All of today’s advanced radio networks, including mobile radio networks (3GPP UMTS with LTE) and wireless networks (WLAN), are subject to continuously growing expectations for data rates. Yet classic transmission techniques such as higher-order modulation schemes and larger bandwidths have reached their respective limits. A very complex technology known as multiple-input multiple-output (MIMO), though, can be used instead.
MIMO is associated with heightened requirements for the test equipment for transmitters and receivers. It employs various techniques as it is implemented in a host of different standards. Designers also should be aware of the range of test instruments that’s needed for the different radio standards to get mobile stations and basestations ready for MIMO.
HOW MIMO WORKS
Multipath propagation and shadow effects are associated with strong fluctuations in propagation conditions in the mobile radio channel during radio transmissions. Various diversity techniques are used to make radio transmission as robust as possible:
- Time diversity: Different timeslots and channel encoding
- Frequency diversity: Different channels, spread spectrum, and orthogonal frequency-division multiplexing (OFDM)
- Spatial diversity
Multiple antennas are used on the transmitting or receiving end. Multiple antenna systems of this sort are known as MIMO systems. Along with more robust transmission, MIMO also attempts to increase the data rate using what’s known as spatial multiplexing. In actual practice, depending on the condition of the radio channel, both spatial diversity and spatial multiplexing (or a combination of these two techniques) are used.
A MIMO system consists of m transmit antennas and n receive antennas (Fig. 1). Since the same channel is used, each antenna receives the direct component intended for it as well as the indirect components for the other antennas. Picture a narrowband channel with independent timing. The direct connection from antenna 1 to 1 is characterized by h11 and so forth, while the indirect connection from antenna 1 to 2 is characterized by cross component h21 and so forth.
The conventional case with one transmit antenna and one receive antenna is known as single-input single-output (SISO) in MIMO terminology. If, in the case of spatial multiplexing, an increase in the data rate benefits a single receiver, this is known as single-user MIMO (SU-MIMO). If the individual transmit paths are allocated to different users, this is known as multi-user MIMO (MU-MIMO). This technique is most practical on the uplink, since the use of a single transmit antenna can minimize the complexity required in the mobile station. MU-MIMO is also known as collaborative MIMO.
For some time now, spatial diversity has been well established in various radio standards, enabling (as suggested above) more robust data transmission. In RX diversity, there are more antennas on the receiving end than on the transmitting end. The simplest case involves two RX antennas and one TX antenna (SIMO, 1x2).
Since no special encoding techniques are involved, this case is very easy to implement. The receiver only requires two RF paths. Due to the different propagation paths, the receiver “sees” two differently faded signals. Using suitable techniques in the receiver, the signal-to-noise ratio can be increased.
If there are more TX antennas than RX antennas, the term TX diversity is used. The simplest case uses two TX antennas and one RX antenna (MISO, 2x1). Here, the same data is transmitted in a redundant manner via two antennas. The benefit of this technique is that the multiple antennas and redundant encoding are shifted from the mobile station to the basestation. There, the implementation is simpler and more cost-effective.
To generate a redundant signal, space-time codes are used. Siavash Alamouti developed the first codes for two antennas. The copy of the signal is transmitted via a different antenna and at a different time. Space-time codes combine spatial and temporal copies (Fig. 2). Signals s1 and s2 are divided into two data streams. Then, an encoded copy of the signals is added in each path.
More extensive pseudo-Alamouti codes have been developed for use with multiple antennas. The encoding can also be implemented in the frequency domain (space-frequency coding).
Instead of increasing the robustness of transmission, spatial multiplexing is used to increase the data rate. The data is divided into independent data streams and transmitted independently via different antennas. Since transmission takes place on the same channel, transmissions are influenced by cross components not equal to zero. In the worst case, the influence is so great that it is not possible to detect the data streams in the receiver.
If the transfer matrix H is known, the cross components can be eliminated computationally in the receiver. Spatial multiplexing is typically used in conjunction with precoding on the transmitting end. The data streams to be transmitted are precoded to maximize the probability of successful transmission.
Using the open-loop technique, the precoding is performed in a predefined manner that is also known to the receiver. Using the closed-loop technique, the receiver reports the channel status to the transmitter via a special return channel. This allows the transmitter to respond to changing conditions and modify the precoding if necessary.
Since the (de)coding also takes place in the mobile station with spatial multiplexing, more computing power is required from the processors, which affects the battery operating time in the mobile stations.
MIMO 3GPP UMTS
The mobile radio standard 3GPP UMTS has undergone continuous further development. In the beginning, there was wideband code-division multiple access (WCDMA). This was followed by data acceleration techniques such as high-speed downlink packet access (HSDPA) and high-speed uplink packet access (HSUPA). The latest specifications include HSPA+ and Long-Term Evolution (LTE).
A transmit diversity mode was already part of Release 99 (WCDMA). Release 7 of the 3GPP specification (HSPA+) expanded this approach to MIMO and increased the data rate again. Using 64QAM modulation plus MIMO on the downlink, a maximum data rate of 28 Mbits/s (Rel. 7) can be achieved. Although MIMO and 64QAM were not yet available simultaneously at that time, parallel usage was introduced starting with Release 8, and a maximum data rate of 42 Mbits/s was the result. Currently, MIMO is not yet available in the uplink for HSPA+.
HSPA+ MIMO modes include a classic single antenna (SISO), transmit diversity, and closed-loop spatial multiplexing with return signaling required from the mobile station, as well as a maximum of two TX antennas.
Release 8 introduced UMTS LTE. It was designed to enable higher data rates, lower latencies, and optimized transmission of packet-oriented services. The basic concept behind LTE involves the use of orthogonal frequency-division multiple access (OFDMA) on the downlink. MIMO techniques are also an integral part of LTE. The result includes maximum data rates of up to 303 Mbits/s on the downlink and 75 Mbits/s on the uplink.
LTE MIMO modes on the downlink include classic single antenna (SISO), transmit diversity, open-loop spatial multiplexing with no return signaling required from the mobile station, closed-loop spatial multiplexing with return signaling required from the mobile station, multi-user MIMO (more than one mobile station is addressed per resource), beam forming, and a maximum of four TX antennas.
To minimize the complexity on the mobile end, MU-MIMO is used on the uplink. This means that multiple mobile stations with only one antenna each use the same channel.
WiMAXTM (IEEE 802.16e-2005) promises a maximum data rate of 74 Mbits/s for up to 20-MHz bandwidth. WiMAXTM MIMO modes on the downlink include classic single antenna (SISO), transmit diversity, closed-loop spatial multiplexing with return signaling required from the mobile station, multi-user MIMO (more than one mobile station is addressed per resource), beam forming, and a maximum of four TX antennas (beam forming: eight antennas). No special coding is provided on the uplink for SU-MIMO. Only different pilots are used. MU-MIMO is also defined for different users.
WLAN (IEEE 802.11n) delivers a maximum data rate of up to 600 Mbits/s with a 40-MHz bandwidth. It is backward compatible with the older 802.11 a/b/g standards.
WLAN MIMO modes on the downlink include classic single antenna (SISO), transmit diversity, closed-loop spatial multiplexing with return signaling required from the mobile station, beam forming, and a maximum of four TX antennas.
The MIMO implementation directly influences the performance in the described standards in terms of the throughput and data rates. This is why versatile and easy-to-use test instruments are needed to verify proper functioning of the various MIMO modes.
FUTURE TEST SOLUTIONS
The increased requirements involved in the development of basestations and mobile stations with MIMO, such as battery life, are also associated with increased expectations for the necessary test equipment. As described above, it is necessary to simultaneously generate and measure multiple RF paths when developing and testing MIMO systems. Rohde & Schwarz offers various test instruments and systems that cover the different radio standards such as 3GPP UMTS, HSPA, HSPA+, LTE, CDMA2000, 1xEV-DO, WLAN, and WiMAXTM.
The spectrum/signal analyzers from Rohde & Schwarz are useful for MIMO transmitter tests. Classic spectrum measurements as well as simple MIMO measurements, like TX diversity, can still be performed with a single analyzer. However, multiple analyzers are needed simultaneously for tests involving the demodulation of complex MIMO signals (spatial multiplexing).
Multiple baseband signals must be generated for MIMO receiver tests. Since MIMO achieves its gain through spatial separation of the individual channels, simulation of the individual fading channels in real time is crucial.
Rohde & Schwarz signal generators can simulate four fading channels for a 2x2 MIMO solution in a single instrument (Fig 3). Impairments such as additive white Gaussian noise (AWGN) must also be available.
Several generators can be connected together to handle more than two paths. Here too, the phase relationship (e.g., for beam-forming) can be controlled. Baseband fading with predefined models and AWGN is integrated into the generator.
Rohde & Schwarz also offers radio communication testers that combine transmitter and receiver test capabilities with signaling in a single instrument (Fig. 4). They suit all types of applications ranging from development to conformance testing of mobile stations. They can be used as RF testers and as protocol testers, and they handle layers 1 to 3 in the ISO model. MIMO tests are also possible here, such as for data rate measurements based on “real” applications such as FTP and TCP/IP.
MIMO plays an important role in RF conformance tests and the certification of mobile stations as well. Rohde & Schwarz provides expandable RF test systems for WiMAXTM, HSPA+, and LTE that can be used for everything from development through complete conformance testing. This means that the complete spectrum of tests and measurements needed for MIMO is covered.
Future radio standards will also use MIMO technology. Work is currently underway on LTE Advanced and WiMAXTM Advanced (802.16m) with MIMO. Of greatest interest here is a further increase in the number of transmit and receive antennas to boost the data rate even more. Spatial multiplexing will also be used on the uplink.