MIMOS or the Malaysian Institute of Microelectronic Systems is interested in implementing Multiple Input, Multiple Output or MIMO antenna technology in its future communication systems. These include the WiWi or WiMAX-WiFi networks its developing – which use WiMAX connections in the backhaul and affordable WiFi coverage of multiple local hotspots or hotzones -- and provide wireless broadband coverage, especially in outlying areas not already served by existing networks and it will use the mobile switching centre (MSC) and base station controller (BSC) concept for handoff. MIMOS expects to come out with its own designed base station at the end of the year or in early 2009.

MIMOS hosted a series of talks on MIMO in April by Dr. David Gesbert (right), professor of the Mobile Communications Department at Eurecom, http://www.eurecom.fr/, a research lab in France owned by a group of top universities and top companies across Europe and it also produces PhDs. “The underlying concept behind MIMO was invented in Stanford University in 1994 and was first patented by Professor Paul Raj Arogyaswami. Its main application at the time was for use in high-definition TV systems,” said Prof. Gesbert.

In 1996, Bell Labs created the first prototype of a MIMO chip for wireless Internet use based on 16 antennas in a notebook PC talking to an access point using a proprietary technology back then and it showed a 16 times higher data rate over single antenna systems. In 1998 both Paul Raj, Gesbert and two others co-founded Gigabit Wireless to promote MIMO OFDM (orthogonal frequency division multiplexing) and Gigabit (under the new name of Iospan) was sold to Intel in 2002. Intel now promotes this technology for use in the WiMax standard. The advantages of MIMO are two-fold: First it enables the increase of data rates by transmission of several independent multiplexed data streams on the different transmit antennas.

Second, it can enable robust communications, especially in challenging environments for radio propagation, by sending instead redundant information over the multiple antennas. Multiple data streams enable higher data speeds, while with redundancy under less radio-friendly conditions, if one signal is disrupted by interference, the receiver can recover all data from the other, a benefit known as “diversity”.

“A space-time coding algorithm invented by Bell Labs in 1996 enable multi-antenna systems to spread the information across several antennas by sending a different combination of the same or independent bits over different antennas and the receiver applies a decoding technique which recovers lost data due to interruption of the signals,” said Gesbert. The main requirement for MIMO diversity is either two transmitting antennas and one receiving or one transmitting antenna and two receiving.

For multiplexing, the minimum setup is two transmit two receive. The multiple antennas must be spaced half a wavelength apart (at minimum, in the best case of widely scattered multipath) from each other. Radio waves travel at the speed of light or 299,792,458 metres per second in free space (a little slower in the atmosphere) and a 2.3GHz WiMAX signal would have a wavelength of about 13 cm, so the MIMO antennas would have to be optimally spaced about 6.5 cm apart at the mobile side. At the base side (above the clutter), more spacing is required due to less scattered and diverse multipath. MIMO can also be employed in mobile phones using one vertical internal antenna in conjunction with a patch antenna to simulate the required half wavelength separation.

Lots of theoretical research and development work was conducted on MIMO between 1996 and today. For applications, the focus initially shifted towards use of MIMO with WiFi and WiMAX, since it was much easier to integrate MIMO into emerging technologies, than into the 3G world where the standard is dominated by fewer heavier players.

However now, 3G LTE (long-term evolution) is adopting MIMO and LTE is expected to be available in about two years. One of the benefits of using the 2.4 GHz and 5.8 GHz unlicensed bands is that it provides an opportunity to reduce the revenue of the big giants and to compete with or outrun the 3G techno-politics. OFDM (Orthogonal Frequency Division Multiplexing) is regarded as better technology than CDMA (Code Division Multiple Access) and it has a less complicated patent regime and is more consistent with IP-based communications.

One of OFDM’s main advantages is its greater scalability to support wider and wider bandwidths, more advanced services and higher data rates. “WiMAX now can operate with a 20MHz channel and next up to 100MHz channel and there is a direct correlation between channel bandwidth and the data rate, and MIMO technology, which combines ideally with OFDM, can enable even greater capacity from the same bandwidth,” said Gesbert. Its scalability with respect to bandwidth is based on a mathematical algorithm known as Fast Fourier Transform and with OFDM, twice the bandwidth requires an algorithm of twice the complexity, hence twice the computational power to process it.However the complexity involved in doubling the bandwidth with a GSM-like physical layer increases exponentially with bandwidth, hence requires and exponential increase in processing power.

Another advancement of OFDM, called orthogonal frequency division multiple access (ODFMA) allows for the placement of different users in different sub-channels or it could have redundant sub-channels for greater robustness. MIMO sits on top of OFDMA and each sub-channel operates using a MIMO transmission technique. There’s now a revival of interest in MIMO due to new discoveries in multi-user MIMO, with the main aim to serve multiple users.

The set of antennas used in MIMO systems based on TDMA (time-division multiple access) and OFDMA techniques can only serve one user at a time, while the set of antennas in MIMO systems using spatial division multiple access (SDMA) can serve as many different users as the number of antenna at the base station.

“The key advantage of this is that users don’t require expensive handsets with multiple antennas, since the base station can multiplex users in the space domain only thanks to its own multiple antennas (typically 4 in LTE context) with a minimal increase in operational costs,” said Gesbert. While single-user MIMO is available for use in WiFi, WiMAX and LTE, only the first prototypes of multi-user MIMO equipment exist and its use in WiMAX and LTE are still under discussion.

High Bandwidth Wireless?

On high bandwidth wireless, the cooperative communications concept is growing where transceivers will help each other by sharing spectrum, resources, frequencies, battery power and so on. For example, relay protocols would enable range extension of cellular coverage by adopting well-known technologies from ad-hoc networks into cellular networks and allow mobile phones within a base station’s coverage to relay signals between each other and the base station. Virtual MIMO Another technique is virtual MIMO, where two distinct base stations operate as if they are a virtual array of MIMO antennas to reduce the inter-cell interference effect and turn interference into an advantage for the system. This domain forms a hot area of research in groups such as Dr. Gesbert’s in Eurecom.