MIMO is an acronym for Multiple Input, Multiple Output and is a significant departure in the architecture and technology from the current SISO (Single Input, Single Output) WLAN products. This is accomplished through the implementation of multiple transmitters and receivers in a single WLAN station, AP or client.
A principle problem in wireless systems is multipath interference. Existing WLAN technologies attempt to compensate for multipath by implementing a rake receiver to allow for specific amounts of delay spread. As delay spread increases, the multipath situation worsens and the arriving symbols begin to interfere with each other beyond the capability of the receiver to recover. This results in common complaints to the help desk with the generic “wireless doesn’t work” intermittent reports which cannot be reliably recreated.
Further, the throughput capability of the WLAN with MIMO is extended to be more comparable with wired LANs. The current IEEE 802.11n draft includes link rates up to 600 Mbps. The commonly used value for link overhead in WLAN systems is 40% giving a throughput potential of about 360 Mbps in a single cell. This is an idealized value and is highly dependent on the operating environment and the composition of clients associated with a particular AP.
A basis for MIMO was created at Bell Labs in the 1997 to 2002 period called BLAST for Bell Labs Layered Space-Time. Using MIMO-like techniques, multiple transmitters and receivers, BLAST exploits multipath to gain very high spectral efficiencies (10s of bits/sec/Hz were measured).
Multiple‐input/multiple‐output (MIMO) technology offers tremendous performance gains for wireless LANs (WLANs) at relatively low cost. Any system with multiple inputs into the receiver and multiple outputs to the transmitter is a MIMO system, but implementing such a system involves several distinctly different radio techniques. A fully compatible MIMO approach offers straightforward choices for both WLAN users and equipment vendors. This paper explains the background and tradeoffs involved in using MIMO techniques and shows how Atheros’ VLocity technology achieves today’s best MIMO performance while maintaining full compatibility with existing 802.11 standards.
THE ORIGINS OF MIMO
MIMO concepts have been under development for many years for both wireless and wire‐line systems. One of the earliest MIMO‐to‐wireless communications applications came in 1984 with groundbreaking developments by Jack Winters of Bell Laboratories. This MIMO pioneer described ways to send data from multiple users on the same frequency/time channel using multiple antennas at the transmitter and receiver. Since then, several academics and engineers have made significant contributions in the field of MIMO.
Many WLAN, Wi‐Max, and cellular companies offer (or are planning to offer) MIMO‐based solutions. Existing applications include multiple‐antenna systems, Code Division Multiple Access (CDMA) systems used in 3G cellular systems, and even Digital Subscriber Lines (DSL) with multiple telephone lines experiencing crosstalk.
LEVERAGING MIMO IN WIDE-AREA NETWORKS
A growing number of current and prospective wide-area wireless network operators are adopting strategies that include mobile broadband access and rich multimedia services. These strategies present significant challenges to their wireless networks — requiring large improvements in network capacity, subscriber data rates, range, and coverage quality in order to build and sustain viable business models. The potential performance gains offered by smart antenna technologies such as MIMO are of increasing interest to operators as they grapple with these challenges to network economics.
Many inherent characteristics of local-area networks that have driven MIMO success in that domain differ substantially in wide-area environments, so the technology transfer must be handled with care. In the following brief overview of wide-area MIMO application, we highlight interference and limited scattering as the most important of these differences and recommend key considerations in implementation. The good news for wireless operators is that a large portion of the theoretical gains from MIMO can indeed be achieved in the wide area by network aware solutions designed to minimize interference in multi-cell environments and maintain robust operation in limited-scattering situations.
MIMO WIRELESS TECHNOLOGY
Consider a system with a single antenna at each end of the link. Although the signal is transmitted in all directions (typically within a 120º sector), a particular wireless channel may only have two dominant paths, as illustrated in Figure 1. We show here an example of an elevated base station communicating with a mobile handset down at street level, where the bulk of the received signal comes from reflections off neighbouring buildings. This corresponds to a single-input single output (SISO) channel.
Figure 1:A wireless channel with two dominant propagation paths between a base station (BS) and client device (CD), represented by the arrows, overlaid on the base station’s nominal 120º-sector transmission pattern.
Here we can introduce the simplest and currently most common form of smart antennas. 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 physics are 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 and transmit.
Providing multiple antennas at both ends of the link corresponds to a MIMO (multiple-input, multiple output) scenario. In this case, we can exploit the two paths much more efficiently — as we illustrate in Figure 2. 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.