The Application of MIMO Technology in Mobile WiMAX NetworkInternal

Technical ResearchThe Application of MIMO Technology in Mobile WiMAX NetworkPrepared by: Xieguozhu Huawei Technologies Co.,Ltd.MIMOMultiple Input Multiple OutputTechnology, as the great breakthrough in the telecommunication industry, has been grabbing the attention of the vast researchers all around the world. This article discusses and analyzes the application of MIMO technology in the mobile WiMAX network, from the angle of the improvement of network coverage, the enhancement of system capacity, the reduction of network construction cost and etc, with the reference of Huawei system simulation and field test results.In the recent years, the number of bandwidth subscribers in the traditional wired network is increasing significantly, so is the 3G mobile network subscribers. The increasing requirement of the market about the mobile bandwidth usage provides the wireless broadband access technology an enormous room of expansion.WiMAX 802.16e has been chosen by more and more operator as the primary access technology for the fixed and mobile broadband network, and hence to provide the abundance of high-broadband multimedia services. This phenomenon imposed the great challenges to the wireless network operators where they have to ameliorate their network capacity, subscribers throughput, and network coverage quality, in order to establish and maintain their business model. Nevertheless, the application of MIMO technology helps the WiMAX 802.16e to confront the challenges and hence to expand the WiMAX network development. MIMO indicates the multiple antennas in the transmitter and receiver sides where the signal is transmitted and received through the multiple transmitters and receivers and in the end, improve the service quality of every subscriber. From the point of view of a traditional single antenna system, MIMO technology can improve the spectral efficiency significantly, and thus enable the system to transmit the higher rate of data services using the limited wireless bandwidth. The figure below shows the MIMO system with N transmitters and M receivers:

MIMO System with N Transmitters and M ReceiversWiMAX 802.16e defines 3 types of MIMO technology, which are STTDSpace Time Transmit Diversity, SMSpatial Multiplexingand Adaptive MIMO Switch, and 3 types of encoding matrixes, which are Matrix A, Matrix B and Matrix C. The WiMAX 802.16 system of Huawei are currently support MIMO Matrix A, and will support MIMO Matrix B and MIMO Matrix C in the September of 2008.MIMO technology can be implemented together with OFDMA technology in the WiMAX 802.16e system. This implementation can enhance the network coverage significantly and improve the WiMAX system capacity, and hence reduce the cost of network construction and maintenance. The Perfect Integration of MIMO & OFDMAMIMO can be implemented in all of the wireless communication technologies. The perfect integration of MIMO and OFDMA Orthogonal Frequency Division Multiple Access enable the advantages of MIMO technology to be implemented more explicitly.MIMO system can overcome the multi-path loss, but still helpless in the frequency selective fading, where the other communication systems normally implement the balancing technology to solve the frequency selective fading in the MIMO system. On the other hand, the OFDMA technology can overcome the frequency selective fading. The mobile communication of next generation requires the technology with high spectral efficiency, but the OFDMA has a limited capability in enhancing the spectral efficiency. Hence, by integrating the MIMO technology with the OFDMA technology, the spectral efficiency can be enhanced without increasing the system bandwidth. In conclusion, the integration of MIMO and OFDMA technologies can provide the data transmission with higher rate, and also improve the system reliability and stability thorough the diversity. In addition, OFDMA has the strong ability to overcome the multi-path inference because of its low coding rate and the embedded time guard interval, where the system would not be interfered by the inter-coding interference provided the multi-path latency is smaller than the guard interval. This can enable the single frequency network which utilizing the broadband OFDMA and MIMO system to overcome the shadow fading, and finally to realize the seamless network coverage. Enhance the Network Coverage SignificantlyThe transmission loss of WiMAX 802.16e system is higher than the other mobile network systems due to its higher frequency band used. Hence, how to enhance the network coverage becomes one of the great challenges WiMAX encounters. And here, MIMO technology indeed helps a lot in enhancing the network coverage. For the MIMO technology under STTD model, the cell coverage radius can be improved through the STTD gain; For the MIMO technology under SM model, the cell coverage radius can be improved indirectly through STTD gain from the enhancement of cell edge data rate; For the MIMO technology under the Adaptive MIMO Switch model, its coverage gain is similar with the gain of STTDs because the cell edge works under the STTD model. From the Huawei system simulation result, the system of 2T2R2 Transmitters 2 Receivers MIMO has a coverage gain of 2 ~ 10dB compared with SISOSingle Input Single Outputunder the STTD and Adaptive MIMO Switch models, where the coverage radius improves 5090; under the SM model, the cell edge can obtain the coverage gain of 3~5dB indirectly. On the other hand, MIMO can enhance the cell edge coverage probability as well.In August of year 2007, Huawei incorporate with a local operator JCOM in Tokyo, Japan, to run the field test of MIMO on-site implementation effect. The field test results show that, the coverage radius of WiMAX 802.16e system can be enhanced by utilizing MIMO Matrix A technology, and hence the number of base station needed for the network construction can be reduced in a large extent.Improve the System CapacityWiMAX 802.16e system provides a high data throughput with high mobility, which enable the subscribers to stay online all the time, and enjoy the broadband experience even at the mobility situation. The MIMO technology under the SM model can double the system throughput and spectral efficiency, and double the single user data peak rate as well; For the MIMO technology under the STTD model, the system throughput and spectral efficiency can be improved by utilizing the high modulation scheme; For the MIMO technology under the Adaptive MIMO Switch model, the cell center works on the SM model while the cell edge works on the STTD model, hence the improvement of system throughput and spectral efficiency are between the effect of STTD and SM. The implementation of OFDMA and MIMO enable the WiMAX 802.16e system to enhance the spectral efficiency up to the maximum extent and to provide the high quality mobile video and television services which requires a high speed and wide bandwidth. From the Huawei system simulation result, it shows that the system throughput can be enhanced around 30 ~ 60% after implementing MIMO 2x2 Matrix B technology, and at the same time, double the data peak rate of subscriber. Reduce the Cost of Network Construction and MaintenanceIn the dense urban and CBD areas, there are more high-end subscribers and hence the requirement of system throughput and data peak is higher as well, which may lead to the capacity-limited phenomenon. However, after utilizing the MIMO Matrix B technology, the WiMAX system downlink throughput was improved 55%, while uplink throughput was improved up to 33%, and in the capacity-limited cases, the number of base station can be reduced around 25%. MIMO has more advantages of the capacity gain in the city areas compared with the other multiple antenna technologies, such as AASAdaptive Antenna System, or Advanced Antenna System, which helps a lot in reducing the cost of high traffic network construction or network expansion.In the cases of coverage-limited, the coverage radius can be improved at least 50% and the coverage area of a single can be improved approaching 100% by utilizing the MIMO technology. This helps to save up to 40% ~ 60% number of base station, and also helps to provide a low-cost network construction solution in the earlier stage in suburban and rural areas by providing the biggest coverage with least number of base station. On the other hand, from the installation and maintenance wise, AAS requires the antenna with at least 4 beamwidth and bigger space. AAS antenna also requires more feeder compared to the traditional antenna, and this will lead to more installation works and wider cable rack. Besides, AAS antenna weight more than the common antenna, which require the tougher antenna installation work, and its bulky volume performs weak in the areas with strong wind due to its poor counter-wind ability. MIMO requires less number of antenna compared with AAS, where one dual-polarization antenna with 45 degree is sufficient to support a 2x2 MIMO technology, hence it has a lower requirement in antenna installation work, antenna space consumption, in addition, its easier to be planned and maintained, and hence it saves the cost of antenna installation and maintenance effectively. ConclusionThe rapid development of broadband data subscribers and mobile communication subscribers, the trend of moving from narrow band voice communication to broadband high speed data communication, the way to improve the network capacity in a fixed spectral resources, and the way of enhancing the network coverage with the fixed number of base station, are the main problems need to be concerned in the future communication network construction. WiMAX 802.16e system introduces the MIMO technology, with the integration of OFDMA, to enhance the spectral efficiency and improve the network coverage and capacity at the same time, by strengthen the system differentiation ability in space. And finally, it can reduce the cost of network construction and maintenance, and enable WiMAX 802.16e system to become the most competitive wireless broadband access technology.

2007-10-17Huawei Confidential Page 1