automatically detects the number of transmit antenna ports and measures various signal characteristics including power levels, modulation quality, control channels, and physical channel allocation. It presents the LTE signals across several views to analyze aspects like frequency response, power levels over time, and signal constellations.

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Analyzing 4G LTE Signals using Sage UCTT Renshou Dai May 15, 2013 Abstract This white paper provides the technical background for all the LTE performance metrics mea- sured by Sage Instrument’s 8901A UCTT (Universal Cellular Test Tool). With its broad analysis bandwidth and deep acquisition memory space, Sage UCTT continuously performs rapid analysis on LTE signals of all bandwidths (1.4, 3, 5, 10, 15 and 20 MHz) over a minimum duration of 10 ms frame period. The measurement throughput is as high as 10 times a second. With synchronized data capturing, Sage UCTT can also perform seamless analysis on aggregated LTE channels (up to N × 20). UCTT automatically detects the number of TX antenna ports present at a given cell site, and automatically performs corresponding N × 1 transmit diversity decoupling using the N sets of channel models obtained from the N sets of Reference Signals associated with each specific TX antenna port. The metrics reported by UCTT are: total-power, frequency-offset, cell-ID, RSSI, RSRP, RSRQ, power and EVMs of all 3 signals (Reference, Primary and Secondary Synchronization Signals), power and EVMs of all downlink control channels (PBCH, PCFICH, PHICH and PDCCH), and it also keeps track of and measures the dynamically-allocated PDSCH channel where the actual user payload data reside. UCTT also scans for all available cell-IDs in an area (reports up to 9). For a given sector, UCTT also reports the number of TX antenna ports detected, and the RS (Reference Signal) level and quality associated with each TX antenna port are reported. The relative delays among them are measured. The Reference Signal from any antenna port selected by the user is also used to estimate the impulse response of the multi-path environment. UCTT presents 5 different views of the LTE signals: the spectral view, the Resource-Block-power frame view, a view of all subcarriers for any symbol, a view of specific subcarriers in the entire 10 ms frame, and the IQ constellation of all the 3 signals, 4 control channels and PDSCH channel. SEM (Spectral Emission Mask) and ACLR (Adjacent-Channel-Level-Ratio) are also measured. By decoding key control chan- nels such as PBCH, UCTT also reports the data content of the MIB (Master-Information-Block) and the number of detected bit errors, correctible errors and uncorrectible errors by exploiting the 3-1 convolutional encoding structure and the CRC checking. 1 Overview of LTE Signal Characteristics At the downlink direction, LTE uses OFDM (Orthogonal Frequency Division Multiplexing) modulation. In time domain, the signal consists of discrete symbols. Each symbol is the sum of N orthogonal subcarriers. Each subcarrier’s phase and amplitude carries information about the modulating bits. Knowledge of the subcarrier spacing, symbol duration, frame, timeslot, subframe, resource block and resource element is essential for quantitative understanding LTE signals at the physical layer. 1.1 Time and frequency characteristics The OFDM subcarrier spacing is 15 KHz. To maintain frequency orthogonality, the minimum symbol duration must be 1 15 ms. In fact, this is exactly the symbol duration used in LTE. Within a 10 ms frame period, there theoretically should be 150 symbols. But to avoid inter-symbol interference due to multi-path propagation, a fraction of each symbol from the end is copied and attached to the beginning 1
of the same symbol (that is, each symbol is cyclically extended and prefixed to the beginning). Adding CP (Cyclix Prefix) effectively increases the symbol duration, hence reduces the number of symbols per 10 ms frame to 140 (from theoretical 150). With extended CP, the number further comes down to 120. The 10 ms frame is divided into 10 subframes, each 1 ms long, and 20 timeslots, each 0.5 ms long. Each subframe contains 2 timeslots. With normal CP length, each timeslot contains 7 symbols. With extended CP, each timeslot contains 6 symbols. Table 1 summarizes the durations of frame, subframe, timeslot, symbol and CPs. Table 1: LTE signal time characteristics Frame duration 10 ms Subframe duration 1 ms Timeslot duration 0.5 ms Symbol duration 1 15 ms Normal CP duration 3 640ms Normal CP of 1st symbol of each timeslot 1 192ms Extended CP duration 1 60 ms Each symbol consists of N number of subcarriers, depending on the operation bandwidth. The midcarrier (corresponding to the actual operating carrier frequency of 751 MHz, for example) is skipped (not used) to avoid carrier leak-through, a common RF hardware problem. The number of subcarriers is always a multiple of 12. 12 subcarriers within a time-slot period forms an RB (Resource Block). Table 2 lists the operating bandwidth, number of RBs and sub-carriers. Table 2: Possible LTE channel bandwidth configugrations Nominal Bandwidth (MHz) 1.4 3 5 10 15 20 Resource Blocks 6 15 25 50 75 100 Subcarriers 72 180 300 600 900 1200 Actual BW (MHz) 1.08 2.7 4.5 9 13.5 18 1.2 Resource element (RE), Resource element group (Reg) and Resource Block Each subcarrier within a symbol is a Resource Element (RE). This is the atomic unit (can not be divided further) that is modulated (phase and amplitude wise) by the information bits. All LTE physical signals and downlink channel’s power are normalized to the average power per RE. The Primary and Secondary Synchronization Signals (PSS, SSS, details later) are mapped to the center 62 REs every 5 ms. The Reference Signal (RS) from antenna port 0 or 1 uses every 6 REs on symbol 0 and 4 of each timeslot; the RS from antenna port 2 or 3 uses every 6 REs on symbol 1 of each timeslot. The leading 1, 2, 3 or 4 symbols of each subframe contains control channels. Each control channel is mapped to a group of REs according to 3GPP TS36-211, section 6.2.4. The group of REs is Resource element group (Reg). The REs not assigned to PSS, SSS, RS and control channels are used for PDSCH channel where user data payload actually resides. The remaining available REs for PDSCH are organized into Resource Blocks (RB). An RB is 12 consecutive subcarriers wide by one timeslot long (7 or 6 symbols, depending 2
on CP length). In reality, the PDSCH for an user is always mapped to 2 adjacent (time-wise) RBs within a single subframe (2 timeslots). 1.3 PSS, SSS signals The PSS and SSS (Primary and Secondary Synchronization Signals) not only provide the framing (synchronization) information for the UE (User Equipment), they also encode the cell ID number. There are 3 possible PSS signals, generated from the frequency-domain Zadoff-Chu sequences with 3 different root indices. Each index encodes one of the 3 sector IDs (N (2) ID = 0, 1, 2). For FDD (structure type 1), the PSS is mapped to the center 62 REs at the last symbol of timeslot 0 and 10. For TDD (structure type 2), the PSS is mapped to the 3rd symbol of subframes 1 and 6. The 5 REs below and 5 REs above the PSS REs are intentionally reserved and not used. There are 168 possible SSS sequences, each encoding one of the 168 possible cell identity group ID (N (1) ID = 0, 1, . . . , 167). For FDD, the SSS is mapped to the center 62 REs at the symbol before the last of timeslots 0 and 10. For TDD, the SSS is mapped to the center 62 REs of the last symbol of timeslots 1 and 11. Like PSS, the 5 REs below and 5 REs above the SSS are reserved and not used. The cell ID is then 3N (1) ID + N (2) ID . The PSS repeats every 5 ms (occurs twice every 10 ms frame). The SSS also occurs every 5 ms, but the SSS sequence at the first 5 ms and the SSS sequence at the second 5 ms are different. This is designed so that the UE can not only decode the cell ID, it can also determine where the true 10 ms frame boundary is. 1.4 RS signals The cell-specific reference signals (RS) are indispensable for the normal functionality of LTE’s OFDM modulation. With signal TX antenna port, the UE needs the RS signal for channel estimation so that it can combat the frequency selective fading. With multiple TX antenna ports, the RS signals from each antenna port are used to form different sets of channel responses (from different TX antenna ports), and the responses are then used to ”decouple” the ”mixing” effects caused by transmit diversity or spatial multiplexing. The cell ID not only controls the RS signal’s random sequence generation, it also controls the RE mapping of the RS signals. That’s why the UE device must first detect the PSS and SSS to decode the cell ID and then it can go ahead find out where the RS signals are and what random signal sequences are needed to descramble them. For Sage UCTT, the RS signals are not only used for channel estimations, they are also used for finer synchronization (after coarse sync via PSS and SSS), also used for carrier frequency offset estimation and signal to noise and RSSI, RSRP and RSRQ measurements. The RS signals from a single TX antenna port are also used to estimate the multi-path impulse response. The RS signals are different TX antenna ports are used to estimate relative delays (synchronizations) among different TX antenna ports. Sage UCTT automatically detects the number of TX antenna ports being used via the RS signals associated with each antenna port (1, 2 or 4), and then forms the channel response estimation for each antenna port. 1.5 Physical Downlink Control Channels Sage UCTT provides measurements on all of the following downlink control channels: • PBCH: Physical Broadcast Channel • PCFICH: Physical Control Format Indicator Channel 3
• PHICH: Physical Hybrid ARQ Indicator Channel • PDCCH: Physical Downlink Control Channel All the above control channels are mapped to specific group of Regs (Resource element groups). For example, PBCH is mapped to the center 72 REs of the first 4 consecutive symbols of timeslot 1 that are not used for RS signals. Taking away the REs used by RS at symbol 0 and 1, there are a total 240 REs allocated for PBCH. PCFICH and PHICH are mapped to the Regs (controlled by cell ID) at the first symbol of each subframe. The PDCCH channel is mapped to Regs at up to the first 3 symbols of each subframe. The exact number of symbols used is indicated by the PCFICH channel at that subframe. All control channels are QPSK modulated except PHICH, which is BPSK. Transmit diversity is used if more than one TX antenna port is used. Sage UCTT goes through all the Regs associated with each channel, measure their powers averaged by the number of REs used. UCTT also uses the estimated channels responses from each TX antenna port to decouple the transmit diversity effect so that the original QPSK or BPSK modulation pattern will be revealed, and EVM measurements will be performed on each of them. Sage UCTT also decodes the PCFICH channel to determine the number of symbols used for PD- CCH, and also decodes the PBCH channel to uncover the MIB (Master Information Block), which indicates information about the actual number of Resource Blocks (bandwidth) used, the Ng num- ber which controls the mapping of PHICH channel and the frame number. By exploiting the built-in repetition of the MIB block inside PBCH and the 3-1 convolutional encoding structure and the CRC checking, UCTT also reports the detected number of bit errors, correctible errors and uncorrectible errors etc. 1.6 PDSCH channel The physical downlink shared channel (PDSCH) contains the actual user payload data. This is actually the most relevant channel from a user point of view, but it is also the most challenging one for a test instrument to track due to its dynamic nature. The PDSCH channel consists of blocks of data from multiple users. Each block is dynamically assigned with varying modulation schemes (QPSK, QAM16 or QAM64) and varying MIMO methods (could be single antenna port, 2 ot 4 TX antenna port transmit diversity or spatial multiplexity). Theoretically, all these information are contained within the PDCCH channel. UCTT’s ultimate goal is to decode the PDCCH channel so that it can firmly track the PDSCH. For now, UCTT searches through all busy Resource Blocks (two adjacent RBs within every subframe), and detects if they are transmitted via transmit diversity. If yes, UCTT will decouple them and the perform power end EVM measurements on the RBs used by PDSCH. 2 Measurement definitions We now go through each of the measurement screen of Sage UCTT’s LTE feature and provide detailed technical definitions for all the parameters. 2.1 Spectral view The spectral view is the first screen presented when a user enters the LTE measurement feature. Figure 1 and Figure 2 show two examples of actual 10 MHz LTE signal from the air. This screen presents the following measurement results: 4
• LTE detection indicator: When a valid LTE signal is present within 5 KHz from the center frequency, UCTT will display LTE Detected. If the signal is not a valid LTE signal, UCTT will display LTE signal NOT Detected. • Spectra: Two spectra are presented (yellow color and light green color). Both spectra have same 15 KHz resolution bandwidth, and both are obtained by averaging the whole 10 ms frame period with this crucial difference: the yellow-colored spectrum is ”unsynchronized” to the LTE symbol, whereas the light-green-colored spectrum is synchronized to the symbol interval. More specifically, the yellow one represents the typical spectrum seen by a typical spectrum analyzer. The green one is obtained by only transforming and averaging through LTE symbols. To achieve this, UCTT internally has determined the frame boundary via PSS, SSS and RS signals, and then analyzed and averaged the 140 symbol periods only (for FDD with normal CP). One see sharper edge at the green spectrum and the obvious mid-carrier that is not used. The purpose of the spectra is to show you the actual bandwidth of the signal, making sure a user has selected the correct bandwidth. For example, if a user selected ”10MHz LTE”, but the spectra indicate effective bandwidth less than 5 MHz, then obviously something is wrong. Either the user made wrong selection (should be 5 MHz LTE, for instance), or indeed, the cell site is transmitting signal of incorrect bandwidth. The other purpose is to quickly indicate the severity of frequency- selective fading. Figure 11 here is connected to a roof-top antenna through long cable. The spectra are more or less flat, indicating the fading is not an issue since the antenna is on the roof. Figure 2, on the other hand, was obtained from an indoor directional antenna. The complex indoor propagation environment shows much higher severity of the frequency-selective fading problem as the spectra are no long flat. • Frequency offset: Sage UCTT uses all the RS signals within the 10 ms to provide a reliable estimation of the carrier frequency offset. Since LTE uses the OFDM modulation, the subcarrier orthogonality is vital to its success. Carrier frequency offset or carrier frequency instability will affect the signal’s orthogonality, hence reducing the performance. Table 3 lists the standard requirements. Table 3: LTE signal minimum frequency offset requirement adapted from TS36.104 [4] BS class relative offset offset in Hz at 750 MHz Wide Area BS ±0.05 ppm ±37.5 Hz Local Area BS ±0.1 ppm ± 75 Hz Home BS ±0.25 ppm ±187.5 Hz • Span: • Bandwidth Spectral view offers a quick view of the frequency-selective fading problem and obvious technology bandwidth. Figure 1 shows the spectrum obtained from an antenna located on roof-top through a long cable, whereas Figure 2 shows the spectrum from an indoor directional antena. 5
Figure 1: Spectral view of an actual 10MHz LTE signal captured from a roof-top antenna. Figure 2: Spectral view of an actual 10MHz LTE signal captured from an in door directional antenna. One can see the obvious frequency-selective fading issue. 2.2 Summary screen 2.3 Cell-ID scanner References [1] 3GPP TS 36.211, ”Physical Channels and Modulation.” 6
[2] 3GPP TS 36.212, ”Multiplexing and channel coding.” [3] SGPP TS 36.141, ”Base Station conformance testing.” [4] SGPP TS 36.104, ”Base Station radio transmission and reception.” 7

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ltewp

  • 1. Analyzing 4G LTE Signals using Sage UCTT Renshou Dai May 15, 2013 Abstract This white paper provides the technical background for all the LTE performance metrics mea- sured by Sage Instrument’s 8901A UCTT (Universal Cellular Test Tool). With its broad analysis bandwidth and deep acquisition memory space, Sage UCTT continuously performs rapid analysis on LTE signals of all bandwidths (1.4, 3, 5, 10, 15 and 20 MHz) over a minimum duration of 10 ms frame period. The measurement throughput is as high as 10 times a second. With synchronized data capturing, Sage UCTT can also perform seamless analysis on aggregated LTE channels (up to N × 20). UCTT automatically detects the number of TX antenna ports present at a given cell site, and automatically performs corresponding N × 1 transmit diversity decoupling using the N sets of channel models obtained from the N sets of Reference Signals associated with each specific TX antenna port. The metrics reported by UCTT are: total-power, frequency-offset, cell-ID, RSSI, RSRP, RSRQ, power and EVMs of all 3 signals (Reference, Primary and Secondary Synchronization Signals), power and EVMs of all downlink control channels (PBCH, PCFICH, PHICH and PDCCH), and it also keeps track of and measures the dynamically-allocated PDSCH channel where the actual user payload data reside. UCTT also scans for all available cell-IDs in an area (reports up to 9). For a given sector, UCTT also reports the number of TX antenna ports detected, and the RS (Reference Signal) level and quality associated with each TX antenna port are reported. The relative delays among them are measured. The Reference Signal from any antenna port selected by the user is also used to estimate the impulse response of the multi-path environment. UCTT presents 5 different views of the LTE signals: the spectral view, the Resource-Block-power frame view, a view of all subcarriers for any symbol, a view of specific subcarriers in the entire 10 ms frame, and the IQ constellation of all the 3 signals, 4 control channels and PDSCH channel. SEM (Spectral Emission Mask) and ACLR (Adjacent-Channel-Level-Ratio) are also measured. By decoding key control chan- nels such as PBCH, UCTT also reports the data content of the MIB (Master-Information-Block) and the number of detected bit errors, correctible errors and uncorrectible errors by exploiting the 3-1 convolutional encoding structure and the CRC checking. 1 Overview of LTE Signal Characteristics At the downlink direction, LTE uses OFDM (Orthogonal Frequency Division Multiplexing) modulation. In time domain, the signal consists of discrete symbols. Each symbol is the sum of N orthogonal subcarriers. Each subcarrier’s phase and amplitude carries information about the modulating bits. Knowledge of the subcarrier spacing, symbol duration, frame, timeslot, subframe, resource block and resource element is essential for quantitative understanding LTE signals at the physical layer. 1.1 Time and frequency characteristics The OFDM subcarrier spacing is 15 KHz. To maintain frequency orthogonality, the minimum symbol duration must be 1 15 ms. In fact, this is exactly the symbol duration used in LTE. Within a 10 ms frame period, there theoretically should be 150 symbols. But to avoid inter-symbol interference due to multi-path propagation, a fraction of each symbol from the end is copied and attached to the beginning 1
  • 2. of the same symbol (that is, each symbol is cyclically extended and prefixed to the beginning). Adding CP (Cyclix Prefix) effectively increases the symbol duration, hence reduces the number of symbols per 10 ms frame to 140 (from theoretical 150). With extended CP, the number further comes down to 120. The 10 ms frame is divided into 10 subframes, each 1 ms long, and 20 timeslots, each 0.5 ms long. Each subframe contains 2 timeslots. With normal CP length, each timeslot contains 7 symbols. With extended CP, each timeslot contains 6 symbols. Table 1 summarizes the durations of frame, subframe, timeslot, symbol and CPs. Table 1: LTE signal time characteristics Frame duration 10 ms Subframe duration 1 ms Timeslot duration 0.5 ms Symbol duration 1 15 ms Normal CP duration 3 640ms Normal CP of 1st symbol of each timeslot 1 192ms Extended CP duration 1 60 ms Each symbol consists of N number of subcarriers, depending on the operation bandwidth. The midcarrier (corresponding to the actual operating carrier frequency of 751 MHz, for example) is skipped (not used) to avoid carrier leak-through, a common RF hardware problem. The number of subcarriers is always a multiple of 12. 12 subcarriers within a time-slot period forms an RB (Resource Block). Table 2 lists the operating bandwidth, number of RBs and sub-carriers. Table 2: Possible LTE channel bandwidth configugrations Nominal Bandwidth (MHz) 1.4 3 5 10 15 20 Resource Blocks 6 15 25 50 75 100 Subcarriers 72 180 300 600 900 1200 Actual BW (MHz) 1.08 2.7 4.5 9 13.5 18 1.2 Resource element (RE), Resource element group (Reg) and Resource Block Each subcarrier within a symbol is a Resource Element (RE). This is the atomic unit (can not be divided further) that is modulated (phase and amplitude wise) by the information bits. All LTE physical signals and downlink channel’s power are normalized to the average power per RE. The Primary and Secondary Synchronization Signals (PSS, SSS, details later) are mapped to the center 62 REs every 5 ms. The Reference Signal (RS) from antenna port 0 or 1 uses every 6 REs on symbol 0 and 4 of each timeslot; the RS from antenna port 2 or 3 uses every 6 REs on symbol 1 of each timeslot. The leading 1, 2, 3 or 4 symbols of each subframe contains control channels. Each control channel is mapped to a group of REs according to 3GPP TS36-211, section 6.2.4. The group of REs is Resource element group (Reg). The REs not assigned to PSS, SSS, RS and control channels are used for PDSCH channel where user data payload actually resides. The remaining available REs for PDSCH are organized into Resource Blocks (RB). An RB is 12 consecutive subcarriers wide by one timeslot long (7 or 6 symbols, depending 2
  • 3. on CP length). In reality, the PDSCH for an user is always mapped to 2 adjacent (time-wise) RBs within a single subframe (2 timeslots). 1.3 PSS, SSS signals The PSS and SSS (Primary and Secondary Synchronization Signals) not only provide the framing (synchronization) information for the UE (User Equipment), they also encode the cell ID number. There are 3 possible PSS signals, generated from the frequency-domain Zadoff-Chu sequences with 3 different root indices. Each index encodes one of the 3 sector IDs (N (2) ID = 0, 1, 2). For FDD (structure type 1), the PSS is mapped to the center 62 REs at the last symbol of timeslot 0 and 10. For TDD (structure type 2), the PSS is mapped to the 3rd symbol of subframes 1 and 6. The 5 REs below and 5 REs above the PSS REs are intentionally reserved and not used. There are 168 possible SSS sequences, each encoding one of the 168 possible cell identity group ID (N (1) ID = 0, 1, . . . , 167). For FDD, the SSS is mapped to the center 62 REs at the symbol before the last of timeslots 0 and 10. For TDD, the SSS is mapped to the center 62 REs of the last symbol of timeslots 1 and 11. Like PSS, the 5 REs below and 5 REs above the SSS are reserved and not used. The cell ID is then 3N (1) ID + N (2) ID . The PSS repeats every 5 ms (occurs twice every 10 ms frame). The SSS also occurs every 5 ms, but the SSS sequence at the first 5 ms and the SSS sequence at the second 5 ms are different. This is designed so that the UE can not only decode the cell ID, it can also determine where the true 10 ms frame boundary is. 1.4 RS signals The cell-specific reference signals (RS) are indispensable for the normal functionality of LTE’s OFDM modulation. With signal TX antenna port, the UE needs the RS signal for channel estimation so that it can combat the frequency selective fading. With multiple TX antenna ports, the RS signals from each antenna port are used to form different sets of channel responses (from different TX antenna ports), and the responses are then used to ”decouple” the ”mixing” effects caused by transmit diversity or spatial multiplexing. The cell ID not only controls the RS signal’s random sequence generation, it also controls the RE mapping of the RS signals. That’s why the UE device must first detect the PSS and SSS to decode the cell ID and then it can go ahead find out where the RS signals are and what random signal sequences are needed to descramble them. For Sage UCTT, the RS signals are not only used for channel estimations, they are also used for finer synchronization (after coarse sync via PSS and SSS), also used for carrier frequency offset estimation and signal to noise and RSSI, RSRP and RSRQ measurements. The RS signals from a single TX antenna port are also used to estimate the multi-path impulse response. The RS signals are different TX antenna ports are used to estimate relative delays (synchronizations) among different TX antenna ports. Sage UCTT automatically detects the number of TX antenna ports being used via the RS signals associated with each antenna port (1, 2 or 4), and then forms the channel response estimation for each antenna port. 1.5 Physical Downlink Control Channels Sage UCTT provides measurements on all of the following downlink control channels: • PBCH: Physical Broadcast Channel • PCFICH: Physical Control Format Indicator Channel 3
  • 4. • PHICH: Physical Hybrid ARQ Indicator Channel • PDCCH: Physical Downlink Control Channel All the above control channels are mapped to specific group of Regs (Resource element groups). For example, PBCH is mapped to the center 72 REs of the first 4 consecutive symbols of timeslot 1 that are not used for RS signals. Taking away the REs used by RS at symbol 0 and 1, there are a total 240 REs allocated for PBCH. PCFICH and PHICH are mapped to the Regs (controlled by cell ID) at the first symbol of each subframe. The PDCCH channel is mapped to Regs at up to the first 3 symbols of each subframe. The exact number of symbols used is indicated by the PCFICH channel at that subframe. All control channels are QPSK modulated except PHICH, which is BPSK. Transmit diversity is used if more than one TX antenna port is used. Sage UCTT goes through all the Regs associated with each channel, measure their powers averaged by the number of REs used. UCTT also uses the estimated channels responses from each TX antenna port to decouple the transmit diversity effect so that the original QPSK or BPSK modulation pattern will be revealed, and EVM measurements will be performed on each of them. Sage UCTT also decodes the PCFICH channel to determine the number of symbols used for PD- CCH, and also decodes the PBCH channel to uncover the MIB (Master Information Block), which indicates information about the actual number of Resource Blocks (bandwidth) used, the Ng num- ber which controls the mapping of PHICH channel and the frame number. By exploiting the built-in repetition of the MIB block inside PBCH and the 3-1 convolutional encoding structure and the CRC checking, UCTT also reports the detected number of bit errors, correctible errors and uncorrectible errors etc. 1.6 PDSCH channel The physical downlink shared channel (PDSCH) contains the actual user payload data. This is actually the most relevant channel from a user point of view, but it is also the most challenging one for a test instrument to track due to its dynamic nature. The PDSCH channel consists of blocks of data from multiple users. Each block is dynamically assigned with varying modulation schemes (QPSK, QAM16 or QAM64) and varying MIMO methods (could be single antenna port, 2 ot 4 TX antenna port transmit diversity or spatial multiplexity). Theoretically, all these information are contained within the PDCCH channel. UCTT’s ultimate goal is to decode the PDCCH channel so that it can firmly track the PDSCH. For now, UCTT searches through all busy Resource Blocks (two adjacent RBs within every subframe), and detects if they are transmitted via transmit diversity. If yes, UCTT will decouple them and the perform power end EVM measurements on the RBs used by PDSCH. 2 Measurement definitions We now go through each of the measurement screen of Sage UCTT’s LTE feature and provide detailed technical definitions for all the parameters. 2.1 Spectral view The spectral view is the first screen presented when a user enters the LTE measurement feature. Figure 1 and Figure 2 show two examples of actual 10 MHz LTE signal from the air. This screen presents the following measurement results: 4
  • 5. • LTE detection indicator: When a valid LTE signal is present within 5 KHz from the center frequency, UCTT will display LTE Detected. If the signal is not a valid LTE signal, UCTT will display LTE signal NOT Detected. • Spectra: Two spectra are presented (yellow color and light green color). Both spectra have same 15 KHz resolution bandwidth, and both are obtained by averaging the whole 10 ms frame period with this crucial difference: the yellow-colored spectrum is ”unsynchronized” to the LTE symbol, whereas the light-green-colored spectrum is synchronized to the symbol interval. More specifically, the yellow one represents the typical spectrum seen by a typical spectrum analyzer. The green one is obtained by only transforming and averaging through LTE symbols. To achieve this, UCTT internally has determined the frame boundary via PSS, SSS and RS signals, and then analyzed and averaged the 140 symbol periods only (for FDD with normal CP). One see sharper edge at the green spectrum and the obvious mid-carrier that is not used. The purpose of the spectra is to show you the actual bandwidth of the signal, making sure a user has selected the correct bandwidth. For example, if a user selected ”10MHz LTE”, but the spectra indicate effective bandwidth less than 5 MHz, then obviously something is wrong. Either the user made wrong selection (should be 5 MHz LTE, for instance), or indeed, the cell site is transmitting signal of incorrect bandwidth. The other purpose is to quickly indicate the severity of frequency- selective fading. Figure 11 here is connected to a roof-top antenna through long cable. The spectra are more or less flat, indicating the fading is not an issue since the antenna is on the roof. Figure 2, on the other hand, was obtained from an indoor directional antenna. The complex indoor propagation environment shows much higher severity of the frequency-selective fading problem as the spectra are no long flat. • Frequency offset: Sage UCTT uses all the RS signals within the 10 ms to provide a reliable estimation of the carrier frequency offset. Since LTE uses the OFDM modulation, the subcarrier orthogonality is vital to its success. Carrier frequency offset or carrier frequency instability will affect the signal’s orthogonality, hence reducing the performance. Table 3 lists the standard requirements. Table 3: LTE signal minimum frequency offset requirement adapted from TS36.104 [4] BS class relative offset offset in Hz at 750 MHz Wide Area BS ±0.05 ppm ±37.5 Hz Local Area BS ±0.1 ppm ± 75 Hz Home BS ±0.25 ppm ±187.5 Hz • Span: • Bandwidth Spectral view offers a quick view of the frequency-selective fading problem and obvious technology bandwidth. Figure 1 shows the spectrum obtained from an antenna located on roof-top through a long cable, whereas Figure 2 shows the spectrum from an indoor directional antena. 5
  • 6. Figure 1: Spectral view of an actual 10MHz LTE signal captured from a roof-top antenna. Figure 2: Spectral view of an actual 10MHz LTE signal captured from an in door directional antenna. One can see the obvious frequency-selective fading issue. 2.2 Summary screen 2.3 Cell-ID scanner References [1] 3GPP TS 36.211, ”Physical Channels and Modulation.” 6
  • 7. [2] 3GPP TS 36.212, ”Multiplexing and channel coding.” [3] SGPP TS 36.141, ”Base Station conformance testing.” [4] SGPP TS 36.104, ”Base Station radio transmission and reception.” 7