Distributed Antenna Systems and MIMO Technology

Transcription

1 With the explosive growth in wireless usage due in part to smart phones, tablet computers and a growing applications developer base, wireless operators are constantly looking for ways to increase the spectral efficiency of their networks. Multiple input/ multiple output (MIMO) technology, which made its first broad commercial appearance in n systems, is now gaining substantial momentum in Mobile Wireless Wide Area Network (Mobile WWAN) with the launch of WiMAX and LTE networks. MIMO is a key technology in these networks which substantially improves network throughput, capacity and coverage. In this paper, Tyco Electronics (TE) provides an introduction to MIMO technology and reports the results of tests it performed to characterize the relative performance of MIMO and single input/single output (SISO) transmission schemes.

2 About MIMO MIMO stands for multi-input and multi-output. Input refers to the number of transmitters and output refers to the number of receivers. SISO mode (single input/single output) is the more conventional mode of communication, where there is one antenna transmitting and one antenna receiving. There are also Multiple Input/Single Output systems (MISO), which is like having multiple base stations transmitting to the user device at the same time as done in downlink soft-handoff in CDMA systems. There are also the Single Input/Multi Output systems, which are used in traditional cellular uplinks, where the device has one antenna but there is receive diversity on the base station. Figure 1 For MIMO to work, a rich scattering environment, with many different paths between transmitter and receiver, as well as a high signal-to-noise ratio are needed. Rather than being a detriment to network performance, multi-path is actually exploited by MIMO processing to increase the capacity or the coverage of the network. The key is that each path must be independent and look different to the receiver. The differences in the multipath are used to create orthogonal communication channels analogous to the orthogonal spreading codes in CDMA-based systems. In addition to being required for the higher orders of modulation, such as 16-QAM and 64-QAM, a high signal-to-noise ratio is also required to properly model the wireless channels that allow the MIMO systems to algorithmically separate the multiple paths which overlap one another in frequency and time. An in-building DAS system is ideal for MIMO because it provides very good signal-to-noise ratio, and in-building environments provide a rich scattering environment. MIMO Types Multiple Anntena Usage MIMO uses multiple transmitters and multiple receivers. Multi-antenna configurations have been around for years, but with advances in signal processing and silicon, MIMO is now economically possible in many small form factor devices such as handsets and data cards. While initial WiMAX and LTE networks use 2x2 MIMO (where there are two transmit antennas and two receive antennas), future LTE systems will use 4x4 MIMO and even higher dimensions of antenna configurations. It is projected that LTE Advanced systems will have the ability to use up to 8x8 MIMO antenna configurations. There are two major categories of MIMO spatial diversity, in which the same data is transmitted over each of the multiple paths, and spatial multiplexing, in which each of the paths carries different data. In 2x2 MIMO with spatial diversity, for example, each of the two antennas is essentially transmitting and receiving the same data although the data is coded differently. This mode is primarily used to improve signal quality, or to increase the coverage area. In 2x2 MIMO with spatial multiplexing, two different streams of data are transmitted over each of the two channels, which theoretically doubles the system throughput. Spatial multiplexing is the mode that really takes advantage of the capacity improvement capabilities of MIMO. The system throughput can be increased linearly with the number of transmit antennas without using any additional spectrum resources. Given the scarcity and cost of the wireless operator s spectrum, improving the spectral efficiency is a critical goal for improving the overall financial operating margins for the wireless operator. It is important to note that commercial MIMO systems switch dynamically between SISO, MIMO diversity, and MIMO multiplexing modes, depending on a variety of factors including the channel environment and signal quality. For example, if the signal quality is very high the system uses spatial multiplexing, and if not, it automatically switches to spatial diversity mode or even to SISO mode. Page 2

3 MIMO Testing A WiMAX MIMO system was used to test MIMO performance in TE s San Jose, California facility. The test used a pre-commercial WiMAX base station and CPE device that could be statically set to remain in spatial diversity, spatial multiplexing, or SISO mode. Throughput tests were then conducted at various points in the building using each of these modes. Two phases of testing were conducted. The first phase focused primarily on gaining comfort and experience with the system and with MIMO testing. Throughput tests were conducted in variety of environments throughout the facility to get a flavor for the type of MIMO performance that could be achieved. The second phase of the testing focused on a more uniform and homogenous environment which provided a more consistent basis on which to analyze the performance. The testing was conducted in the 2.6 GHz WiMAX TDD band. While the testing was conducted on a WiMAX system, the results are extendable to LTE MIMO performance since the air interfaces of WiMAX and LTE are very similar in that they both use OFDM as the basic modulation scheme, and they both incorporate MIMO. The principal difference is that the current commercial deployments of LTE operate in frequency division duplex mode (FDD), while the majority of commercial WiMAX deployments operate in time division duplex mode (TDD). In certain implementations of MIMO, TDD operation provides better performance as the channel models in the downlink and uplink should be nearly identical. However, with closed loop MIMO that is specified for WiMAX and LTE, there should be minimal differences in performance. In addition to statically setting the MIMO mode, the modulation and coding scheme was also statically set in the system. This allowed for the comparison of the throughput performance between each MIMO mode at specific modulation and coding schemes. Modulation schemes ranging from QPSK up to 64QAM at different coding rates were tested. Only downlink testing was performed. The test environment was a 2-story structure about 300 feet long. The floor plan of the first floor of the facility is shown in Figure 2. In the first phase of the testing, the base station was used to drive a single MIMO-capable remote antenna unit, while in the second phase two MIMO-capable remote antenna units were used. The antennas were spatially separated by 6 wavelengths, which is approximately 27 inches at 2.6 GHz. Figure 2 1 MIMO RAU was Deployed Approx. 200 ft Test Locations Location A 70 ft Location G 236 ft Location B 100 ft Location H 270 ft Location C 115 ft Location I 42 ft Location D 187 ft Location J 6 ft Location E 55 ft Location K 40 ft Location F 47 ft Location L 210 ft SISO 64QAM 2/3 does not work First Floor Initial attempt at measuring SISO/MIMO throughput Data taken in a variety of environments and distances RAU and Antenna (Antenna spacing: 6 wavelength) Base Station and MH, EH Page 3

4 Antenna Separation Antenna separation is critical with MIMO systems as MIMO performance requires that the multiple paths between each of the transmit and receive antennas are highly de-correlated. The decorrelation of the antenna paths in turn has a heavy dependence on the angular separation of the paths when viewed from the perspective of the receiving antenna. Angular separation improves as the distance increases between the antennas in the system. The minimum spacing between antennas specified in industry literature ranges widely, from 3 to 7 wavelengths. In theory, one wavelength of separation should be sufficient, but it depends on the environment and the amount of signal scattering. Current LTE operator guidelines for spatial separation are on the order of 2 to 3 wavelengths, or 2 7 to 3 11 at 750 MHz. TE recommends 4 to 6 wavelengths of separation where possible (roughly 5 to 8 spacing), and an absolute minimum of 2 to 3 wavelengths when tight spaces dictate. As an alternative to spatially separated antennas, dual-pole antennas are increasingly being used in MIMO deployments, as the measured performance differences between the two antenna configurations are negligible in real world applications. Dual-pole antennas are a great solution in situations where only a single antenna housing can be installed. Test Results Phase I For the first phase of testing, the RAU was installed on the first floor of the western side of the building as indicated in Figure 2. Because throughput measurements were taken with the CPE device at various locations throughout the building, the transmit power was set to +15dBm per antenna. A variety of test environments were selected, such as a cubicle area, a warehouse, a private office, small and large conference rooms as well as a large training room. Measurements were also taken on the second floor of the building. The colored squares in Figure 2 indicate the test locations. The primary goal of this first phase was to quantitatively measure the performance improvements that could be achieved with MIMO in these various environments. Figure 3 shows the throughput results for SISO, MIMO spatial diversity, and MIMO spatial multiplexing for each of the tested locations. The table in Figure 3 is organized into three major sections, each corresponding to one of the antenna configuration modes. The top section contains the results for SISO mode, the middle section, labeled MIMO-A, contains the results for spatial diversity mode, while the bottom section, labeled MIMO-B, contains the results for spatial diversity mode. Each section is further sub-divided into rows which represent the different modulation and coding schemes. Each column represents a different CPE location in the building, Figure 3 2nd Floor DL Type Modulation Mbps Mbps Mbps Mbps Mbps Mbps Mbps Mbps Mbps Mbps Mbps Mbps Mbps Mbps Avg Location J-6ft K-40ft I-42ft F-47ft E-55ft A-70ft B-100ft C-115ft D-187ft N-205ft M-206ft L-210ft G-236ft H-270ft DL(Mbps) QPSK 1/ SISO QPSK 3/ QAM 1/ QAM 3/ QAM 2/ Location J-6ft K-40ft I-42ft F-47ft E-55ft A-70ft B-100ft C-115ft D-187ft N-205ft M-206ft L-210ft G-236ft H-270ft DL(Mbps) QPSK 1/ MIMO-A QPSK 3/ QAM 1/ QAM 3/ QAM 2/ Location J-6ft K-40ft I-42ft F-47ft E-55ft A-70ft B-100ft C-115ft D-187ft N-205ft M-206ft L-210ft G-236ft H-270ft DL(Mbps) QPSK 1/ MIMO-B QPSK 3/ QAM 1/ QAM 3/ QAM 2/ MIMO-A: MIMO-B: 64QAM 2/3 and 16QAM 3/4 works at more locations and has higher data rates than SISO. Typically doubles data throughput. Page 4

5 with increasing distance from the RAU, moving from left to right. For example, the left-most column results were obtained with the CPE about 6 feet from the RAU while the right-most column results were obtained at about 270 feet from the RAU. The results are presented in megabits per second. The maximum downlink throughput of the system was limited to 11 Mbps by the firmware release of the base station and the CPE device. As expected, for those locations that were close to the RAU or where there was good signal quality, the SISO results and the MIMO diversity results were similar because the same data was transmitted over both antenna paths (no additional data is being transmitted in MIMO diversity versus SISO). When the signal quality was sufficiently high such that SISO was able to achieve the maximum data rate, MIMO diversity was not able to improve the signal quality any further and as a result no improvement in throughput was observed as is evident in the first 9 columns of Figure 3. However, in areas farther from the RAU where the signal quality is not as good, MIMO diversity provided the maximum throughput where in some cases the system could not maintain a link in SISO mode. For example, with 16 QAM 3/4 rate coding, the system could not sustain a link at test location N (column 10) while in SISO mode. However, in MIMO diversity mode the system achieved a throughput of 4.09 Mbps. Likewise, with 64 QAM 2/3 rate coding, the system could not sustain a link at test location L (column 12) in SISO mode but achieved a throughput of 5.47 Mbps while in MIMO diversity mode. This shows how MIMO diversity mode can improve signal quality. The yellow-shaded cells show areas where the throughput was lower than expected; here again, MIMO diversity mode shows an improvement in throughput over SISO because of the improved signal quality. The key point is that MIMO diversity mode can improve signal quality in poor environments and offer better throughput than SISO in those cases. While the improvements achieved by MIMO diversity are great, what really excites the wireless community about MIMO is the throughput improvements achievable with MIMO spatial multiplexing. It is with spatial multiplexing that the throughput can be increased linearly with the number of the transmit antennas. Thus, with 2x2 MIMO the throughput can be doubled over SISO without any additional spectrum resources. The MIMO spatial multiplexing results in Figure 3 show a near doubling of throughput relative to MIMO diversity and SISO. In this mode, a unique stream of data was transmitted from each of the two MIMO antennas, which effectively doubled the overall throughput of the system. For example, in the first nine columns of Figure 3, SISO and MIMO diversity delivered throughput of roughly 5.46 Mbps for 64QAM modulation with 2/3 rate coding, while MIMO spatial multiplexing delivered roughly Mbps of throughput with the same modulation and coding scheme. In these locations the signal quality was sufficiently high that MIMO was able to provide the maximum throughput expected. Even in other locations where the signal quality may not have been high enough to provide 64 QAM performance, spatial multiplexing doubled the achievable throughput for the lower modulation and coding schemes which still provides a substantial improvement in overall network throughput. Averaging the throughput over all test locations resulted in an 85% increase in system throughput. MIMO spatial multiplexing means that mobile operators can now use spectrum resources much more efficiently. The increased overall network throughput can be used to increase system capacity by servicing users more efficiently and getting users on and off the network more quickly, which allows the operator to service more users. Notice that there were areas where the signal quality was sufficiently poor (the orange cells) that a link could not be sustained with spatial multiplexing at the higher modulation and coding schemes. This underscores the requirement that high signal quality is essential for proper spatial multiplexing operation, especially for higher orders of modulation. Test Results Phase II The primary purpose of the second phase of testing was to observe the variability in MIMO performance over a more uniform and homogeneous environment. The testing was conducted over a 6000 square foot area consisting mainly of half height cubicles, roughly 53 inches in height. Two RAUs were deployed, as shown in Figure 4, in order to obtain more data with the same test setup. Due to the relatively small test area, the output power of each RAU was reduced to roughly +2dBm per antenna such that each RAU would barely cover the entire test area. The RAUs were not transmitting simultaneously but were tested separately. Page 5

6 Figure 4 88 feet RAU #1 6λ ant.spacing RAU #2 6λ ant.spacing feet A uniform grid of 20 measurement points, arranged in a 5 x 4 grid, was selected in order to uniformly map the performance across the test area. As with the first phase of testing, 6-wavelength spacing was used for the MIMO antennas at the RAUs. The CPE was placed on a cart 30 inches off the ground, which is roughly the height of the desks. The BTS firmware and the CPE device used in the second phase of testing limited the maximum throughput to roughly 5 Mbps. The results for the second phase of testing, as shown in Figure 5, are arranged into a 5 x 4 grid to graphically correlate with the 20 test locations. Figure 5 contains two grids, one for each RAU tested. The top grid lists the throughput results for the testing conducted with the first RAU while the bottom grid lists the results for the testing conducted with the second RAU. In the top grid, the RAU can be envisioned to be located to the left of the grid and the results for the first column of data are associated with locations that are closer to the RAU. The test locations for the results in the second through fourth columns are increasingly farther from the first RAU with the test locations associated with the results in the fourth column being roughly 66 feet from the first RAU. In the bottom grid, the RAU can be envisioned to be located on the right side of the grid and the results for first column of data are associated with locations that are furthest from the second RAU. The test locations for the results in the second through fourth columns are decreasingly closer to the second RAU. As a point of reference, the test locations corresponding to the results in the first column are roughly 88 ft. from the second RAU. Page 6

7 Figure 5 Throughput performance RAU #1 SISO MIMOA MIMOB RSSI SISO MIMOA MIMOB RSSI SISO MIMOA MIMOB RSSI SISO MIMOA MIMOB RSSI Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Throughput performance for RAU #2 ~66 ft. distance from RAU #1 SISO MIMOA MIMOB RSSI SISO MIMOA MIMOB RSSI SISO MIMOA MIMOB RSSI SISO MIMOA MIMOB RSSI Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm Loc # dBm ~88 ft. distance from RAU #2 ~66 ft. distance from RAU #2 Throughput numbers in blue indicate operation at a lower modulation-coding level than 64-QAM 3/4. Varies from 16-QAM 1/2 to 64-QAM 1/2 Averaged the throughput over the entire area for each antenna configuration SISO: 2.41 Mbps MIMO-A: 2.43 Mbps MIMO-B: 4.17 Mbps (173% of SISO) Once again, throughput performance was measured for SISO, MIMO spatial diversity, and MIMO spatial multiplexing modes. As in the first phase of testing, the results for MIMO diversity are in the columns labeled MIMO-A while the results for spatial multiplexing are listed in the columns labeled MIMO-B. The results again show that throughput nearly doubles with MIMO spatial multiplexing as opposed to MIMO diversity and SISO. Note, however, that at the farthest distances from the RAU, the MIMO spatial multiplexing throughput was slightly reduced, but it is still substantially higher than the throughput for SISO or MIMO diversity. In some of these instances, a lower degree of modulation and coding was required to maintain the link, as indicated by the results in red, but the overall throughput is still much higher with spatial multiplexing. It was estimated that the received signal level at these extreme locations was on the order of -71 dbm which is approximately the receive sensitivity level for 64-QAM operation. Averaging the throughput results over all 40 test points, MIMO spatial multiplexing mode delivered a 73% increase in throughput over MIMO spatial diversity and SISO modes. These tests demonstrate that MIMO spatial multiplexing is capable of delivering nearly double the throughput of MIMO spatial diversity and SISO, making MIMO an important transmission mode for mobile operators seeking to maximize throughput and capacity in their networks without requiring additional spectrum. TE DAS Built for MIMO TE s InterReach Fusion and InterReach Spectrum indoor DAS systems are uniquely suited for MIMO operation. TE DAS systems convert Radio Frequency (RF) signals to Intermediate Frequencies (IF) for distribution, while competitors keep the signal at RF for distribution. This means that in a TE system, both MIMO bands can be transported over a single cabling infrastructure (i.e. the same optical or CATV cable), and because the RF frequencies are mapped to different IF frequencies, they can be transported and then transmitted from independent MIMO antennas at the RAU. Competitors use the native RF frequencies to transport signals over coax or optical cable and cannot separate the MIMO signals, so they cannot use just one cabling infrastructure to transport the MIMO signals they need two cables to transmit each of the two MIMO signals. Page 7

8 TE s MIMO Experience TE has several years of experience in working with MIMO. It began with development of the Fusion WiMAX system, in 2008, which is the only MIMO DAS approved for use by Sprint and Clearwire. The first Fusion WiMAX systems were deployed at Dulles and Reagan National Airports, and have been on the air since early The systems operate in the 2.6 GHz WiMAX band using 10 MHz RF channels. TE has shipped well over 200 LTE systems to date, which includes over 3000 RAUs. The majority of these are MIMO-based systems (80%). Some of the more noteworthy installations include: The headquarters of a major wireless carrier (delivering 38 Mbps on the downlink and 12 Mbps on the uplink) A very large corporate campus in Silicon Valley (>20 Mbps downlink and >15 Mbps on the uplink) A mid-sized southwestern airport (this is a SISO mode deployment that delivers about 20 Mbps on the downlink) A Las Vegas casino and convention center (this was installed to support the 2011 Consumer Electronics Show, and delivers over 20 Mbps on the downlink and 6 Mbps on the uplink). Conclusion TE s internal testing demonstrates the practical benefits of MIMO, which improves signal quality with MIMO spatial diversity, or throughput with MIMO spatial multiplexing. However, it is the MIMO spatial multiplexing mode that has created the latest buzz in the Wireless Wide Area Network industry as it delivers significantly higher throughput for mobile networks without the need for additional spectrum. At a time when 4G network bandwidth will be a key advantage in a service offering, MIMO makes sense for all mobile operators. Figure 6 2x2 MIMO signals overlap at the same RF frequencies RF 1 RF 1 RF 1 Intermediate Frequency (IF) multiplexing separates MIMO signals to different IF frequencies IF 1 IF 2 MIMO signals interfere with each other for RF distribution over coaxial cable. IF multiplexing transports 2x2 MIMO signals over a single low bandwidth medium (CATV cabling) WHITE PAPER Contact us: Tyco Electronics Corporation P.O. Box 1101 Minneapolis, Minnesota USA Tel: Tel: Fax: TE Connectivity, TE connectivity (logo), Tyco Electronics, and TE (logo) are trademarks of the TE Connectivity Ltd. family of companies and its licensors. While TE Connectivity has made every reasonable effort to ensure the accuracy of the information in this document, TE Connectivity does not guarantee that it is error-free, nor does TE Connectivity make any other representation, warranty or guarantee that the information is accurate, correct, reliable or current. TE Connectivity reserves the right to make any adjustments to the information contained herein at any time without notice. TE Connectivity expressly disclaims all implied warranties regarding the information contained herein, including, but not limited to, any implied warranties of merchantability or fitness for a particular purpose. The dimensions in this document are for reference purposes only and are subject to change without notice. Specifications are subject to change without notice. Consult TE Connectivity for the latest dimensions and design specifications Tyco Electronics Corporation, a TE Connectivity Ltd. Company. All Rights Reserved AE 4/11 Revision 2011