MIMO and its meaning for WiMAX

Much has been made over the years of using multiple antennas in wireless communications systems as a way to allow network operators to increase the capacity of their networks while maintaining a high quality of service. More than likely, the first time consumers experienced the Multiple Input Multiple Output (MIMO) technique was in their cars, where “receive diversity” is used to provide better reception under widely varying mobile signal conditions. However, it is also employed in professional wireless microphones and wireless guitar systems. Based on the needs of coming generations of extremely-high-speed wireless networks, of which WiMAX is one, MIMO will become a standard feature in these systems in the future.

MIMO is a fundamental part of the WiMAX IEEE 802.16-2004 and IEEE 802.16e-2005 standards that are based on OFDM and OFDMA respectively. While IEEE 802.16-2004 is primarily focusing on transmit diversity, spatial multiplexing has additionally become an essential part of IEEE 802.16e-2005 standard. WiMAX MIMO modes are referred to as Matrix A and Matrix B. Matrix A is the WiMAX downlink transmit diversity scheme based on space-time coding according to Alamouti. Each information symbol of the data stream is sent out of the two transmit antennas, but interleaved over time and with a predefined coding applied. This provides the benefit of making the link more robust and increasing reception probability.

Matrix B refers to spatial multiplexing, which can use either single or multiple code word transmission, called vertical or horizontal encoding respectively. Feedback signaling from the terminal to the base station for MIMO mode selection and channel quality reporting is also likely to be employed in WiMAX. Switching between Matrices A and B is possible based on channel conditions, and the network informs the terminal how long a certain MIMO mode is active.

The WiMAX specifications go even further, to include transmit and receive beamforming based on detailed signals, with up to eight antennas. On the uplink side, WiMAX proposes a Transmit Collaborative MIMO principle similar to the uplink MU-MIMO scheme proposed for LTE. Different mobile stations are assigned to the same sub-channel and simultaneously transmit over the same radio resource using a different pilot pattern. Only one transmit antenna is needed on terminal side. The base station determines uplink resource allocation.

The meaning of MIMO

The term “MIMO” is a very broad term, since in its most elemental form simply refers to the use of more than one antenna for transmitting signals, receiving them, or both, along with multiple input and output signals. MIMO exploits the spatial dimension of the radio channel. Systems with one transmit and one receive antenna are called SISO (Single Input Single Output) systems, and systems with one transmit antenna and two receive antennas are called SIMO (Single Input Multiple Output) systems. A SISO system can exploit the benefits of receive diversity. A system with two transmit antennas and one receive antenna can also be referred to as MISO system (Multiple Input Single Output), which can exploit transmit diversity. The most likely scenario for MIMO in WiMAX applications is the “2 x 2 MIMO” approach, in which two transmit and two receive antennas are employed.

MIMO technology can be applied to either the downlink or uplink of a system, the downlink being the base station transmitter, and the mobile (or fixed) terminal being the receiver. The uplink is just the reverse of this. From the perspective of user equipment, cost as well as available “real estate” constrains the incorporation of MIMO, so it is more common for base stations to have more antennas than user equipment.

How MIMO benefits performance

Extensive research has shown that MIMO can significantly enhance the performance of data transmission. Diversity gains can increase the quality of data transmission, and spatial multiplexing gains can increase the throughput of data transmission. In practical application however, these two benefits can conflict, so that MIMO algorithms either provide diversity or multiplexing gains but not both, the result being determined by the type of signal processing employed by the system.

Diversity gain is accomplished by receiving the same data stream over multiple (ideally independent) propagation paths. In a transmit diversity or MISO system, multiple transmit antennas simultaneously send the same data stream. In a receive diversity or SIMO system, multiple receive antennas receive the data stream sent by one transmit antenna. In either case, the receiver captures multiple copies of the same signal. The effects of fading can be minimized because all received signals are not likely to be affected by fading in the same way.

While the presence of multipath fading channels is important to exploit diversity gains, the proximity of the antennas at the transmitter and receiver require that the fading channels are correlated in amplitude and phase. The higher the correlation of the fading channels, the lower the diversity gain. The job of the receiver is to combine “versions” of the same signal by maximum ratio combining or selective combining. Diversity gain can improve the quality of the received signal and also increase coverage area provided by a base station, reducing the number of sites required by a network. There is however, a number of antennas above which diversity gain saturates and no further gains are possible.

Spatial multiplexing gain is accomplished by simultaneously sending different data streams over the same radio resource, which can dramatically increase throughput and bandwidth efficiency. The streams are only discriminated by the spatial dimension – each stream is sent on a different “layer” of the radio channel together with a specific pilot or reference signal sequence. The receiver can distinguish the data streams sent from the different transmit antennas by their pilot sequence and can perform channel estimation for each stream separately.

While it is theoretically possible to increase the channel capacity of a MIMO system linearly with the addition of antennas, in reality the actual number of antennas is limited by the complexity of signal processing algorithms and RF subsystems. It is also a challenge to optimally array multiple antenna elements, especially on end user equipment. Radio channel characteristics do not always allow optimum capacity gains, and spatial multiplexing requires a minimum channel quality so it is not applicable anywhere at any time.

It is important to understand the difference between single-user and multi-user MIMO systems (SU- and MU-MIMO). In SU-MIMO, the data streams transmitted simultaneously belong to one user, so the data rate this user can achieve can be dramatically increased. The information symbols transmitted to this user are split into independent data streams that are emitted by the different antennas. Conversely, in MU-MIMO systems, the data streams transmitted simultaneously belong to different users, so the total capacity of the system is increased because of spatial multiplexing gain.

Generally speaking, spatial multiplexing gain can only be fully exploited if data streams can be detected and recovered correctly in the receiver, which must solve a linear system of equations (for 2x2 MIMO two equations with two unknowns). This is possible if the channel matrix H has full rank, which is achieved if each antenna receives a different channel as is true in strong multipath environments with spatially uncorrelated fading.

In summary, MIMO systems can exploit the multipath characteristics of the radio channel, which makes them a good choice for WiMAX and other wireless communication systems. The maximum number of data streams that can be transmitted over a radio channel is equal to the rank of the channel matrix. Since the radio channel is time-varying, the characteristics of the channel matrix must be evaluated continuously and when the channel matrix does not have full rank, the full spatial multiplexing gain will not be achievable, so transmit diversity may provide a good alternative to a MIMO scheme like transmit diversity.

MIMO and protocol layers

While MIMO falls into the discipline of RF technology, its implementation also affects protocol layers and their interaction with the physical layer. It is interesting to note that WiMAX and the future paths of “traditional” wireless services such as UMTS Long Term Evolution (LTE) have many features in common. Like WiMAX, these systems must also be optimized for packet data services with high data rates. So instead of conventional dedicated channel operation (like circuit switched voice services), shared-channel operation will be employed. In the dedicated channel approach, a radio resource is allocated for the duration of a call, while shared channels are assigned more flexibly and the available radio resources are shared dynamically between the subscribers in a cell. A user is allocated a radio resource only when data packets are actually to be transmitted, which is more efficient for packet services.

Adaptive modulation and coding (AMC) is a key feature to optimize packet data transmission. The shared channels carrying user data do not operate with a fixed modulation and coding scheme but rather are flexibly adapted based on channel conditions. A robust modulation and coding scheme must be selected in difficult radio conditions, while a more sensitive modulation scheme like Quadrature Amplitude Modulation (64QAM) can be selected when conditions are optimum. The decision over which modulation and coding scheme to apply for a data packet is performed in the network based on the channel quality measurement reports from the terminal. The decision is part of the scheduling algorithm in the base station. Efficient scheduling is essential to meet quality of service requirements for packet data services and timing requirements for scheduling are very demanding.

The relationship between the hybrid ARQ (automatic repeat request) retransmission protocol and MIMO is another interesting feature of MIMO systems. Hybrid ARQ (HARQ) schemes can greatly benefit packet data services by increasing the performance of data transmission in both the uplink and downlink. In an HARQ scheme, the receiver can request retransmissions of incorrectly received data packets by sending a negative acknowledgement (NACK). Correctly received packets are acknowledged (ACK). The transmitter may select a different channel coding version, sometimes called redundancy version, for the retransmission of the packet in order to increase the likelihood for reception. In the simplest case, the MIMO scheme does not work with the HARQ protocol but it is possible to have independent operation of the HARQ protocol on each antenna. In this case, the receiver must separately acknowledge the data packets received from each antenna, possibly including the use of different coding versions on each antenna.

MIMO operation also affects protocol Layer 3 control messages, which must include MIMO-specific information elements for setting up, reconfiguring, and releasing calls. Terminal capability indications are required to inform the network about the MIMO modes supported in the terminal or about the antenna configuration of the terminal. Configuration of MIMO operation is often also part of Layer 3 messages, such as restricting the reporting interval for MIMO-specific terminal measurements.