Combination of MIMO and Beamforming technology for WIMAX

Ⅰ Overview

The so-called multi-antenna technology is a wireless communication technology which uses more than one antennas in both Base Station (BS) and Mobile Station (MS). It is becoming one of the rapidly developmental hotspots among broadband wireless access technologies. It’s also one of the key technologies in many wireless communication standards, such as 16e,16m,LTE,LTE-A. The application of the multiple antenna technology brings Power Gain, Space Diversity Gain, Spatial Multiplexing Gain, Array Gain, Co-channel Interference Reduction Gain. Therefore, it is used to improve the system coverage, enhance the link reliability and increase system capacity, and what’s more, these performances can be achieved without obvious cost increase in wireless communication systems.

Generally speaking, Space-Time Block Coding (STBC) is the proper technology to achieve the Spatial Diversity Gain while Space Multiplexing (SM) is for the Multiplexing Gain in MIMO system. As shown in Figure 1(a), STBC offers redundancy in the spatial dimensions by transmitting a signal on more than one antenna during two time slot. For the spatial Multiplexing, as shown in Figure 1 (b), it sends a different signal on each time-frequency resources of each antenna, could multiply spectrum efficiency without additional spectrum resources.

Figure 1 MIMO system

Array Gain and Co-channel Interference Reduction Gain can be achieved by Beamforming (BF) technology. As shown in Figure 2, by weighting the signal streams, the BS forms a narrow wave beams which points to the direction of aim user while suppress the interference signal from non-aim user, as a result, both Array Gain and Co-channel Interference Reduction Gain could be implemented.

Traditional BF technology is based on estimating the DOA (Direction of Arrival) of beamforming phased-array and calculating the beamforming weights. For WiMAX system, the Beam is usually formed based on channel coefficient matrix which is used to obtain the beamforming weights. The Beam Forming technology is also called “MIMO-BF” or “MIMO BF” in WiMAX industry.Different with MIMO+BF, MIMO-BF or MIMO BF is solely Beam Forming without being combined with MIMO Matrix A or MIMO Matrix B.

Figure 2 BF Systems

As we know, BF transmits only one data stream at a time and there is no multiplexing gain and diversity gain. Especially when the channel condition is good, multiplexing gain is the main bottleneck for system and BF is not the best choice to improve the system throughput. Therefore, in order to further enhance the throughput, we could combine BF and MIMO together and come into a MIMO+BF technology which including STBC+BF and SM+BF. MIMO+BF has the advantages of both MIMO and BF. Therefore, it not only improves the coverage, but also increase system throughput remarkably.

Ⅱ. MIMO + BF implement schemes

In general, there are two ways to implement MIMO+BF scheme, one bases on the antennasub-array, another bases on the entire antenna array.

　　　　 Scheme 1, bases on the antennasub-array

　　　　  The antennas of BS are divided into two sub-arrays; each sub-array formed a beam towards MS.

　　　　  The data streams after MIMO coding (STBC or SM) are mapped onto sub arrays separately.

　　　　  The system architecture shown in Figure 3.

Figure 3 MIMO+BF Scheme 1- based on the antennasub-array

Take SM+BF as example, the way of transmitting data streams for each antenna is shown in Figure 4, wherein, represents different data streams,  represents beamforming weight.

Figure 4 Data transmission scheme of SM+BF based on antenna sub-array

　　　　 Scheme 2, based on the entire antenna array

　　　　  All antennas of BS form 2 beams towards MS.

　　　　  The data streams after MIMO coding (STBC or SM) are mapped onto beams separately.

　　　　  The system architecture shown in Figure 5

Figure 5 MIMO+BF Scheme 2- based on the entire antenna array

Take SM+BF as example, the way of transmitting data streams for each antenna is shown in Figure 6, represents different data streams, represents beamforming weight. In this scheme, each antenna sends the superposition of data streams s1 and s2.

Figure 6 Data transmission scheme of SM+BF based on entire antenna array

Ⅲ. Technical Requirement

In order to implement the MIMO+BF, the system needs to satisfy the following condition：

²　　　　  Both the BS and MS should support the dedicated pilot

²　　　　  Both the BS and MS should support the MIMO technology

²　　　　  The MS is configured more than one receive antennas in the SM+BF scenario.

²　　　　  To get better performance, the MS should support one or two antennas to send Sounding signal. (At present, there is no MS support the Sounding technology.)

At present, ZTE has proposed different solutions to meet the various MS, regardless of the MS whether to support the Sounding.

Ⅳ.Performance

Both ZTE Corporation and WIMAX Forum’s simulation results show, MIMO+BF has the following advantages:

　　　　  Increase the received signal strength of each stream.

 MIMO+BF can enhance system coverage and improve the cell edge user throughput, which reduce network construction and maintenance costs greatly.

　　　　  Improve the system’s CINR

 When the total transmits power keep constant, the CINR of 4Tx2Rx SM+BF has 5 ~ 7db CINR gain compared with the 2Tx2Rx SM.

　　　　  De-Correlate the received signals of MS’s antenna 1 and 2 (reduce the relevance of the two received signal), which is beneficial to SM.

　　　　  Increase system’s throughput

　When the total transmit power keep constant, the throughput of 4Tx2Rx SM+BF is improved 50% compared with the 2Tx2Rx SM, which reduce the cost to meet the need of download data at high speed.

Ⅴ. Conclusion

MIMO+BF holds advantages of both MIMO and BF. It can not only obtain spatial diversity gain like STBC, but also can get Array Gain as the BF does. Thereby, it can be utilized to increase the coverage and improve the cell edge user throughput effectively, and reducing network construction and maintenance costs greatly. It brings space multiplexing gain likes SM, and also reduces the co-channel interference as the BF does, thereby, it can be utilized to improve the spectrum efficiency and system throughput in order to meeting customers demand for high-speed download need.

ZTE Corporation has been studying MIMO+BF for a long time, and achieved remarkable progress. We are one of communications equipment which firstly master the MIMO+BF technology. Considering that  current configuration of the terminals are various, some supports only one antenna to send data, while others support two antennas such as Tile Switched Diversity, it is indeed a big challenges to implement MIMO+BF. But after ZTE's long-term efforts, finally we successfully conquer it and have proposed different solutions to meet the diversity of the terminal.