MIMO Technology for the Next-Generation Wireless Services

MIMO stands for Multiple Input Multiple Output and is fast maturing as a revolutionary broadband wireless communication technology. The future for MIMO is very bright according to the researchers and industry experts. The future chipsets for wireless network interface cards (NICs), adaptors and access points (APs) are MIMO-equipped, enabled, and empowered. Wherever higher data transmission rates or more robust connections are important, MIMO will be the technology of choice. Worldwide standards forums, agencies and organizations, academic institutions and research labs are in unison focusing on the various aspects of this disruptive technology in order to make it ready for the future wireless world. Its acceptance and adoption rates are simply phenomenal and tremendous. Service providers are seriously and keenly looking for the standards-based MIMO products so that they can fulfill the aspirations of their customers and to conceive new value-added and differentiated wireless network services in order to retain them and to devise new revenue-generation means. In this paper, we are to concentrate on this trend-setting and path-breaking technology, its beginning, evolution, benefits, application domains, issues, challenges, etc.

1. Wireless Networks & Challenges

Wireless communication is bringing an unprecedented paradigm shift in people’s daily lives at delivering anytime, anywhere and any device voice communication and remote information access. Thus the fast proliferation of look and feel, handheld, trendy & tiny, and mobile devices with wireless communication capabilities enable us to be extremely productive even while we are traveling, shopping, staying in hotel, sipping our coffee, wandering at the lobby, waiting for a flight, relaxing at beaches and resorts, etc. Novel, QoS-compliant and multimedia data services are being conceived and delivered to the subscribing users at highly affordable and enticing costs. Ultimately the mobile Internet or the wireless web is poised to see the light with the arrival of high-speed wireless connectivity standards, technologies and best-of-breed solutions. Content creators, game developers, wireless service providers, chip makers, handset manufacturers, leaders of network infrastructure solutions and even standard bodies are working relentlessly, collectively, and collaboratively to take our wireless experiences to still greater heights in the days to come. 

There are diverse application domains for wireless technologies. Enterprises are increasingly empowering their field service teams and marketing executives with smart phones and Blackberries. Other enterprises are exploring the more expansive goal of end-to-end wireless connectivity that allows mobile users to move from personal networks (Bluetooth, & UWB), to local wireless networks (Wi-Fi), to metropolitan wireless networks (WiMAX), and to wide area networks (3G). The leading wireless network types include

· Wireless Personal Area Network (WPAN)
· Wireless Local Area Network (WLAN)
· Wireless Metro Area Network (WMAN)
· Wireless Wide Area Network (WWAN)

There are several specifications, standards, technologies and implementations emerging to fulfill the unique demands of these differing networks. Bluetooth and ultra wideband (UWB) for WPANs, IEEE 802.11 (Wi-Fi) for WLANs, 802.16 WiMAX standards for WMANs, and finally 3G / Super 3G technologies for WWANs are fast maturing and evolving. Of course, there are several research challenges as the free air space is the main wireless data carrier. In the recent past, ad hoc wireless networking using mesh topology concept is becoming popular as several real world and killer applications are being made out of these specialized networks. 

Thus wireless communication from its very humble beginning as an enabler of voice communication today has evolved to provide various kinds of location-aware and people-centric data services. However human expectations never subside and the next-generation mobile applications clearly demand higher throughput, larger coverage, always-on connectivity, reliable and sustainable linkage among the participating nodes and ultra-high data transmission speed. Specialized and time-critical applications such as wireless IP telephony, audio / video and web conferencing while on the move, video on demand (VoD), television programmes on the mobile handsets etc are clearly pushing for ultra-high bandwidth. 

These challenges remain due to the universal property of the radio channel, which has a highly variable and volatile nature. Unlike the relatively stable and durable environment that exists on wire, cable, or fiber, the ability of the air to carry information can and does change over time and often from moment to moment. Given this fundamental vulnerability and variability apart from the overhead inherent in any networking protocol, the actual throughput available is often less than the peak data rate. Thus there are research efforts to improve the performance of wireless networks at the physical layer in order to realize higher throughput. 

2. Throughput-Enhancing Techniques

There are some fundamental and physical limitations for any mobile environment due to the topsy-turvy nature of the radio channel. As we roam around in a mobile environment, the distance between the transmitter and the receiver varies and this in turn degrades the service quality. Also the signals would clash with the fixed objects, physical walls, high-rise buildings, trees, mountains etc resulting in signal reflections, diffractions, and deviations in their original paths. Also there are possibilities for the signals to interact and interfere with other signals from different sources. This results in signals traveling in multiple paths and hence signal fading occurs. That is, signals become so weak that they cannot be reliably captured and even sometimes there is not enough signal to demodulate or even to detect the transmission itself and to extract the communicated information from the carrier precisely. There are several ways and means being explored in order to mitigate this problematic fading and energy loss so that users can experience preferred reliability, speed, clarity, coverage and ultimately better throughput. 

Smart Antenna technology is an excellent mechanism to surmount this problem of fading. A conventional radio uses one antenna to transmit a data stream. A typical smart antenna radio, on the other hand, uses multiple antennas. This design helps to combat distortion and interference. Beam forming (a.k.a. beam switching) concentrates the signal energy on the main path whereas receive combining (a.k.a. diversity) is to capture the strongest signal at any given moment. These two, being the multi-path problem mitigation techniques, do not multiply data throughput over the wireless channel. Both techniques however have demonstrated their power in incrementing performance in point-to-point (P2P) applications such as outdoor wireless backhaul applications.

Another interesting performance-enhancing technique is channel bonding that multiplies throughput by combining two or more radio channels. That is, to gang together multiple radio channels. For example, for attaining 108 Mbps, two 54 Mbps channels can be simultaneously utilized. Compressions and other relevant techniques are being leveraged to gain additional advantages. Assuming the channels are readily available, channel bonding is a promising technique significantly increasing the throughput. However, channel bonding consumes more bandwidth. In some cases, channel bonding may not be an option because the target frequencies have been already allocated to other users or services. Hence the ideal solution for the performance degradation problem is to come out with competent technologies that simply pack more information per unit of bandwidth and time. That is, optimized utilization of channel to attain higher spectral efficiency. MIMO is the all-time favorite broadband wireless technology as illustrated below.

3. MIMO Technology 

The smart antenna techniques discussed above are one-dimensional, whereas MIMO is multi-dimensional. It builds on one-dimensional smart antenna technology by simultaneously transmitting multiple data streams through the same channel, which increases the wireless data capacity significantly. Also all the user-defined and preferred QoS requirements are implicitly and inherently met to the fullest satisfaction of users through the MIMO technology. The picture below indicates the overall structure and behavior of a MIMO system. 



We can think of conventional radio transmission as traveling in a one-lane highway. The speed limit governs the maximum allowable flow of traffic through that lane. Compared with conventional radios, one-dimensional smart antenna systems help move traffic through that lane faster and more reliably so that it travels at a rate closer to the speed limit. However MIMO helps traffic move at the speed limit and opens more lanes. The number of lanes that are opened hence multiplies the rate of traffic flow.

Multi-path phenomenon has been a deterrent for robust reliability and hence is an unwanted impairment and pitfall for dependable and efficient wireless communication. Multi-path actually occurs in non-line-of-sight (NLOS) environments where signals sent from a transmitter reflect off fixed objects such as concrete walls, high-rise buildings, etc. and take many paths to reach the destination. Due to this, the signal would lose its power, vitality, energy, direction, pace etc. in its move. Stanford University researchers Greg Raleigh and VK Jones have successfully leveraged this limiting factor into a blessing for wireless data communication. That is, this multi-path aspect is beneficially exploited to multiplicatively increase the capacity of a dwindling radio communication system. That is, each multi-path route could be treated as a separate viable and virtual channel for additional data transmission.

Towards this, MIMO system need to use multiple and spatially separated antennas at both ends. MIMO encodes a high-speed data stream across multiple antennas. Each antenna carries a separate, lower-speed stream. Each signal transmitted in a multi-path environment travels in multiple routes. This causes the transmitted signals to jumble and cobble together creating a mess. So the MIMO receiver has to have proven and efficient mathematical algorithms implemented inside to come out of this chaos by unscrambling, unraveling and unearthing the received signals precisely. 

MIMO-OFDM combines OFDM and MIMO techniques thereby achieving higher spectral efficiency and performance. A MIMO-OFDM system transmits independently OFDM-modulated data from multiple antennas simultaneously. At the receiver side, after OFDM demodulation, MIMO decoding on each of the sub-channels extracts the data from all the transmit antennas on all the sub-channels. OFDM modulation divides a broadband channel into many parallel sub-channels. The use of an FFT/IFFT pair for modulation and demodulation make it computationally efficient as well. The transmitted signals arrive at the receiver after being reflected from many objects. An OFDM receiver has to sense the channel and correct these distortions on each of the sub-channels before the transmitted data can be extracted. OFDM is effective in correcting such distortions. OFDM’s unique advantage lies in realization of high spectral efficiency measured in bits/sec/Hz. The “Orthogonal” part of the name refers to a precise mathematical relationship between the frequencies of the sub-channels that make up the OFDM system. Each of the frequencies is an integer multiple of a fundamental frequency. This ensures that even though the sub-channels overlap, they do not interfere with each other. This results in high spectral efficiency. 

The prospect of many orders of magnitude improvement in wireless communication performance at no cost of extra spectrum (only hardware and complexity are added) is largely responsible for the success of MIMO as a topic for new research. This has prompted progress in areas as diverse as channel modeling, information theory, coding and modulation theories, signal processing, antenna design, designing fixed as well as mobile terminals with multiple antennas, etc. 

4. MIMO Technology for WPANs

The idea behind MIMO is that the signals on the transmit (TX) antennas at one end and the receive (RX) antennas at the other end are combined in such a way that the data rate (bits/sec)) for each MIMO user will be improved. Such an advantage can be used to increase both the network’s QoS and the operator’s revenues significantly. MIMO products employ diversity to mitigate multi-path fading, beam forming technique to reduce the amount of energy lost to multi-path propagation, and multiple channels to increase bandwidth. The upshot is that MIMO systems multiply spectral efficiency and capacity by encoding, transmitting, receiving, and decoding multiple signals.

Today we have several varieties of handhelds, smart phones, wearables, and pocketables with us. When we enter into a new environment, our digital companions would initiate a conversation with the devices found in the environment. Thus a communication network is being created instantly in order to share files and to fulfill our requirements. Thus forming personal area networks especially using wireless concepts has become an ordinary act. Bluetooth and ultra wide band (UWB) technologies are the dominant ones in this space. By imbedding multiple antennas in the personal devices, we can realize better coverage, high-speed data transmission and reliability. Thus the MIMO technology is all set to penetrate into the WPANs soon. 

5. MIMO Technology for WLANs 

Increasingly WLANs are being utilized for sharing, storing and carrying audio / video, voice, and other multimedia content. This clearly demands for increased speed, capacity, clarity and coverage. But the frequency spectrum for WLAN communication is finite and fixed. Thus the only option is to realize enhanced spectral efficiency otherwise all the available frequency channels would be overused resulting in interferences, collisions and clashes resulting in nullification of signals. Unfortunately today’s WLAN standards do not do justice on the range requirements encountered in apartments, small and medium businesses (SMBs), retail stores, and hotels. For example, a consumer may be frustrated to discover that a wireless LAN cannot reach both an upstairs bedroom and a home office in a corner of the basement. A small business may be disappointed to discover that it needs three or four access points (APs) interconnected via a wired backbone to cover its modest-sized premises. 

Further on, existing wireless LAN standards also lack the required throughput levels for a suite of emerging and evolving applications such as home entertainment. For example, a single High Definition TV (HDTV) stream requires at least 20-24 Mbps of sustained throughput throughout the home. Today’s IEEE 802.11-compliant systems could not deliver this data rate as significant portion of the bandwidth is reserved for protocol overhead such as commands, status messages, and error control mechanisms. Also there are many users and applications sharing the bandwidth and hence the actual user throughput is very less near the edges of the home coverage area. With this low data transmission rate and lack of in-built QoS mechanisms, establishing “wired quality” video streaming becomes impractical in today’s WLAN infrastructure. 

Having understood this, the quest for additional throughput has forced the industry specialists to look towards the evergreen MIMO–OFDM combo technology, which is the cornerstone for the next generation IEEE 802.11n standard. It is sure that 802.11n-compliant MIMO products (expected to hit the market this year) will incorporate several traits such as beam forming, space-time block coding, cyclic delayed diversity, maximal ratio combining, and intelligent antenna selection. These techniques have shown their power and potential in ensuring enhanced WLAN throughput, range, reliability and robustness.

QoS-compliant Video Streaming over MIMO-enabled WLAN Systems – Video files sharing is the key driving force for the widely anticipated convergence of wireless communication technology and consumer electronics. As DVDs, household appliances, and digital home theaters are becoming popular and affordable, anytime anywhere video storage, management, sharing, and viewing using any device in the home is inching towards the grand reality. For this to happen, home area networking (HAN) technology has to provide sufficiently high bit rates to support the distribution of multiple video and HDTV streams from a central location, along with total home coverage. Video applications cannot tolerate bandwidth fluctuations, so guaranteed bandwidth and QoS are the top two essential requirements. Also, a wireless network should provide wire-like performance regardless of changing environmental conditions. These and other challenges in the new home environment cannot be met with existing IEEE 802.11 a/b/g-based wireless standards and products. Thus the realization dawned to unearth revolutionary techniques that facilitate high-quality video applications with higher bandwidth, lower jitter, zero latency and extended coverage. These are supposed to deliver much better immunity to interference, and the means to handle degraded link conditions. Excess bandwidth can also be traded for extended reach and lower power consumption. MIMO, adaptive beam forming and other relevant techniques combine well to fulfill the goal of QoS-compliant video streaming over WLAN networks. 

6. MIMO Technology for WMANs

Wireless broadband promises to bring high-speed data to multitudes of people in various geographical locations where wired transmission is too costly, inconvenient, or unavailable. WiMAX is a wireless MAN technology devoted to making broadband wireless commercially available to the mass market by employing IEEE 802.16 standards. IEEE 802.16-2004 supports several multiple-antenna options including Space-Time Codes (STC), MIMO antenna systems and Adaptive Antenna Systems (AAS). There are several advantages to using multiple-antenna technology over single-antenna technology: 

· Array Gain - This is the gain achieved by using multiple antennas so that the signal adds coherently.
· Diversity Gain - This is the gain achieved by utilizing multiple paths so that one or more bad paths do not limit the ultimate performance. Effectively, diversity gain refers to techniques at the transmitter or receiver to achieve multiple “looks” at the fading channel. These schemes improve performance by increasing the stability of the received signal strength in the presence of wireless signal fading. Diversity may be exploited in the spatial (antenna), temporal (time), or spectral (frequency) dimensions.
· Co-channel Interference Rejection (CCIR) - This is the rejection of signals by making use of the different channel response of the interferers.

A common scheme that exhibits both array gain and diversity gain is maximal ratio combining that combines multiple receive paths to maximize Signal to Noise Ratio (SNR). Selection diversity, on the other hand, primarily exhibits diversity gain. That is, the signals are not combined and instead the signal from the best antenna is chosen. 

For AAS, multiple overlapped signals can be transmitted simultaneously using Space Division Multiple Access (SDMA), which is a technique that exploits the spatial dimension to transmit multiple beams that are spatially separated. SDMA makes use of CCIR, diversity gain, and array gain. For MIMO systems, spatial multiplexing is often employed. Spatial multiplexing transmits coded data streams across different spatial domains. Some techniques, such as BLAST do not require feedback, while others, such as vector coding on the modes of the channel do. MIMO techniques can also make use of CCIR, diversity gain, and array gain. The higher performance and lower interference capabilities of MIMO and AAS make them attractive over other high-rate techniques for WiMAX systems in costly, licensed bands. 

For WiMAX, the simplest MIMO system is actually a Multiple-Input Single-Output (MISO) STC code called the Alamouti code. This requires two antennas at the Base Station (BS). The Alamouti code provides maximal transmit diversity gain for two antennas. Another transmit diversity scheme is cyclic delay diversity. A key advantage of transmit diversity is that it can be implemented at the BS, which can absorb higher costs of multiple antennas and associated RF chains. This shifts cost away from the SS, which enables faster market penetration of 802.16 products. 
Recognizing that smart antenna technologies are essential to meet operator performance requirements for mobile broadband wireless services, the WiMAX community has selected a system architecture that incorporates both MIMO and AAS (adaptive antenna systems, also known as beam forming). MIMO increases subscriber data rates and AAS improves cell-edge link budgets, manages interference, and maximizes overall network capacity. Used in combination, MIMO and AAS yield a significant performance edge for WiMAX. Thus broadband solution providers are exploring all possibilities to enhance the wireless network performance through beamforming, interference mitigation, and spatial multiplexing. 

7. MIMO Technology for WWANs

The concept of Wireless web / the mobile Internet is increasingly becoming hot and popular nowadays. Several wireless network operators and service providers are adopting standardized technologies and formulating strategies in order to leverage the innovations happening in the broadband wireless access (BWA) domain to deliver rich multimedia services to their customers. These strategies however present significant challenges to their wireless network infrastructures — requiring large and vast improvements in network capacity, subscriber data rates, range, and coverage quality in order to build and sustain viable business models. 

Smart antenna deployments in commercial wide-area networks to date have used multiple antennas only on the base station side of the link, with a single antenna on the client device. Attaching multiple antennas at client devices is not affordable to many subscribers. However, system on a chip (SoC) and miniaturization technologies is growing fast and hence the cost of adding smart antennas comes down sharply. Thus the realization of multiple antennas at both ends allows for many new and unprecedented transmission and performance-incrementing techniques that are not possible previously. 

Consider a system with a single antenna at each end of the link. Although the signal is transmitted in all directions, a particular wireless channel may only have two dominant paths, as illustrated in Figure A. This corresponds to a single input single output (SISO) channel. If the receiver has more than one antenna, it can intelligently combine the signals from the different antennas and recognize that the signal indeed is arriving from two main directions. It can do this because the two paths have different spatial characteristics or different spatial signatures. Since the receiver recognizes that there are two different spatial signatures, it can combine the signals from the two antennas such that they add coherently resulting in a stronger combined signal. This corresponds to a single input [to the channel], multiple output [from the channel] (or SIMO) scenario and this is the well-known case of receiver diversity. Receive diversity is used widely in 2G and now 3G cellular networks on the base station side of the link.

If instead, the transmitter has multiple antennas while the receiver has only one antenna, the signal still travels along the same paths since the environment is the same. This corresponds to a multiple-input single output (MISO) scenario. The main difference compared to SIMO is that the combining has to be done at the transmitter instead of the receiver. By weighting the transmit antennas appropriately, the two paths can be made to add coherently in the same way as for the SIMO case. This approach is used widely in PHS and HC-SDMA systems with multiple antennas on the base station side, for both receive (working in SIMO mode) and transmit (working in MISO mode).



Providing multiple antennas at both ends of the link corresponds to a MIMO scenario. The two paths can be efficiently and intelligently exploited to achieve desired performance as indicated in the figure B. The transmitter can weight its antennas so that one stream of information, shown in blue, is sent along the first path (i.e., spatial signature) and another stream of information, shown in orange, on the other path. Since the receiver also has multiple antennas, it can separate the two streams by detecting that they have different spatial signatures. In this case, two entirely 



different data streams can be sent, potentially doubling the data rate. This MIMO advantage can be achieved without requiring extra bandwidth or power. The multi-path propagation, which generally impairs the performance of single-antenna links, is instead exploited in the MIMO case to increase the channel efficiency and quality. 

MIMO systems exploit multi-path propagation provided that these spatial dimensions exist in the propagation environment. In the figures above, there are four antennas and only two dominant paths. Hence, in this case only two data streams can be formed even though there are four antennas. Therefore, the performance of MIMO is closely related to enough scattering and the multi-path richness of the environment where the system is employed. Thus MIMO is to enhance the capacity, capability, and connectivity of wide area networks.

MIMO Technology for Mobile Ad-Hoc Networks (MANETs)

There are several real-world applications emerging based on the innovations in mesh topology and ad hoc networking technologies. It is impractical to create, deploy, operate, monitor, fine tune, and manage infrastructure such as APs and base stations in battle fields, rough & tough, and remote environments. In these spaces, wireless meshes need to be constructed, leveraged and destructed on need basis. 

A MANET consists of a group of mobile nodes that may communicate with each other without fixed wireless infrastructure. That is, there is no master-slave relationship between nodes such as “base station to mobile users”. Communication between nodes can be supported by direct connection or multiple hop relays. The nodes have the responsibility of self-organizing so that the network is robust to the variations in network topology due to node mobility as well as the fluctuations of the signal quality in the wireless environment. In order to enable strong link reliability, availability, and survivability, commercial standards such as Bluetooth are empowered with ad-hoc networking capabilities. MIMO, being the next-generation wireless technology, is being touted as the prime driver for the future wireless meshes of varying sizes, structures, capabilities, ranges, powers, etc. 



Summary

MIMO technology is a sophisticated signal processing and smart antenna technique for transmitting multiple data streams in parallel through multiple antennas thereby we can realize higher data transmission rate, extended coverage, better reliability and spectral efficiency. MIMO is a multi-dimensional technology capable of guaranteeing all the QoS requirements of wireless networks. It sends an independent data stream through each antenna, increasing the wireless spectrum utilization by a factor equivalent to the number of transmit streams. Thus all the available channel capacity is optimally utilized for achieving maximum throughput. MIMO is bringing a number of breakthroughs by surmounting the bottlenecks in wireless systems. As we are keenly anticipating for the true mobile Internet, the roles and contributions of MIMO technology is getting bigger, larger and deeper. In just a few years time, the technology will become so common in our handhelds and will penetrate into the complete wireless ecosystem in a big way.

Communication Details

Dr. C. Pethuru Raj, Senior Consultant, Wipro Technologies, MG Road, Bangalore, 560001, www.wipro.com

Rathnakar Acharya, Professor of Information Technology, Alliance Business Academy, BTM, 2nd Stage, Bangalore, 560076, www.alliancebschool.ac.in 

E-mail: pethuru.chelliah@wipro.com
rathnakar.a@alliancebschool.ac.in