Building future networks with MIMO and OFDM

Fast-rising consumer appetite for wireless broadband and the high-bandwidth applications it’s beginning to deliver, including games, photos and video, creates a technology dilemma for today’s network equipment makers and wireless carriers.

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.

Getting ready, from a carrier perspective

It is important to note that the OFDM-MIMO evolution being suggested here for 3GPP and 3GPP2 is an evolution of existing 3G networks – not a new network. This means existing sites and network architecture could be re-used, allowing wireless operators to preserve their previous investments in existing networks like today’s 3G networks. On the other hand, the use of OFDM-MIMO for WiMAX 802.16e, which is progressing very rapidly, would require a new network.

The building of large-scale OFDM-MIMO networks will take several years and it will not be without challenges. For instance, at the cell site, the use of cross pole antennas will allow service providers to avoid having to install additional large antennas, which can cause installation and zoning problems. However, service providers and equipment manufacturers will need cabling from the antennas to the tower bases. Devices will also be impacted, requiring extra built-in antennas and more stringent component tolerances. Fortunately, significant work is in progress to address these issues.

In summary

There is considerable evidence that the appetite for wireless broadband is only just beginning and that wireless broadband will become the largest growth area within the wireless industry. The starting point for this growth is increasing consumer reliance on laptop computers. Today, more laptops are sold in the United States than desktop PCs. As this trend continues, so will users’ demand for the same type of broadband service no matter where they are – at home, at the office, or on the road.

WiFi and existing high-speed cellular networks being deployed today meet some of these needs, but OFDM-MIMO, used by WiMAX 802.16e or beyond 3G, is the technology needed to allow for economic and scalable wireless broadband. The scalability furnished by these technologies becomes especially important once MP3 players, PDAs, portable games devices and other handheld devices are equipped to provide wireless broadband, giving users a broadband connection wherever they go.

Going back to our earlier analogy, if the wireless industry fast-forwards the acceptance of MIMO and OFDM technologies into the refinement of our highways, we will be able to put the latest, greatest automobiles--applications--on those highways and allow people to get places faster and more efficiently than ever before. In this way, MIMO and OFDM are key technologies for enabling the wireless industry to deliver on the vast potential and promise of wireless broadband.

John Hoadley is vice president, Next Wireless Generation Access, for Nortel.

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