Multiple antenna configurations can be used to overcome the detrimental effects of signal multipath and fading when trying to achieve high data throughput in limited-bandwidth channels.
Multiple-input, multiple-output (MIMO) antenna systems are used in modern wireless standards, including in IEEE 802.11n, 3GPP LTE, and mobile WiMAX systems. The technique supports enhanced data throughput even under conditions of interference, signal fading, and multipath. The demand for higher data rates over longer distances has been one of the primary motivations behind the development of MIMOorthogonal- frequency-division-multiplexing (OFDM) communications systems. For years, engineers assumed that the theoretical channel capacity limits were defined by the Shannon- Hartley theorem1 illustrated in Eq. 1.
Capacity = BW × log2(1 + SNR) (1)
As Eq. 1 shows, an increase in a channel’s SNR results in marginal gains in channel throughput. As a result, the traditional way to achieve higher data rates is by increasing the signal bandwidth. Unfortunately, increasing the signal bandwidth of a communications channel by increasing the symbol rate of a modulated carrier increases its susceptibility to multipath fading. For wide-bandwidth channels, one partial solution to solving the multipath challenge is to use a series of narrowband overlapping subcarriers. Not only does the use of overlapping OFDM subcarriers improve spectral efficiency, but the lower symbol rates used by narrowband subcarriers reduces the impact of multipath signal products.
MIMO communications channels provide an interesting solution to the multipath challenge by requiring multiple signal paths. In effect, MIMO systems use a combination of multiple antennas and multiple signal paths to gain knowledge of the communications channel. By using the spatial dimension of a communications link, MIMO systems can achieve significantly higher data rates than traditional single-input, single-output (SISO) channels.2 In a 2 x 2 MIMO system signals propagate along multiple paths from the transmitter to the receiver antennas.
Using this channel knowledge, a receiver can recover independent streams from each of the transmitter’s antennas. A 2 x 2 MIMO system produces two spatial streams to effectively double the maximum data rate of what might be achieved in a traditional 1 x 1 SISO communications channel.
How can already-scarce spectrum--a limited natural resource that’s nearing maximum saturation--carry more bandwidth-intensive data services, more economically, to an increasingly hungry public?
For an analogy, think of the highway you drive. Imagine its lanes increasingly filled by automobiles getting on at every on-ramp; at a certain point in time, there’s gridlock. That gridlock won’t allow the newer, faster automobiles onto the highway even if technology has enabled those new automobiles to be smaller, more sleek and cheaper. In this case, the high potential demand for the latest, greatest automobile will exceed the ability to get those autos onto the highway. Therefore, the full potential of innovation is lost.
Addressing the challenge of limited spectrum, coupled with increasing consumer demand for bandwidth, requires innovation, so that consumer hunger can be satiated while carriers’ business models perform effectively. New solutions must be developed that:
- use available spectrum with the utmost efficiency to allow higher data throughput over the wireless link
- support a greater number of users within individual cells and significantly enhance the user experience
- reduce the carrier cost of transporting megabit-rate traffic and carry that lower carrier cost through to the consumer.
Researchers and engineers have been tackling these issues on a number of fronts – from air interface design, to advanced antenna technologies, to new radio frequency and hardware solutions. Their efforts have uncovered technologies that will form the key building blocks for next-generation wireless broadband access solutions.
This article will take a look at two key technologies that will increase the likelihood that consumers will be able to access the high-bandwidth applications they want, while on the move and in a cost-effective way. We’ll also take a look at some of the obstacles to getting there, some of the issues carriers will face, and the fast-changing future of wireless broadband.
OFDM-MIMO: Key to greater performance
One building block for next-generation wireless access, MIMO (multiple-input, multiple-output), is an advanced antenna technology that can carry 4 to 5 times more data traffic than today’s most advanced UMTS-HSDPA-ready (3G) networks. A network design incorporating MIMO technology provides the scalability needed to quickly deliver multimedia content to the mass market. With MIMO, for example, a ½ megabit picture can be downloaded in a half second or a 30-megabit video in half a minute.
MIMO works by creating multiple parallel data streams between the multiple transmit and receive antennas (see figure below). Using the multi-path phenomenon, it can differentiate the separate signal paths from each MIMO antenna. Thinking back to the highway example, MIMO effectively adds several new highways.Another radio technology with tremendous potential for helping solve spectrum challenges is OFDM (Orthogonal Frequency Division Multiplexing). OFDM is a modulation technique, depicted in the following graphic, which uses many sub-carriers, or tones, to carry a signal.
OFDM has some key advantages over the common wireless access technology known as CDMA, which is used in many of today’s 3G cellular networks. To begin with, it is more robust, which means that it provides better performance in cluttered areas with many reflections (multipath). It also allows for simpler receivers.
Perhaps most important, OFDM is more amenable to MIMO technologies. A trial conducted in Nortel’s Wireless Technology Lab in early 2005 offers an example of this synergistic nature. During the trial, a mobile user had the ability to view two live streaming videos simultaneously while downloading a 264 MB file at 37 Mb/s over a standard 5MHz PCS band. Using OFDM-MIMO, the download was achieved in less than a minute compared to the 90 minutes that would be required with today’s networks. This is roughly 10 times the 3.6 Mbps enabled by the first generation of HSDPA devices.
OFDM is a logical next step in broadband radio evolution. It is already being applied in IEEE standards like IEEE 802.11 and 802.16, also referred to as Wi-Fi and WiMAX, respectively. Standards groups are currently working to standardize OFDM-MIMO as it relates to Wi-Fi and WiMAX. At this time, OFDM-MIMO is not part of the formal evolution path for existing cellular systems based on the 3GPP (UMTS, HSDPA) and 3GPP2 (CDMA 1X, EV-DO) standards; however, standards groups are working to understand its role in providing wireless broadband.