Wireless LAN networks operating at speeds traditionally seen as the preserve of fixed technologies are now on the horizon.  Patrick Molin of Agere Systems explains how the goal posts are being shifted by using MIMO technology.

The proliferation of wireless networking in the office, at home, and in public spaces has generated considerable interest in the technology’s potential for driving seemingly ever-higher transmission speeds.  The current 802.11b standard delivers 11Mbits/s and emerging standards such as 802.11g and 802.11a are promising to increase throughput to 54Mbits/s. But, the need for speed will continue to grow, and many applications, such as the delivery of HDTV signals for entertainment and hospital communications for medical imaging would benefit from throughput rates beyond the 54 Mbits/s mark.  Taking wireless LAN technology beyond the limitations of time and frequency division multiplexing is a new concept under development, one that promises to provide a quantum leap in speed and capacity, bringing wireless capabilities near to those of wired networks.

Late last year, our researchers announced a breakthrough in WLAN technology, demonstrating transmission speeds of 162Mbits/s, roughly 15 times faster than today’s 802.11b wireless data networks.  This innovation was achieved not by augmenting the time and the frequency dimensions, which have become increasingly congested, but by adding a third degree of freedom — space.  The ability to take existing channels — in this case, three 54Mbit/s channels — and operate them in parallel using multiple transmit and receive antennas, allowed the researchers to demonstrate increased speed and to show that the improvement in data throughput rates increases proportionally to the number of transmit antennas.

Multiple Input/Output

Given the bandwidth restrictions imposed by frequency allocations, the growing congestion from numerous types of devices, and the signal-to-noise ratios in today’s home and office communications environments, our researchers began exploring avenues to provide dramatic improvements to wireless data communications.  These engineers and scientists developed the principle of adding a third dimension to the traditional frequency-time continuum:  space.

The basic concept of this is to transmit multiple streams of data on multiple transmit antennas, at the same frequency.  Typically, multiple receive antennas are used as well since this configuration dramatically improves the overall performance of the system.  This principle is called MIMO —  Multiple Input Multiple Output.

The use of the MIMO principle for WLAN was first demonstrated in a public forum by Agere at Eindhoven University of Technology in the Netherlands using a platform developed by Agere which combines MIMO with OFDM.

The demonstration, based on 802.11a technology in the 5GHz band, paired three transmit and three receive antennas to achieve 162Mbits/s over a wireless connection. This high data rate was achieved through the MIMO/ODFM transmission combina-tion and even with this enhancement, the technology remains compliant with the IEEE 802.11 standards specifica-tions. Similar rates are expected to be achieved for 802.11g-based transmis-sions using a dual-band radio solution (supporting both 2.4GHz and 5GHz).

MIMO Processing

The demonstration at Eindhoven University provided a proof point and verified that a data stream could be time-division multiplexed across multiple spatial pipes, with each one operating at the same frequency.  While the three transmissions do interfere, the three streams can indeed be separated through the use of MIMO processing on the receiving side.

While the most basic configuration for MIMO utilizes two transmit antennas and one receive antenna, the signal-to-noise ratio post-MIMO processing is low in this situation, resulting in inferior performance and a sub-optimal radio architecture.  The research team therefore decided to use paired antennas: three transmit and three receive to boost overall system throughput, achieve an acceptable signal-to-noise ratio, and validate the signal processing algorithms.  Theoretically, MIMO does not require the number of receiving antennas to equal the number of transmitting antennas, but the MIMO signal processing algorithms simplify when the variables are matched.  The MIMO principle is illustrated where three transmitters S1, S2, and S3 are transmitting across a fading wireless medium with transmission properties Hxx to three receivers R1, R2, R3.

Hence, an essential aspect of MIMO processing is the determination of the medium transmission matrix Hxx.  In the IEEE 802.11a standard, a preamble training sequence is sent by the transmitter, which aids the receiver in estimating the channel matrix Hxx. During payload reception, the matrix Hxx is refined using known pilot tones, that are also sent by the transmitter.

The MIMO concept suggests that bandwidth can be increased linearly as the number of transmitting antennas increases. If three transmitters can provide 162Mbits/s bandwidth, then six transmitters should be able to provide 324Mbits/s.  Indeed, the un-mixing process to separate out signals is acheived usning simple linear matrix algebra well within the capabilities of today’s processors. 

In theory, MIMO could work with a single receive antenna, but in principle, the bit-error rate improves dramatically when the number of receive antennas is increased.  And in practice, the algorithms are less complex when the number of receivers equals the number of transmitters.  When the number of receivers is greater than the number of transmitters, the robustness of the algorithm is further improved at the expense of additional hardware and computational complexity.

Independent of the number of receive antennas, deciding on the number of transmitters is one of the key parameters in any practical application. One key factor is the number of ‘reflectors’ or ‘scatters’ in the medium.  The medium must be dispersive for MIMO to be effective; MIMO is less efficient when only direct line-of-sight transmission exists. For example, in a three transmitter/three receiver configuration, it is permissible to have one line-of-sight transmission as long as there are two independent reflections that allow the transmission matrix Hxx to maintain ‘full rank.’  This constraint of independent reflections represents an effective limit to the throughput enhancements possible with MIMO — a ten-by-ten configuration would require nine independent reflections; a scenario with limited probability of occurrence.

Our researchers believe that an 8×8 array is the effective upper limit to practicality; and a 3×3 configuration will work in nearly all home, office, and industrial environments and ultimately provide a cost-effective solution that meets both market and application bandwidth needs.

Another practical limit to MIMO deployment is that the inherent need for multiple antennas and receiver/ transmitters will further tax the power consumption, heat dissipation, size, battery life, and cost constraints for portable devices.  Therefore a balance must be struck to optimise the overall price/performance metrics of the system to yield optimum user experiences.

Independent force

The MIMO principle is independent of frequency bands and modulation schemes. In fact, for 802.11g (2.4GHz) where bandwidth is limited and congestion is higher, the ability to add the third dimension of space to time and frequency (and thereby improve the system throughput) may prove to be even more valuable than at 5GHz.  The demonstration at Eindhoven selected OFDM at 5GHz since 802.11a is already a ratified IEEE standard. MIMO, however, could also be applicable to GSM/GPRS systems, although the current power requirements make it less attractive to apply multiple antennas in portable cell phones, since the market attention for mobile phones is focused on decreasing size and weight and increasing battery life.

MIMO futures

While mass deployment of MIMO-based systems for the consumer and enterprise segments is probably several years away, it is expected that improvement and refinement of the systems will continue to take place in the interim.  A MIMO-OFDM system would require IC chips designed to perform multiple frequency down-conversions, OFDM demodulation, and MIMO interference cancellation.  Such processing is possible with today’s ASICs, but the power consumption may yet be too high for many applications in the near term. However, MIMO does demonstrate the future potential for WLAN technology to deliver bandwidths typical of today’s wireline implementations (100 Mbits/s).  Such a performance breakthrough would have implications for 3G wireless hotspots, public wireless data access, in-home delivery of HDTV signals, and many other applications in consumer, medical, industrial, and enterprise data communications.

The SIM card has been an inherent part of GSM systems from the beginning. However, according to Mark Melling of Intuwave, thinking laterally about the SIM as a smartcard adds utility and value to the mobile proposition, especially when it comes to securing data.

Mobile operators seeking to increase mobile data revenues by simply perpetuating PC-based usage models, rather than exploiting the intrinsic capabilities of the mobile device, are missing a trick. For example, if the SIM is viewed as a smartcard, it opens up security possibilities that resonate far beyond the mobile world. However, operators will have to change their approach in order to take full advantage.

The fixed-line Internet is often regarded as the model for how mobile data services should develop. In this scenario, the mobile handset is seen as a PC replacement and simply providing the same kind of enterprise access that a laptop might need is key to success. However, Intuwave has always argued that the ‘always with you’, ‘always on’ and ‘intermittently connected’ nature of the mobile device means that usage patterns are sufficiently dissimilar from those of the PC, to require new revenue models to be developed if full advantage of its capabilities is to be made.

An excellent example of the kind of thinking that is required concerns the SIM card. It is becoming clear that the SIM — a feature unique to the mobile world — has applications far beyond those for which it was originally designed. The clue is in the name — Subscriber Identity Module. It was created to remotely authenticate users to the network and to the billing systems that allow operators to generate revenues from voice traffic.

In practice, however, the SIM is effectively a mass-market smartcard. By combining stored evidence of identity (such as a key) with personal information only the user will know (a password, for example), it offers the same two-tier authorisation provided by smartcards. This ability of the SIM card to hold personal data that can authenticate users to a variety of systems is an ideal solution for enterprises and content developers grappling with the exploding issues of digital rights management (DRM).

Built-in benefits

As a smartcard, the SIM’s mobile heritage has many advantages. Firstly, SIM cards can be remotely configured to enable deployment of security keys and to ease blocking of transactions in the event of loss or theft. They also offer secure storage of sensitive data and keys in a way that PCs do not. And crucially, the SIM is tied directly to billing mechanisms for the purposes of micropayments.

The mobile phone is also the one device that we take with us wherever we go and its small size and sheer portability means that locating a smartcard on this device is a practical step. In essence, the SIM is a smartcard to which we have all ‘opted in.’ As operators and financial services organisations now see the combination of simple security devices (passwords) and a smartcard as acceptable security, there is no reason why the SIM cannot gain wider acceptance as an authentication tool, irrespective of the actual device (PC, kiosk, etc) used to access information.

Intuwave believes that the most immediate impact of the mobile phone as smartcard will be in the implementation of effective DRM for the mobile world with a business usage case revolving around access to corporate data and a consumer scenario around intellectual property, access to which incurs royalty payments.

In the business world, the smartcard might be used to encrypt and decrypt information enclosed in e-mails. Today, there is virtually no way of controlling access to information once it is sent out via e-mail: the sender merely hopes it reaches only the addressee to which it has been sent and is not forwarded to any other parties. If access rights to that e-mail can be tied to the user information contained on a SIM card, then a higher level of security is achieved. Also, it would be possible to freeze access to information on a PC if users left their desks, meaning that even an open email system would still be protected. The SIM might also authenticate access to a whole host of enterprise systems.

In a broader sense, there are a number of initiatives, particularly to do with recorded music, where intellectual property is openly available on a commercial basis over the internet. Again, the issue is how to control the onward distribution of this content. Using the SIM card as a basis of authenticating content to individual users might be an effective way of tackling this problem. Currently, this usage case is about controlling illegal distribution — in the medium term, the automatic link between the SIM card and the operator’s billing system might authenticate onward recipients to access the intellectual property by adding any due royalties to their mobile phone bill.

There are, of course, a variety of security initiatives currently underway but, far from being at odds with these, the approach outlined is complementary to many of the solutions now being advanced. For example, the Open Mobile Alliance is implementing a mobile DRM solution that doesn’t exploit the smartcard capabilities of the SIM — but its efforts would be significantly enhanced by doing so.

Reality not fiction

At the moment, the SIM as smartcard scenario might sound like science fiction but Intuwave is working with a variety of different organisations that have the experience and industry weight to usher such solutions into being. However, while we believe the technical challenges can be solved, there is one issue that will need to be resolved if the SIM is to become an effective smartcard solution.

At the moment, the SIM card is owned by the mobile operator that issues it with the phone and its ownership rights are set out in the small print of every service contract. While this remains the case, there are limited incentives for other parties, particularly corporate entities, to invest resources in a solution that remains the property of another. After all, any enterprise wishing to download sensitive security keys to a SIM card will want the security of knowing that they can assert some claims over its ownership.

Concrete steps

Some steps are already being taken to address this. For example, the EC’s Trusted Transaction Roaming (T2R) project brings together operators Orange and Vodafone, technology providers Gemplus, SmartTrust, and Ubizen and wireless trust organisation Radicchio. It aims to leverage the available GSM infrastructure, particularly the SIM card, to provide mass-market authentication and end-user consent services. This will allow third parties, such as governments, financial institutions and businesses, to use mobile handset as a complementary authentication and consent channel.
In the same way that an individual with a private car can be insured to carry commercial property belonging to his company or a physical property is bought by one party to be leased to another, so we believe operators can find some way for another’s intellectual property to be carried on its SIM cards without compromising the latter’s rights. Initiatives such as T2P are an important step in the process towards that.

Overall, mobile operators looking only at PC usage models for inspiration will short-change themselves and Intuwave believes that the SIM card is an excellent case in point. Its utility and value as a smartcard solution has no parallel in the PC world and is an essential feature of its ‘mobileness’. However, its ubiquity, portability and convenience together with its ability to store key personal data and link to billing systems means it might have been specifically designed to function as a smartcard. Moreover, as it already exists in the mobile handsets carried by all of us, it minimises many of the barriers to adoption of smartcards. In the creation of mobile data service revenues, it’s time to think out of the (PC) box.