Analysis of the application of smart antenna technology MIMO in wide area wireless networks

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Analysis of the application of smart antenna technology MIMO in wide area wireless networks [Copy link]

Wide-area wireless network operators are increasingly involved in mobile broadband access and rich multimedia services. These services pose great challenges to wireless networks. Operators need to make significant improvements in network capacity, user data rate, distance and coverage quality. The potential performance gains provided by Multiple-Input Multiple-Output (MIMO) smart antenna technology can effectively address these challenges.

Wide area wireless network operators are increasingly adopting mobile broadband access strategies and rich multimedia services, which pose great challenges to their wireless networks. In order to establish and maintain a profitable business model, significant improvements in network capacity, user data rates, distance and coverage quality are required. Operators are increasingly interested in the potential performance gains provided by smart antenna technologies such as MIMO, as these technologies can meet these challenges and bring network development. The existing practical application of MIMO in the field of wireless local area networks (WLANs) and the recent continuous advancement of client device technology will promote the popularization of MIMO applications in wide area networks.

Many of the inherent characteristics of MIMO that have made it successful in the LAN space are very different from those in the WAN environment, so we must be careful about the transition between these applications. In the following brief description of WAN MIMO applications, we will focus on interference and limited dispersion characteristics as the most important differences and important considerations in implementation. The good news for wireless operators is that most of the theoretical gains of MIMO can indeed be achieved in WANs, provided that network-aware solutions are used that can reduce interference in multi-cell environments and maintain stable operation under limited dispersion conditions. It is also worth noting that since these performance gains can be achieved without any changes to existing wireless protocols, MIMO in WANs is easier to implement than is generally believed.

Figure 1: A wireless channel with two dominant propagation paths between a base station (BS) and a customer device (CD), as indicated by the arrows, superimposed on the base station’s nominal 120° sector transmission pattern.

Defining MIMO Technology

Since user-end devices are highly cost-sensitive, the current smart antenna configuration in commercial WANs uses multiple antennas on the base station side of the link, while the client device only has one antenna. As the pressure to improve WAN economics continues to increase, as well as the increased integration of client device chips and the reduced edge costs of adding smart antenna processing to the client, operators are increasingly interested in solutions that use smart antennas on both ends of the link.

Using multiple antennas at both ends will enable the adoption of many new transmission technologies that are not feasible in systems that only use multiple antennas at one end. In most cases, applying these technologies will provide more system performance gains.

The industry discussion of smart antennas includes very different definitions of the terms used in various implementations, so it is worth briefly discussing the applicable classification method. As a starting point for the simplest example, consider a system with only one antenna at each end of the link. Although the signal is sent in all directions (generally within a 120° sector), a particular wireless channel may have only two dominant paths, as shown in Figure 1. The example shown in this article is a communication between a high-mounted base station and a mobile phone (more broadly referred to as a "client device" because it may be a mobile computing platform) low on the road. Most of the received signal comes from reflections from nearby buildings. This is a single-input single-output (SISO) channel. [Note: The terms "input" and "output" in the field of wireless communications refer to the channel itself, not the devices at either end of the channel]

This article discusses the simplest and most common smart antenna. If the receiver has more than one antenna, it can intelligently combine the signals received from the different antennas and recognize that the signal is indeed coming from two main directions. The reason it can do this is because the two paths have different spatial characteristics or different spatial signatures. Since the receiver can recognize these two different spatial signatures, it can combine the signals from the two antennas and add them together to form a stronger combined signal. This approach is called single input [to channel 1] multiple output [from channel 1] (or SIMO), which is the famous receiver diversity scheme. Receive diversity technology is widely used on the link base station side of 2G and now 3G cellular networks.

On the other hand, if the transmitter has multiple antennas and the receiver has only one antenna, the signal will still propagate along the same path because the physical environment has not changed (the building is still there). This propagation mode is called Multiple Input Single Output (MISO). The biggest difference of MISO compared to SIMO is that the signal combination must be done at the transmitting end, not at the receiving end. By carefully adjusting the transmitting antenna, the two paths can be superimposed in the same way as SIMO. This method is widely used in PHS and HC-SDMA (High Capacity Space Division Multiple Access) systems, where the base station side of this system has multiple antennas for receiving (operating in SIMO mode) and transmitting (operating in MISO mode).

Providing multiple antennas at both ends of the link is known as MIMO. In this case, the two paths can be used more efficiently, as shown in Figure 2. The transmitter can adjust its antennas so that the information stream shown in blue in Figure 2 is sent along the first path (i.e., spatial signature), while the other information stream shown in orange is sent along the other path. Because the receiver also has multiple antennas, it can separate the two streams by detecting different spatial signatures. In this case, the transmitter can send two completely different data streams, which is equivalent to doubling the data rate from the user's perspective. This approach has material advantages over MISO or SIMO processing alone in the best case, and this MIMO advantage is achieved without adding additional bandwidth and power. Multipath transmission, which normally degrades the performance of a single antenna link, actually improves channel efficiency and quality in MIMO.

It is important to understand that the ability of a MIMO system to exploit multipath propagation presupposes the presence of these spatial dimensions in the propagation environment. In Figure 2, there are four antennas, but only two dominant paths. Even with four antennas, only two data streams can be formed in this case. Therefore, MIMO performance is closely related to the richness of multipath in the environment in which the system is used. Fortunately, in many environments there is enough scattering and multipath propagation to support multiple parallel data streams.

Information theory studies have shown that if multiple antennas are used at both ends of the link, the system capacity, which represents the upper limit of the data rate, will increase linearly with the number of antennas (given the given channel and keeping the overall power constant). The theoretical capacity of different MIMO systems with the same number of transmit and receive antennas is shown in Figure 3.

Figure 2: A communication channel with two dominant propagation paths can double the user data rate in a MIMO fashion. Note that multiple antenna processing can perform beam shaping so that the signal propagates along the channel of interest while the other dominant channel is silent.

The capacity of a MIMO system (i.e., 8 antennas at each end of the link) can be up to 8 times that of a single-antenna system. Considering all network operating and capital expenses, MIMO technology provides much higher performance and economic benefits than a single-antenna system. Especially for high-data-rate services, such as true broadband access, IPTV, and large file transfers, where limited bandwidth can cause serious problems, MIMO technology is a promising solution.

The predicted values in Figure 3 only represent the performance limits of an ideal system. Information theory does not provide much practical guidance on how to reach these limits, and actual systems face the challenge of making full use of the spatial dimensions provided by the channel. There are generally three main recommended channel utilization methods, the first two focusing on the performance of a single link, and the third focusing on the performance of the entire network:

1. Increase data rates

The techniques discussed above (as shown in Figure 2) are generally referred to as spatial multiplexing. For channels with rich scattering environments, data rates can be increased by sending independent information streams on each antenna, and using more sophisticated receiver technology, the different data streams can be separated and decoded separately. For example, a system using four transmit and four receive antennas will have a capacity four times that of a single antenna system.

2. Improve service quality through diversity technology

On the contrary, if the same signal is sent on multiple symbols on multiple antennas, the reliability of the transmission can be improved instead of increasing the data rate. This technique of sending multiple copies of the signal on different antennas and at different points in time actually provides space-time diversity. The technique of spreading or encoding information symbols in space and time at the same time is called space-time coding.

3. Higher data rates and better quality of service by mitigating interference

Another approach to exploiting the spatial dimension in MIMO systems that is suitable for more interference environments is to optimize the distribution of RF energy throughout the system to minimize the generation and sensitivity of co-channel interference in the network. This approach will be discussed in detail in the last section of this article. This approach can provide higher data rates and more robust links by taking advantage of higher SINR (higher SINR allows higher modulation levels and therefore higher link data rates) and classical diversity (which increases link stability). Just as in a MISO system, the base station uses multiple spatial channels to achieve consistent combined energy for the client device, these channels are used by the client to improve the effective sensitivity in these spatial 'directions' (like in a SIMO system), reducing the power required by the base station to transmit. The reverse process is completed on the uplink. The base station and client devices automatically work in unison to reduce the interference level in the system. As will be discussed later, overall network performance is a key aspect of wide area network system optimization, and interference reduction is the main driver for improving broadband network performance.

Research laboratories around the world have proven the practical feasibility of MIMO technology in early wireless LAN applications, with system capacity very close to the theoretical predictions that can be achieved by using both spatial multiplexing and space-time coding techniques in the laboratory. Due to the huge performance gains achieved in the initial applications, MIMO technology quickly moved out of the laboratory and was applied to actual WLAN products.

MIMO's Early Success on WiFi

The most publicized MIMO implementation is in the fixed WLAN environment, where the biggest benefit of MIMO is increased throughput for individual user devices. In particular, home and enterprise WLANs have several characteristics that make them ideal candidates for early MIMO adoption, including:

1. Rich scattering

Most WiFi systems are located in environments with a lot of scattering conditions, such as indoors or between dense urban buildings. In these environments, there are usually multiple propagation paths or spatial dimensions that can be used to form multiple streams. In fact, indoor environments are very similar to the conditions required to achieve the linear growth of capacity with the number of antennas shown in Figure 3.

2. Independent deployment

An important factor in achieving rapid deployment is that WiFi equipment is usually purchased by end users themselves and deployed independently in their own networks. Interoperability of different MIMO WiFi solutions is not an issue, as shown by the success of IEEE 802.11n products before public MIMO standards were agreed upon, allowing for rapid deployment of MIMO technology without having to wait for standards to be unified.

3. Limited distractions

It is also critical that the WiFi environment closely matches the theoretical assumptions for studying MIMO technology. Due to the short-range and dynamic channel assignment characteristics of WiFi networks, MIMO receivers generally operate without significant co-channel interference. The performance of these solutions will quickly degrade if operated in an environment with uncompensated co-channel interference.

The successful deployment of MIMO in WiFi demonstrates that the potential performance improvements provided by MIMO are real. The fact that it took only a few years to go from lab results to actual WiFi products means that the chances of repeating the success are very high for wide area wireless network operators.

Challenges facing WAN

The same performance advantages that have made MIMO a successful technology for WiFi products also make it a likely technology choice for wide-area mobile wireless environments. However, mobile, multi-cellular environments differ fundamentally from WiFi RF environments in some ways, so mobile environments present many configuration challenges.

1. Interference

Figure 3: Theoretical average capacity versus signal-to-noise ratio (SNR) for a MIMO system with N transmit and receive antennas while keeping the total transmit power constant.

Interference is particularly severe in wide area environments due to dense and large cell deployments. In such environments, both interference mitigation and high throughput performance are required. Therefore, in order to extend the successful application experience of MIMO in WLAN to wide area networks and mobile broadband data services, new MIMO solutions must be adopted, taking into account both interference and data rate.

2. Limited scattering

In some cases, a wide-area scattering environment can only have one or two dominant paths. For example, if it is line-of-sight (LOS) propagation, there is only one dominant propagation path, which limits the use of spatial multiplexing technology.

3. Interoperability

In wide area networks, all users need to communicate seamlessly with base stations across large networks (across regions and operators), so interoperability must be supported. Solutions like the ones mentioned above that use spatial multiplexing or space-time coding techniques require protocol modifications, which greatly increases the time to market for MIMO solutions in wide area networks. For example, the receiver needs to know the space-time code used by the transmitter to correctly decode the data. Work to incorporate MIMO into mobile systems has been carried out in multiple standardization organizations, such as the IEEE 802.16e standard, but it will take quite some time for robust commercial products to be officially available.

These factors make the adoption of MIMO in WANs more challenging than WiFi, requiring new solutions that can address the unique properties of large multi-cellular networks. The successful implementation of MIMO in WANs will depend on the following two key properties:

Interference mitigation. To reduce interference in wide area networks, the additional degrees of freedom gained through antenna arrays at both ends of the link are at least partially exploited. Performing interference mitigation at both the transmitter and receiver can significantly reduce network interference compared to systems that only perform interference mitigation at one end.

Robust solutions. Solutions need to be developed that can account for the limited number of dominant propagation paths, and even in channels with limited scattering, significant performance gains can be achieved by combining signals at the transmitter and receiver. Recent studies have shown that even in channels with only one dominant propagation path (also called a key-hole channel), significant performance gains can be achieved by using smart antenna technology at both ends of the link.

MIMO for Wide Area Networks

Significant MIMO gains can be achieved in existing WANs without modifying existing protocols or waiting for new protocols to be completed. Significant performance improvements can be achieved using adaptive array processing techniques in base stations and similar processing techniques in mobile terminals, which is the third basic MIMO approach mentioned above. In fact, theoretical research also shows that this is the best approach to take under the multi-channel conditions that are most common in WANs. Improving both signal strength and interference suppression performance is particularly important to promote the development of WANs and support operators' increasingly higher bandwidth and multimedia service goals.

The following is a solution that balances interference mitigation and throughput. The base station minimizes interference on the base station side by calculating the combined weights of the antenna array. Similarly, the mobile terminal uses its antenna array to reduce interference on the handset side. Since no special link coding is required on either the base station or the client device, the implementation and operation of MIMO processing can be completely independent of each device. The result is a self-organizing and self-optimizing system that continuously adapts to the changing interference environment and the changing service needs of the user. Since the devices at both ends of the link are independent of each other, this MIMO approach can provide excellent performance even in heterogeneous networks or networks that are undergoing upgrading and changing conditions (networks where not all base stations and client devices are equipped with multiple antennas). Single-antenna terminals can simply join such a grid using SIMO (transmitting) or MISO (receiving) channels and work with multi-antenna terminals. The overall network performance provided by this interference-minimizing MIMO technology will increase as more multi-antenna devices are added to the system.

Conclusion

The performance gains provided by MIMO technology provide a promising impetus for the next step in wireless communications. MIMO devices that provide performance enhancements for the WiFi market and wide area networks will soon be available. However, the RF environment in wide area mobile wireless systems is completely different from WiFi, and interference is the biggest challenge. Fortunately, wide area MIMO solutions based on adaptive antenna processing technology are now available that can provide huge performance gains in single antenna systems. These solutions can fully meet interference and throughput requirements through multiple antennas and the inherent spatial dimensionality of the channel. Moreover, most of the performance gains can be achieved without modifying the protocol, and it is believed that these solutions will soon be widely used in the near future. Therefore, wide area MIMO applications may be easier to implement than imagined.

By Steven Glapa

Marketing Director

ArrayComm