Field trials of LTE with 4 4 MIMO

Transcription

1 Addressing network capacity demands with LTE MIMO Field trials of LTE with 4 4 MIMO Multi-antenna reception and transmission is a key enabler of the high performance offered by LTE. JOHAN FURUSKOG, KARL WERNER, MATHIAS RIBACK AND BO HAGERMAN The Long Term Evolution (LTE) standard for mobile broadband includes multi-antenna transmission modes that improve performance in terms of coverage, spectral efficiency and peak throughput. To fully exploit the potential of these techniques, the antenna system at the LTE base station, or enodeb (enb), and in the user equipment (UE) must be designed with the intended performance profile in mind. Here we present a comparison of the performance of several multi-antenna setups in field trials. Multi-antenna technologies in LTE Multiple antennas for reception and transmission at the enb and in UEs are a key enabler of the high performance offered by 3rd Generation Partnership Project (3GPP) LTE. 1 The standard supports multi-antenna technologies that improve both link- and system-level performance in a wide range of scenarios. 2 In LTE, data is mapped to layers after having been encoded and modulated. The number of transmitted layers is called the transmission rank. 3GPP LTE Release 8 (Rel-8) permits up to four layers to be transmitted in the downlink. 2 These layers are precoded and mapped to antenna ports in a procedure that is semi-statically configured to use one of seven transmission modes (see Table 1). 3 Transmission modes 1 and 7 are identical from a UE perspective: a single layer is transmitted in both cases. In mode 1, the layer is transmitted from one antenna port, whereas in mode 7, the layer is transmitted from one antenna port, or transparently to the UE, a combination of antenna ports. In mode 2, a single layer is encoded with a space-frequency block code (SFBC) based on the Alamouti code, and transmitted from multiple antennas. Transmission mode 3 is identical to mode 2 when the rank is one. For higher ranks, a predefined codebook of pre coder matrices is cycled across the frequency band, along with a layer permutation designed to give each layer similar average channel quality. Apart from the rank indicator (number of layers), no feedback is required to select the precoders in modes 2 and 3. These modes are thus suitable in scenarios where timely channeldependent feedback cannot be made available for instance, in high-speed scenarios. Transmission mode 5 (multi-user BOX B Field Trials The measurement campaign was performed using the Ericsson Research LTE test bed, which comprised a single mobile UE and a single enb. The trials addressed downlink performance only. MIMO) enables single-layer transmission to several users who simultaneously share the same frequency allocation. In closed-loop spatial multiplexing (mode 4), one or more layers are transmitted using a precoder matrix that is selected from a predefined codebook. The precoder selections are typically based on channel measurements made by the UE. The precoders are applied to the layers transmitted from the enb with the goal of maximizing performance according to the instantaneous channel conditions. Depending on the configured uplink feedback mode, the UE may feed back multiple preferred precoder matrix indicators (PMIs), where each PMI is valid for a particular sub-band (frequency-selective precoding); or one PMI that is valid for all sub-bands (wide-band precoding). Transmission mode 6 is similar to transmission mode 4, except that it is restricted to rank-one transmission. The various transmission modes specified in the standard may be used with any antenna setup, and the codebooks are designed for a wide range of antenna setups and scenarios. Nevertheless, the choice of antenna setup is critical, and the best choice is dependent on the target performance profile and propagation environment. BOX A Terms and abbreviations 3GPP 3rd Generation Partnership Project CDD cyclic delay diversity cumulative distribution function CQI channel-quality indicator enb enodeb, LTE base station FDD frequency division duplex HARQ hybrid automatic repeat request HSPA high-speed packet access LTE MIMO MMSE PMI SFBC SIMO SNR UE Long Term Evolution multiple input, multiple output minimum mean square error precoder matrix indicator space-frequency block code single input, multiple output signal-to-noise ratio user equipment Field trial campaign LTE test bed The measurement campaign was performed using the Ericsson Research LTE test bed, which comprised a single mobile UE and a single enb. The test bed supported up to 4 4 ([number of transmit antennas]x[number of receive antennas]) MIMO in the downlink. The trials addressed downlink per formance only, and the frequency bandwidth for ERICSSON RE VIE W 1 21

2 downlink transmission was set at 1MHz. The test bed incorporated the basic LTE frequency division duplex (FDD) principles and had similar parameters. 4-5 The relevant multi-antenna techniques were aligned with 3GPP Rel-8, and therefore it was anticipated that the measured results would be representative of the relative throughput gains achievable for LTE with multiple transmit and receive antennas in the scenarios considered. 2 Table 2 shows some basic para meters of the test bed. Given that the test bed was a prototype, the absolute performance is not necessarily representative of commercial products. The results were obtained using LTE transmission mode 4. The codebooks defined in the standard were used for frequency-selective precoding. 2 Nine precoders are selected over the 1MHz bandwidth, each valid for a subset of the band. The total downlink transmit power was normalized to be independent of transmission rank and number of transmit antennas. Field trials An important goal of the field trials was to measure the relative performance of different antenna setups at the enb. A similar campaign for MIMO high-speed packet access (MIMO-HSPA) was described in MIMO-HSPA Test Bed Performance Measurements. 6 The field trials were conducted in a business district in northern Stockholm, Sweden. The UE was driven along two separate routes in two sectors (Sector 1 and Sector 2) at speeds ranging from 5-3km/h. Figure 1 shows an aerial photo graph of the area with drive routes drawn in. Sector 1 is suburban with flat, open areas and relatively low buildings. Sector 2 is urban and occupied by fiveto nine-floor office buildings on undulating terrain. In each sector, the maximum distance between the enb and UE was around 72m. Also, two power settings were used in each: 34dBm and 18dBm in Sector 1; and 34dBm and 24dBm in Sector 2. Antenna system configuration The enb site installation was designed to imitate a conventional macrocellular site with antennas slightly FIGURE 1 The drive routes in Sector 1 (red) and Sector 2 (yellow) together with views of each sector as seen from behind the antenna installations. 29 Europa Technologies Image 29 Sanborn Sector 1 enb above rooftop height. The enb antennas were prototypes developed by Ericsson specifically for the measurement campaign. Figure 1 shows the enb antenna arrangement in each sector. Each enb antenna enclosure housed four linearly arranged columns with a horizontal spacing of.7λ (~8cm). Each column consisted of a dual-polarized (+45 and -45 degrees), co-localized an tenna pair. In total, there were eight antenna ports per enclosure. Each sector had multiple separate antenna Table 1: Transmission modes in 3GPP LTE Rel-8 2m Sector 2 enclosures mounted on horizontal bars, enabling reconfigurable inter-antenna spacing ranging from 8cm to 3m. The enb was equipped with a dynamic calibration system that ensured coherent transmission from the antennas. The UE antennas were mounted on the roof of a van, arranged in a square pattern with 2cm inter-antenna spacing. Two types of antennas were used: horizontally polarized antennas from SATIMO, and vertically polarized antennas from Kathrein. Both types TRANSMISSION MODE DESCRIPTION 1 Single-antenna port, port 2 Transmit diversity 3 Large-delay CDD (open-loop spatial multiplexing) 4 Closed-loop spatial multiplexing 5 Multi-user MIMO 6 Closed-loop single-layer precoding 7 Single-antenna port, port 5 ERICSSON REVIEW 1 21

3 Addressing network capacity demands with LTE MIMO FIGURE 2 Selection of cumulative distribution functions (s) of throughput achieved during test drives along the measurement routes. Top left: Sector 2, low power setting. Top right: Sector 2, high power setting. Bottom left: Sector 1, low power setting. Bottom right: Sector 2, low power setting Table 2: Parameters of the Ericsson Research LTE test bed PARAMETER Carrier frequency System bandwidth Link adaptation, closed loop operation (3GPP mode 4) [z] 1x2: SIMO [b] 1x4: SIMO [k] 4x2: x x [l] 4x2: //// [c] 4x4: x x [a] 4x4: //// [z] 1x2: SIMO [b] 1x4: SIMO [k] 4x2: x x [l] 4x2: //// [c] 4x4: x x [a] 4x4: //// Hybrid automatic repeat request (HARQ) Receiver LTE duplex mode Number of antennas (downlink) enb: 1, 2 or 4. UE: 2 or [z] 1x4: SIMO [k] 4x2: x x [l] 4x2: //// [c] 4x4: x x [a] 4x4: //// [b] 1x4: SIMO [j] 2x4: x [l] 2x4: // [v] 4x4: xx DESCRIPTION 2.6GHz 1MHz, UE always scheduled over entire bandwidth Rank, PMI and channel-quality indicator (CQI) feedback on millisecond timescale. Time between channel measurement and transmission of the resulting transport format is around 5ms HARQ with chase combining Minimum mean square error (MMSE) receiver FDD were omnidirectional in the horizontal plane. Results Power gain versus multistream gain Below we compare two categories of antenna setups: those intended to create a channel that is correlated at the transmit side, and those intended to create a channel that is uncorrelated at the transmit side. The latter category is typically well suited for multi-stream transmission (spatial multiplexing) to users with good channel conditions that is, to users near the cell center with a high signal-to-noise ratio (SNR). The former is best suited for beamforming toward users with bad channel conditions that is, to users near the cell edge with a low SNR. The correlation level at the transmit side depends not only on the antenna setup but also on the propagation environment. However, for simplicity, this article denotes the two archetypical setup types: correlated and uncorrelated. The correlated configuration was obtained by having closely spaced transmit antennas of equal polarization. In the 4 4 MIMO example, the correlated configuration is denoted with [a] in the figure; in the 4 2 MIMO example, with [l]. Four co-polarized columns were used in a single antenna enclosure, resulting in an inter-element distance of.7λ. The uncorrelated configuration was obtained using two dual-polarized columns with wide spacing. In the 4 4 MIMO example, the uncorrelated configuration is denoted with [c] in the figure; in the 4 2 MIMO example, with [k]. The inter-element distance between the dual-polarized pairs was approximately 25λ. In all four examples described above, the UE antenna configuration comprised antennas of both vertical and horizontal polarization two of each kind for the four-antenna UE setup, and one of each kind for the two-antenna UE setup. Figure 2 shows the performance of the correlated and uncorrelated benchmark configurations for drive tests in Sector 2 with the low power setting (top left), Sector 2 with the high power setting (top right), and in Sector 1 with the low power setting (bottom left). For a ERICSSON RE VIE W 1 21

4 given number of receive antennas in the UE, s corresponding to each benchmark enb configuration clearly intersect for all two- and some four-antenna UE cases. None of the benchmark configurations consistently performed best during all parts of any given route. The uncorrelated setup performed best above the intersection point, in the high throughput region, while the correlated setup performed best below the intersection point in the low throughput region. The level of the intersection point along the vertical axis gives the relative proportion of how often one setup outperformed another along a drive route. The intersection point is dependent on the scenario. In general, s that correspond to test drives with the low power setting intersect at higher points than those for test drives with the high power setting. This may be attributed to the fact that, given single-layer transmission, increased received power gives UEs operating with low receive power and a low SNR a more significant increase in throughput than UEs operating with high receive power and a high SNR. The correlated setup gives a larger power gain than the uncorrelated setup. In the higher power region, it is usually much more beneficial to transmit multiple layers, and the uncorrelated setup creates channels that better support this kind of transmission. For similar reasons, the drive routes in the more challenging Sector 2 produced curves that intersect at higher points than those of the more open Sector 1. The intersection point for s that correspond to the UE configuration with four antennas was lower than those that correspond to the UE configuration with two antennas. In a relative sense, due to its lesser ability to improve SNR by coherently combining the signals received by multiple antennas, the two-antenna UE setup benefits more from the increase in received power provided by the correlated setup. By contrast, the four-antenna UE benefits more from the uncorrelated setup, which improves the ability of the channel to support multiple layers. Increasing the number of receive antennas always improves performance. For a given number of receive FIGURE 3 s of throughput in Sector 1 using the low power setting (left), and corresponding proportion of rank selections (right) x4: enb: xx, UE: - - 4x4: enb: xx, UE: 2x2: enb: x, UE: - 2x2: enb: x, UE: Proportion antennas, multi-antenna transmission using the correlated benchmark enb setup outperforms singleantenna transmission (single input, multiple output or SIMO) in every case. In most cases, the uncorrelated setup shows a gain over SIMO, and in some cases it dramatically improves throughput. Due to the larger reference signal overhead in multi-antenna transmission, SIMO occasionally outperforms the uncorrelated setup. Figure 2 (bottom right) shows results in Sector 2 for the low power setting 5% increase at median 4x4: MIMO 2x2: MIMO enb: xx UE: - enb: xx UE: enb: x UE: - Rank 1 Rank 2 FIGURE 4 s of throughput at cell center (left) and at cell edge (right). enb: x UE: Rank 3 Rank 4 but includes setups with two transmit antennas. Clearly, the closely spaced co-polarized setup [i] performs better than the dual-polarized setup [j] for users with the least favorable channel conditions. On the other hand, the dual-polarized setup performs better for users with better channel conditions. The results also suggest that a four-antenna enb setup with two closely spaced dual-polarized pairs [v] captures the benefits of both the twoantenna enb setups shown. One other visible benefit of that setup for 113% increase at median 4x4: MIMO 2x2: MIMO ERICSSON REVIEW 1 21

5 Vinjett Addressing network capacity demands with LTE MIMO FIGURE 5 The selected parts of the drive route in Sector 2 with a picture of the enb antenna installation overlaid with schematic drawings of the antenna setups. 29 Europa Technologies Image 29 Sanborn 29 Google Gray buildings 28 Sanborn Cell-edge Use-case: Low power a four-antenna UE is the ability to transmit more than two layers when channel conditions allow it. Polarization-matching aspects The UE antenna configurations described thus far were composed of purely vertically and horizontally polarized antennas. Such configurations are well matched to enb antenna configurations and transmission modes that rely on polarization diversity for example, multi-stream transmission with dualpolarized antennas. Note, how ever, that perfectly orthogonal polarizations are hard to obtain in practical UE implementations. The ability to support multistream transmission using polarization diversity diminishes the more similar the UE antennas are to each other in polarization. To illustrate this principle, tests were performed for both 4 4 and 2 2 MIMO using exclusively vertically polarized antennas in the UE. Figure 3 (left) shows the resulting s. In terms of throughput, performance decreases in the high-throughput region with an unmatched UE antenna configuration, due to the reduced ability to sustain multilayer transmission. This is also evident Cell-centre Use-case: High power enb from the bar charts in Figure 3 (right), which show the transmission-rank proportion. The s also suggest that the opposite holds true in the low-throughput region, where singlelayer transmission dominates. This effect may be ascribed to the ability of an enb with dual-polarized antennas to match the effective polarization of the transmission through proper choice of precoder. The polarization can be matched when the UE has antennas with parallel polarization, but not when the polarization is mixed as in the orthogonal case. The intersection point for the 4 4 setups is different from that of the 2 2 setups. The gain from matching polarization is mainly one of SNR; the s for the 2 2 setups intersect at a higher point than those for the 4 4 setups, which is in line with the results reported above. LTE mobile broadband now and tomorrow The first commercial LTE networks are currently being launched. Multiantenna technology is thus already a vital component in the evolution of mobile broadband. And its importance will continue to increase as user traffic grows and puts greater demands on network capacity. Two use cases exemplify the potential of LTE multiantenna technology to meet these increasing capacity demands: a user close to the cell center, limited by cell capacity and peak throughput; and a user close to the cell edge, limited by network coverage. Figure 4 (left) shows the first use case. Those parts of the Sector 2 measurement route that primarily support the transmission of multiple streams were selected with the transmit power set to high. A typical 2 2 MIMO configuration (which represents the first generation of multi-antenna-capable LTE products) is presented together with a 4 4 MIMO system that uses the uncorrelated transmit antenna setup. The second use case is illustrated in Figure 4 (right), comprising the parts of Sector 2 that are closer to the cell edge, with the transmit power set to low. A 4 4 MIMO setup using the correlated transmit antenna configuration is presented together with the reference used in the first case. Figure 5 shows the selected parts of the drive route in Sector 2. Note that 3GPP LTE Rel-8 supports up to twice the frequency bandwidth used in the measure ments. For the cell center case, a 5 percent increase in median throughput was measured; for the cell edge case, the median throughput was more than doubled. These examples clearly illustrate the potential of the multiantenna technologies in LTE Rel-8 to appease growing demands on network capacity. ERICSSON RE VIE W 1 21

6 Johan Furuskog joined Ericsson in 27 after graduating from Uppsala University in Sweden with an MSc in engineering physics. His work as a research engineer at Ericsson Research in Kista, Sweden, has mainly concerned test-bed development and field trials with focus on MIMO channel characteristics and LTE multi-antenna performance. Johan is currently involved in different test-bed projects, at both Ericsson Research and Business Unit Networks, which target the design and deployment of future systems for radio access. Mathias Riback who joined Ericsson in 24, is currently project manager for a future access test-bed at Business Unit Networks, Kista, Sweden. Before this he worked as a senior research engineer at Ericsson Research with the main focus on radio propagation research and test-bed development for both HSPA and LTE with emphasis on multi-antenna aspects. Mathias holds an MSc in electrical engineering from the Royal Institute of Technology, Stockholm, Sweden. Bo Hagerman received an MSc EE, Lic. Tech. EE and PhD in Radio Communication Systems from the Royal Institute of Technology, Stockholm, Sweden in 1987, 1993 and 1995, respectively. From 1987 to 199, he was a member of the techn ical staff in the Ericsson Radio Systems Research and Development depar tment, where he worked in the area of signal processing with applications to GSM receivers. He joined the Radio Access Technologies Research department at Ericsson Research, Stockholm in 1995, where he is currently a Senior Specialist in the area of Advanced Antenna Systems, working with research on multiple antenna systems and heterogeneous deployments in cellular networks. Karl Werner joined Ericsson in 27. As a research engineer at radio access technologies in Stockholm, Sweden, he has mainly worked with test-bed implementation and with field measure ment campaigns targeting performance of antenna systems for LTE. He holds a PhD in Signal Processing and an MSc in computer engineering from the Royal Institute of Techno logy in Stockholm, Sweden, and Lund University, Lund, Sweden, respectively. References 1. Dahlman, E. Parkvall, S. Sköld, J. & Beming, P., 27, 3G Evolution: HSPA and LTE for Mobile Broadband. 2nd ed, Oxford, Academic Press 2. 3GPP TS rd Generation Partnership Project, Technical Specification Group Radio Access Network: Evolved Universal Terrestrial Radio Access (E-UTRA), Physical Channels and Modulation. Release GPP TS rd Generation Partnership Project, Technical Specification Group Radio Access Network: Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer Procedures. Release 8 4. Riback, M. & Karlsson, J., 28. Initial Performance Measurements of LTE. Ericsson Review, Vol. 85 (1), pp Johansson, B. & Sundin, T., 27. LTE test bed. Ericsson Review, Vol. 84 (1), pp Riback, M., Grant, S., Jongren, G., Tynderfeldt, T., Cairns, D. & Fulghum, T., 27: MIMO-HSPA Testbed Performance Measurements in proceedings of PIMRC, Athens ERICSSON REVIEW 1 21