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Lte Fundamentals

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LTE – Long Term Evolution Fundamentals, test and measurement

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Lte Fundamentals

  1. 1. LTE – Long Term Evolution Fundamentals, test and measurement Orlando Moreno omoreno@hotmail.com 408.656.2498
  2. 2. Understanding Long-Term Evolution The Universal Mobile Telecommunication System (UMTS), the successor to GSM, is steadily increasing its penetration of the world’s wireless communications markets, and is likely over time to become the primary standard used by not just the current 85% wireless carriers, but by virtually all of them. Its most promising emerging enhancement, Long-Term Evolution (LTE), has moved further along in the standards-setting process and on to the path of initial deployment. With release of the latest version of Release 8 of the 3rd Generation Partnership Project (3GPP) in June 2009, the core LTE specification has stabilized. 3GPP Release 8 also encompasses a considerable number of interim enhancements embodied in Evolved High-Speed Packet Access (eHSPA), also known as HSPA+. 408.656.2498 omoreno@hotmail.com 2
  3. 3. Understanding Long-Term Evolution Unlike acronyms for many wireless standards, LTE is aptly named, as it is almost certain to be the long-term high-speed data solution for all 3GPP-based wireless networks (i.e. GSM, WCDMA) for many years to come. With the announcement of nearly all major 3GPP2 service providers, using today CDMA®2000 1xRTT and 1xEV-DO Rev. 0 and Rev. A technology, LTE seems to become the major technology for mobile broadband. The other emerging standard is mobile WiMAX, comparable to LTE from a performance perspective, and in contrast to LTE currently deployed in several countries throughout the world. As both technologies have their advantages and disadvantages it is likely that in the years ahead, both WiMAX and LTE will gene rate significant market share. 408.656.2498 omoreno@hotmail.com 3
  4. 4. Understanding Long-Term Evolution WiMAX may be most successful in countries with minimal wired or wireless infrastructure and is currently being deployed in a growing number of markets by Clearwire Communications. The two biggest service providers in the U.S., Verizon Wireless and AT&T will instead use LTE within the 700 MHz spectrum both companies received after the FCC’s spectrum auction. As no infrastructure for WiMAX existed upon which WiMAX could be built, deployment will require significant investment in both hardware and software. LTE has an advantage in this respect, since it can be built on major carriers’ existing infrastructure, e.g. base station towers etc. LTE also requires investment in infrastructure and software. However, there is no doubt, that LTE has the potential to deliver all of the long- promised services for wireless communications, especially high-data-rate streaming of multimedia content and data files, and the transition to an all-Internet Protocol (IP) network to a vast number of users due to the commitment of the wireless industry. 408.656.2498 omoreno@hotmail.com 4
  5. 5. HOST OF NEW TECHNOLOGIES LTE is significantly different in several important ways from current technologies. Together they result in the ability of LTE to deliver improvements in spectral efficiency, network capacity, and higher sustainable data rates further from base stations, lower latency, and comparative simplicity. One of the most significant differences is the use of Multiple Input Multiple Output (MIMO) technology as a core element in the design. On the face of it, MIMO might seem simple, but dig deeper and it quickly becomes obvious that it is very complex intelligent antenna technology that simply could not have been achieved without the capabilities of today’s high-speed digital signal processing devices. In systems employing MIMO, multiple antennas are employed by both the transmitter and receiver to increase data throughput without the need for additional bandwidth or higher transmit power. 408.656.2498 omoreno@hotmail.com 5
  6. 6. HOST OF NEW TECHNOLOGIES The other goal is the achievement of higher transmission robustness by increasing the signal-to-noise ratio (SNR) specifically for terminals at the cell edge. The improvements from MIMO are achieved by its ability to increase spectral efficiency and link reliability. MIMO is implemented through the use of spatial precoding at the transmitter. In LTE, 2x2, 4x2 and later 4x4 MIMO configurations (the numbers indicating the number of transmit and receive antennas) are supported. Early systems are likely to employ the 4x2 version. 408.656.2498 omoreno@hotmail.com 6
  7. 7. Different downlink and uplink modulation schemes The Orthogonal Frequency Division Multiple (OFDM) modulation technique is rapidly replacing other schemes because of its inherently superior characteristics in many respects. LTE employs OFDM Access (OFDMA) in the downlink. OFDMA provides multiple access by assigning subsets of subcarriers to individual users to allow simultaneous multi-user, low-data-rate transmission. The individual assignment of subcarriers allows the best possible quality of service to be maintained by controlling the data rate and error probability individually on a “per user” basis. It can be visualized as an architecture in which resources are allocated on a time-frequency basis, dealing with a number of subcarriers (multiples of 12 subcarriers = 1 resource block) assigned for defined period (Transmit Time Interval, TTI = 1ms). 408.656.2498 omoreno@hotmail.com 7
  8. 8. Different downlink and uplink modulation schemes There are many advantages claimed for OFDMA, the most reasonable one is its ability to adapt to changing channel conditions. Some of OFDMA’s advantages include the ability to more effectively and easily combat multipath distortion and the flexible use of available bandwidth. An entirely new OFDM-derived modulation scheme is used for the uplink called Single Carrier Frequency Division Multiple Access (SC-FDMA). This OFDM-based access scheme combines the advantages low peak-to- average power ratio (PAPR) of single-carrier systems (a benefit to RF power amplifier designers) with the resistance to multipath distortion and inherently flexible sub-carrier frequency allocation offered by OFDMA. SC-FDMA is well suited to use in the uplink path because its lower PAPR, which is achieved by performing another Discrete Fourier Transform (DFT) prior to the subcarrier mapping. 408.656.2498 omoreno@hotmail.com 8
  9. 9. Different downlink and uplink modulation schemes This spreads the modulation symbol over the subcarriers assigned to the UE, which greatly improves the user equipment’s transmit power efficiency. The subcarrier mapping could be either distributed or localized, where LTE uses the localized mode. This gives the system the flexibility of a multi-user scheduling gain in frequency domain versus a robust transmission for control channels and high mobility of the UE, which would be achieved with the distributed mode. In conjunction with development of the LTE standard, 3GPP is creating the System Architecture Evolution (SAE) as its core network architecture. In contrast to the previous GPRS core network, SAE provides an all-IP, simplified architecture, support for higher throughput and low-latency radio access networks (RANs), multiple, heterogeneous RANs, legacy systems such as GPRS, as well as non-3GPP systems. 408.656.2498 omoreno@hotmail.com 9
  10. 10. Different downlink and uplink modulation schemes The resulting overall system architecture consists of Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) that describes user equipment, air interface and base station (enhanced Node B, or eNB), and the Evolved Packet Core (EPC). The EPC comprises entities like the Mobility Management Entity (MME) and Serving and Packet Gateway. 408.656.2498 omoreno@hotmail.com 10
  11. 11. THE LTE TEST AND MEASUREMENT CHALLENGE Testing LTE requires standard instruments such as signal generators and signal and spectrum analyzers for either component (e.g. power amplifier, RF IC) or entity (eNodeB, UE) testing. Additionally, radio communication testers verifying the RF and protocol stack of a device under test (i.e. terminal, User Equipment (UE)) are essential, allowing specific services of interest to run on the IP layer, i.e. E2E testing. MIMO alone presents new challenges, since it has never been deployed as a core technology in both the infrastructure and user equipment of any other personal communications system. As a result, changes to both the test specifications and how the tests are conducted will be an ongoing process as experience is gained during development, manufacturing, installation, and maintenance of LTE infrastructure and user equipment. 408.656.2498 omoreno@hotmail.com 11
  12. 12. THE LTE TEST AND MEASUREMENT CHALLENGE Both OFDMA and SC-FDMA rely on denselystacked, low-data-rate subcarriers to convey information. Every subcarrier has a very narrow bandwidth (15 kHz), so a flat-fading channel is a representation of what the signal experiences. In addition, a cyclic prefix combats intersymbol interference. This scenario makes the physical layer inherently frequency-selective so all procedures defined by the specifications are also frequency-selective. Beyond this, MIMO adds another level of complexity to the measurement challenge. For example, single-user MIMO in the downlink, realized as transmit diversity or as open-loop or closed-loop spatial multiplexing, all of which are covered in the LTE standard, requires specific test set-ups. Consequently, automated testing becomes a necessity in order to verify correct operation of the entire system. 408.656.2498 omoreno@hotmail.com 12
  13. 13. Physical-Layer Testing The physical layer test process starts with low-level block measurements that verify the system’s receiver and transmitter. The strategy adds further processing step by step until all required functional blocks are integrated. Designers must be able to observe internal signals, such as the transmitted and received payload and measurement of the timing offset between the uplink and downlink frames. Without this information, it is difficult to perform system debugging. The system should also be easy to use without modification. For example, the packet-data-convergence-protocol (PDCP) layer should not have to be configured when testing the physical layer, and when testing the radio-link-control (RLC) layer, configuring the physical layer should be as easy as possible. 408.656.2498 omoreno@hotmail.com 13
  14. 14. Physical-Layer Testing Since most of the testing occurs at the same time as development and debugging, it is essential that engineers be able to automate the testing and embed it into a regression environment with as little manual intervention as possible. The test environment should include all components from the test bed, such as fading-channel simulators, and external noise and interference sources. 408.656.2498 omoreno@hotmail.com 14
  15. 15. Data-Path Testing The first step in the data path testing regimen is ensuring that the individual channels operate in an open-loop fashion. This validates the correct implementation of 3GPP technical specifications (TS) 36.211 and 36.212 that define physical channels and modulation respectively multiplexing and channel coding in LTE and therefore the downlink and uplink transmission with forward error correction. In order to verify the correct reception and transmission to and from the user equipment, a radio communication tester provides a downlink signal to the device under test and analyzes the uplink signal. Intermediate points in the encoding and decoding chains of the test equipment must be visible to assist in the debugging process. The process also requires special test features such as the ability to corrupt downlink transmissions, for which standard signal generators and analyzers can be used along with standardsbased software. 408.656.2498 omoreno@hotmail.com 15
  16. 16. Functional Testing Once operation of the individual data paths in a noise-free environment has been established, functional testing can begin. In this case, the feedback procedures within the user equipment are evaluated in a controlled and static environment, removing variables and making behavior of the procedures easier to predict. The tests include channel quality (Channel Quality Indicator, CQI), hybrid ARQ (HARQ), and timing control, all of which are contained in 3GPP TS 36.213. The designer must respond in real time to the control information. For example, in testing downlink HARQ operation, the downlink transmission must be tested in accordance with the received ACK/NACK information. 408.656.2498 omoreno@hotmail.com 16
  17. 17. Functional Testing To test these features efficiently, the radio communication test set has to provide special test features such as built-in interference or channel models. In particular, the channel models that apply static flat channels on a per- subcarrier basis enable test channel estimation and CQI reporting to be tested in a deterministic and repeatable environment. Other special test features include downlink scheduling (which can be used to test HARQ processes), uplink scheduling to stimulate uplink transmission, timing control to verify that the user equipment obeys the timing commands sent within the physical-downlink-shared-channel (PDSCH) payload, and uplink power control to verify the correct behavior of the power-control algorithms in the user equipment. 408.656.2498 omoreno@hotmail.com 17
  18. 18. Functional Testing The system also requires extensive logging and measurement capability to validate the user equipment implementation. Measured parameters include uplink power per subframe, transmission quality, and uplink/downlink timing offset and throughput. The system requires OFDMA channel-noise generation to simulate transmission to other user equipment operating in the network. 408.656.2498 omoreno@hotmail.com 18
  19. 19. Performance Testing After completion of functional testing, designers must evaluate the performance of the user equipment in accordance with 3GPP TS 36.101 that defines user equipment radio transmission and reception along with performance requirements. This step is divided into two parts. In the first, the performance of individual blocks such as receiver and transmitter is measured, and in the second the system performance, including closed-loop operation and user equipment procedures, is measured. A static-channel model must be replaced by a fading-channel simulator, and neighboring channels, blockers, and interferers are added to the downlink signal to simulate an environment that the user equipment would experience in the field. 408.656.2498 omoreno@hotmail.com 19
  20. 20. Performance Testing While performing the block-level performance test for the PDSCH, the designer must see what block-error rate (BLER) changes occur during varying signal power, type and level of interference, and transport format, along with the fading-channel profile. During block-level tests on the physical downlink-control channel (PDCCH), the engineer must pay attention to false-detection ratios. This is necessary to check the performance of the algorithms that detect the wanted signal in various interference scenarios without prior knowledge of the characteristics of the signal. It requires about 40 blind trials within a defined search space. Once the block-level testing is complete, testing moves on to system-level measurements to validate the interaction of various system components. Mandatory tests include throughput with enabled HARQ, CQI-validation, and PDCCH detection. 408.656.2498 omoreno@hotmail.com 20
  21. 21. Performance Testing At the same time, engineers can verify compliance of the uplink transmission with 3GPP TS 36.101 to see if the transmission fulfills the required spectral- emission masks and minimum-quality measurements based on parameters such as error vector modulation (EVM) and spectral flatness. 408.656.2498 omoreno@hotmail.com 21
  22. 22. Production Testing After the equipment has completed development, it must be evaluated in production. Since this process uses some of the tests performed during the earlier stages, it is obviously beneficial if the same test platform and environment can be used for both development and production testing so that design teams can transfer information to make problem-solving easier. Nevertheless, the requirements for production testing are still different from those for development. It is typically not necessary to delve into the great detail as was required during development, but it is essential to create clear pass/fail criteria. The test regimen must be developed with an eye toward efficiency, since reducing production test time has a significant positive impact on manufacturing cost. 408.656.2498 omoreno@hotmail.com 22
  23. 23. Production Testing In addition to the tests described above, there are others as well, including physical-layer MIMO testing and testing of higher layers such as Medium Access Control (MAC) layers, the radio resource control RRC, and Inter-RAT (radio-access technology) handover to other wireless technologies such as GSM and W-CDMA. 408.656.2498 omoreno@hotmail.com 23
  24. 24. SPECIFIC TESTS There are many tests to which components (such as RFICs), subsystems, and systems must be subjected in order to them to meet all performance requirements aspects of LTE system operation. The following sections cover many of the primary tests and how they can be configured, using RFICs as an example. 408.656.2498 omoreno@hotmail.com 24
  25. 25. Bridging the Gap Before discussing specific measurements on LTE-specific RFICs, it is important to note the importance of the interface between the baseband IC (BB IC) and RF IC. The use of analog I and Q data between the baseband IC and the RF IC has been the standard method for verifying both devices for many years, although the industry has been working to define a common serial digital interface between these two widely-different operational environments. If such an interface could be created, it would provide less susceptibility to interference, broader bandwidths, higher data rates, and “plug & play” configurability, while also establishing interoperability between baseband and RFICs from multiple vendors. The interface would also be easier and less expensive to implement than traditional analog approaches, and could potentially reduce complexity, test time, and cost. 408.656.2498 omoreno@hotmail.com 25
  26. 26. Bridging the Gap Of the various efforts underway to realize this interface, the one that has risen to prominence is called the Dig-RF standard, the most recent version of which is Dig-RF v3.09. Dig-RF efforts are conducted under the auspices of the Mobile Processor Industry Interface (MIPI) alliance, which focuses on the physical interface and the protocol on which it runs. The protocol configures the interface and arranges transport of digital data between both circuits. Physical layer parameters such as data rate, system clock speed, and pin usage are also defined. However, the high data rates and average user data throughput, high spectrum efficiency, scalable bandwidths, and use of MIMO technology incorporated in LTE are not supported by the Dig-RF v3.09 standard. 408.656.2498 omoreno@hotmail.com 26
  27. 27. Bridging the Gap Consequently, a working group within MIPI is working on an additional standard (DigRFv4) to address it, but work is not yet complete. Harmonization is expected soon although a first draft of the specification has been publicly available since November 2008. Armed with this preliminary version, manufacturers have been able to develop chipsets for LTE using their own digital IQ formats. With current test methods, the digital baseband signal is converted by a signal generator into an analog baseband signal and fed into the analog I and Q inputs of an adaptor board carrying the RFIC. In the reverse direction, the output of the RF chipset consists of analog I and Q signals sent to the analog baseband input of a signal analyzer. The baseband chipset handles the analog-to-digital and digital-to-analog conversion. With digital IQ, this conversion is performed at the RFIC, so signals between the baseband and RF ICS become digital. 408.656.2498 omoreno@hotmail.com 27
  28. 28. Bridging the Gap In the absence of a standardized digital interface, Rohde & Schwarz created its own, called DIGITAL IQ (TVR290), which provides digital I/Q signals through which standard instruments can be directly interconnected. TVR290 can only be used with Rohde & Schwarz equipment, so to connect equipment from other manufacturers using different digital I/Q standards, the company also developed the R&S EX-IQ-Box digital signal interface module. This component allows the Rohde & Schwarz format to be converted to other digital I/Q formats used in handsets and base stations. Adapting to customer-specific physical connectors is implemented by break- out boards that accommodate other manufacturers’ equipment. For transmit validation of an LTE-capable RF chipset using a digital IQ format, the EXIQ-Box is connected to the digital baseband output of a signal generator, and the output of the EX-IQ-Box is connected directly to the interface of the RFIC, supplying digital I and Q signals. 408.656.2498 omoreno@hotmail.com 28
  29. 29. Bridging the Gap The RFIC performs the digital-to-analog conversion and IQ modulation with up-mixing to the RF frequency. The RF portion of the uplink signal can then be analyzed with a signal analyzer. The down-converted, digitized RF downlink signal is converted by the interface module from the user-specific digital IQ format to the Rohde & Schwarz format and is fed to the digital baseband input of a spectrum analyzer in which signal analysis is performed. Testing of coming generations of RF ICs will almost certainly employ the common interface that results from the Dig-RF efforts. In the interim, the R&S EX-IQ-Box lets manufacturers exploit the advantages of a digital interface between the baseband and RF environments. 408.656.2498 omoreno@hotmail.com 29
  30. 30. Transmission Filters One of the major differences between CDMA-based 3G/3.5G and OFDM- based LTE is the lack of a definition for a transmission filter. In WCDMA/HSPA, a Root-Raised-Cosine (RRC) filter with a roll-off factor of α=0.22 is specified by 3GPP. The use of this filter increases the signal bandwidth of 3.84 MHz to a transmission bandwidth of 4.68 MHz, which explains why a channel bandwidth of 5 MHz is required to achieve the necessary out-of-channel performance so as not to disturb adjacent channels. Without a transmission filter definition in LTE, the filter can be optimized for either in-channel or out-of-channel performance, where the goal of the design engineer is to find a harmonization between both. Improving the in-channel performance (i.e., modulation quality) results in an improved Error Vector Magnitude (EVM). 408.656.2498 omoreno@hotmail.com 30
  31. 31. Transmission Filters Out-of-channel performance is evaluated by measuring Adjacent Channel Power (ACP) and a Spectrum Emission Mask (SEM). Rohde & Schwarz addresses the open filter definition by offering different filter types dedicated to LTE. Two of these filters are designed to improve the in-channel (“Best EVM”) or the out-of-channel (“Best ACP”) performance. The third filter (“Balanced EVM and ACP”) is a trade-off between inchannel and out-of-channel performance that provides good ACP and acceptable modulation quality. Additionally user-specific filtering can be applied to the signal. 408.656.2498 omoreno@hotmail.com 31
  32. 32. Transmit Measurements The first step in the verification process is to measure the output power of the RFIC and record it for different power amplifier gain factors. Another method of measuring power is to use a result summary in which the frame power for the physical channel, demodulation reference, and sounding signals are displayed. Different signal shapes result depending on the filter in use. Figures 5 a, b, and c show the impact of the three different filter types in the R&S SMU200A vector signal generator on the shape of the uplink signal. Figure 5a 408.656.2498 omoreno@hotmail.com 32
  33. 33. Transmit Measurements Figure 5b compared to Figure 5c shows that the signal is falling off more smoothly, resulting in much better EVM performance. The strong filtering applied to the signal by using the “Best ACP” filter (Figure 5c) will decrease modulation quality but improves ACP and minimizes the impact to other channels or signals. Figure 5b Figure 5c 408.656.2498 omoreno@hotmail.com 33
  34. 34. Peak-To-Average Power Ratio With the SC-FDMA modulation scheme, a Discrete Fourier Transform (DFT) is performed before the Inverse Fast Fourier Transform (IFFT). The modulation symbols are spread so that each subcarrier is carrying a part of each modulation symbol. In OFDMA, each subcarrier carries one modulation symbol, where the independent phases of this subcarrier will often combine constructively and end in a high PAPR, also known as crest factor. The crest factor is estimated by calculating the Conditional Cumulative Distribution Function (CCDF), which describes the probability distribution of the signal power. For SC-FDMA the PAPR depends on the filter used as well as on the modulation format. In contrast the downlink uses OFDMA as the transmission scheme which results to a high constant PAPR since different modulations are used for different users in a loaded system. 408.656.2498 omoreno@hotmail.com 34
  35. 35. Frequency Error and Transmit Modulation Frequency error for LTE should be about ±0.1 ppm depending on the carrier frequency and can differ between ±77.7 and ±198 Hz depending on the frequency band. The frequency error is displayed on the signal analyzer in the numeric overview of the measurement results and can be displayed separately for each sub-frame. There is a direct relationship between EVM and the phase noise of the voltage- controlled oscillator (VCO) used in the device’s IQ modulator. Exceeding the limits for EVM is an indication that the VCO and the Phase Locked Loop (PLL) controlling it are generating large amounts of phase noise. The nonlinearities of components used in the device can also negatively affect EVM. Evaluating the spectrum emission mask (SEM) will determine whether the phase noise (only EVM fails) or nonlinearities (EVM and SEM both fail) are causing the problem. 408.656.2498 omoreno@hotmail.com 35
  36. 36. EVM’s Importance Estimation of EVM is essential for evaluating the modulation quality for any single-carrier transmission scheme using digital modulation. EVM measurements have been used since the introduction of 8PSK as a modulation format in second-generation networks such as Enhanced Data Rates for Global Evolution (EDGE). In an EVM measurement, the received signal is compared to a well-defined reference signal and the difference results in an error vector. The magnitude of the error vector should not exceed predefined limits to avoid errors in demodulating the signal. Until now, all cellular standards have used single-carrier transmission schemes, and only one signal is analyzed to estimate modulation quality. However, LTE uses multi-carrier transmission schemes in both the downlink and uplink with the number of orthogonal subcarriers dependent on the allocated resource blocks or transmission bandwidth, rather than available channel bandwidth. 408.656.2498 omoreno@hotmail.com 36
  37. 37. EVM’s Importance For each of these subcarriers the EVM is estimated depending on the modulation scheme used (QPSK, 16QAM or 64QAM). The measured values are compared against defined limits, which in the downlink are 17.5% for QPSK, 12.5% for 16QAM, and 8% for 64QAM. Since support for 64QAM in the uplink is an option for LTE-capable user equipment, there is no tolerance specified yet, as the definition awaits a future version of the standard. Nevertheless, a tolerance of 8% for the downlink might be considered. In the uplink the output power of the device under test needs to be greater than -40 dBm, which is the minimum power defined for LTE. In Figure 6, the graphical result is shown for an EVM-versus-subcarrier measurement on a 10 MHz FDD uplink signal, as peak, average, and minimum value (blue, yellow, green curves). 408.656.2498 omoreno@hotmail.com 37
  38. 38. EVM’s Importance The x-axis shows the frequency, from which the occupied bandwidth can easily be estimated. The 46 allocated resource blocks are equal to 552 subcarriers, which gives a transmission bandwidth of 8.28 MHz (±4.14 MHz). The maximum and minimum EVM based on the average curve are highlighted as a numeric value for the affected subcarriers. EVM can be estimated for consecutive SCFDMA symbols, and the measurement is called “EVM versus symbol”. In the time domain, there are seven SC-FDMA symbols per time slot when a normal cyclic prefix is used and six when the extended cyclic prefix is used. The measurement is carried out for a complete radio frame, i.e., for 140 SCFDMA symbols. This type of measurement is important because there is no specification for a transmit filter. 408.656.2498 omoreno@hotmail.com 38
  39. 39. EVM’s Importance Aggressive filtering will add time distortion to the signal, which will reduce the effects of the cyclic prefix and can cause unwanted intersymbol interference. For this reason it is essential to estimate the EVM over time (for symbol duration). 408.656.2498 omoreno@hotmail.com 39
  40. 40. The Spectrum Emission Mask By analyzing the SEM it is possible to conclude whether the failure of the EVM measurement is related to the VCO or comes from non-linearities generated by components used in the device under test. The 3GPP specification defines exact limits that cannot be exceeded. For a given bandwidth of 10 MHz the limits in Table 1 are defined for an out-of- band transmission. It is important to note that the power values (in dBm) for out-of-band emissions vary depending on the resolution bandwidth of the measurement filter (30 kHz and 1 MHz). The limits in Table 2 are translated into a graphical format in Figure 7. The y-axis shows the measured power value related to 1 MHz resolution bandwidth, although for ±1.0 MHz adjacent to the occupied channel, the resolution bandwidth is specified with 30 kHz. Converting the specified tolerance of -18 dBm/30 kHz to a resolution bandwidth of 1 MHz results in a limit of -2.77 dBm/1 MHz. 408.656.2498 omoreno@hotmail.com 40
  41. 41. The Spectrum Emission Mask Table 1: Key LTE specifications Parameter Values Data rate (SISO, 20 MHz) Downlink: 100 Mb/s. Uplink: 50Mb/s Maximum data rate 300 Mb/s (20 MHz, 4x4 MIMO) 75 Mb/s (20 MHz, 64 QAM) Bandwidths 1.4 through 20 MHz Access method Downlink: OFDMA. Uplink: SC-FDMA Modulation Types Downlink: QPSK, 16QAM and 64QAM Uplink: QPSK, 16QAM, 64QAM (optional) Maximum users per cell 200 Latency below 30 ms Optimum cell size Optimum performance: 5 km Moderate performance: 30 km Minimal performance: 100 km Mobility Optimum: 15 km/h Good performance: 120 km/h Acceptable performance: 320km/h 408.656.2498 omoreno@hotmail.com 41
  42. 42. The Spectrum Emission Mask This new harmonized value is taken as the limit instead of dividing the diagram into two separate areas in which one is related to a resolution of 30 kHz and the other to 1 MHz, as it is for SEM measurement of WCDMA and HSPA. Δf008 (MHZ) +/- +/- +/- +/- +/- +/- +/- +/- Table 2: 0…1 1…2.5 2.5…5 5…6 6…10 10…15 15…20 20…25 Measurements Power (dBm/BW) -18 -10 -10 -13 -13 -25 - - limits for SEM Measurement bandwidth 30 kHz 1 MHz Figure 7: Spectrum emission mask (SEM) normalized to 1 MHz. 408.656.2498 omoreno@hotmail.com 42
  43. 43. In-Band Emission EVM measurements are performed only on resource blocks allocated to the user. In practical terms not all 50 resource blocks in the uplink can be assigned to a single user because of the need for an uplink control channel (PUCCH). This is used by other user equipment to transmit uplink control information as they are not transmitting any user data in the uplink. In this example, 46 resource blocks are assigned at most to a single user for a given bandwidth of 10 MHz. In actual network operation and more realistic terms, fewer than 46 resource blocks will be allocated to one user so the impact of a user on non-allocated resources must be estimated. This ensures that the transmission of user data in the uplink does not interfere with the data transmission of other users as well as with transmitting control information on a control channel. The in-band emission measurement is used for estimating the amount of interference. 408.656.2498 omoreno@hotmail.com 43
  44. 44. In-Band Emission Absolute and relative in-band emissions are measured, and the relative in-band emission is derived from the absolute in-band emission. Both results are expressed in dB. 408.656.2498 omoreno@hotmail.com 44
  45. 45. Spectrum Flatness/Difference and Group Delay EVM assumes that the measurement takes place after the IDFT and is measured by estimating and removing the sampling timing offset and the RF frequency offset. However, this hides problems in the transmitter at the power amplifier and IQ modulator, so additional EVM testing or further measurements are necessary to check for problems. The two additional requirements defined to ensure excellent transmitter performance are spectrum flatness and flatness difference. The spectrum flatness measurement compares the power variations of a subcarrier to the average power of all subcarriers, in decibels (dB). It is an estimation of the frequency response of the transmission channel, in which the flatness difference shows the deviation from this estimation, also in dB. Channel group delay measurement is the derivative of the signal’s phase, so group delay can only be constant when the phase of the signal is linear, which it never is in practice. 408.656.2498 omoreno@hotmail.com 45
  46. 46. Spectrum Flatness/Difference and Group Delay Group delay always exists but only reduces signal quality if it exceeds the guard interval (the cyclic prefix), causing intersymbol interference. Group delay varies depending on the filter used, and a filter optimized for EVM will decrease group delay, while an ACP-optimized filter will increase it. 408.656.2498 omoreno@hotmail.com 46
  47. 47. Adjacent Channel Power (ACP) and Adjacent Channel Leakage Ratio (ACLR) This measurement characterizes the distortion of the transmitter’s power output, which can cause interference to adjacent channels. Since LTE will often use the same frequency bands as WCDMA/HSPA, the two must coexist. Different frequencies within the band may be used for downlink and uplink so it is important to verify the impact of an LTE signal on a WCDMA/HSPA signal with ACLR measurement. For the impact on WCDMA/HSPA, the power of the LTE signal is measured using an RRC filter with a resolution bandwidth of 3.84 MHz and a roll-off factor of α=0.22 in an offset of 5 and 10 MHz from the edge of the allocated bandwidth. The filter is moved to the desired frequency and the power of the signal is integrated for its resolution bandwidth. The resulting power value is then displayed for the desired frequency offset. 408.656.2498 omoreno@hotmail.com 47
  48. 48. An RF IC Transmit Test Example For transmission of signals, the RF chipset determines part of Layer 1 functionality in a wireless device, as well as the IQ modulation/demodulation process, and mapping onto the carrier frequency for the selected frequency band. Verification tests must be performed for each of the supported bands. During the early stages of development, this is performed at multiple frequencies per band and as development progresses at only low, mid, and high regions of the band to save test time. The tests are executed for three different temperature ranges to ensure that the device can operate within an expected range of environmental conditions. In a typical measurement setup for RFIC transmitter verification (using analog IQ signals) using the Rohde & Schwarz R&S 15 SMU200A vector signal generator and R&S FSQ signal analyzer, the signal generator delivers an uplink baseband signal to the analog I and Q inputs of the RFIC. 408.656.2498 omoreno@hotmail.com 48
  49. 49. An RF IC Transmit Test Example The chipset provides the IQ modulation and amplification, upconverting the signal to an RF frequency so that it can be transmitted via the antenna. The signal from the antenna is directly connected to the RF port of the signal analyzer that measures the uplink RF signal and verifies it against the limits in the 3GPP specification. This configuration can validate the entire transmitter chain, although to test specific transmitter components such as the synthesizer, a modified measurement setup must be used. 408.656.2498 omoreno@hotmail.com 49
  50. 50. SUMMARY Requirements for measuring components, subsystems, and systems designed for LTE applications have evolved through the same rigorous process that brought the industry other complex digital standards such as WiMAX, and the previous generations of UMTS. However, ratification of the standards themselves is just the first step in the evolutionary process that unfolds as infrastructure equipment and user equipment is developed. Consequently, much will be learned about how the predicted and actual performance of LTE networks compare as the first systems are deployed. Throughout the process, measurement hardware and software will play a key role in ensuring that all elements of the network meet their required specifications and that they are fully interoperable. Both are available today, and will keep pace with LTE developments so that LTE systems can fulfill their promise for truly high-speed mobile transmission. 408.656.2498 omoreno@hotmail.com 50
  51. 51. Questions ? Orlando Moreno omoreno@hotmail.com 408.656.2498 408.656.2498 omoreno@hotmail.com 51