Why Mimo Makes Sense For Next-Generation Cellular

Why MIMO makes sense for next-generation cellular

OFDM has been gaining ground on CDMA and emerging as the new disruptive technology of the future.

By Gurinder Dhillon, Airgo Networks, Inc.

All wireless technologies face the challenges of signal fading, multi-path, increasing interference and limited spectrum. MIMO (multiple-input-multiple-output) technology exploits multi-path to provide higher data throughput, and simultaneous increase in range and reliability all without consuming extra radio frequency. It solves two of the toughest problems facing any wireless technology today, speed and range.

Digital communication using MIMO processing has emerged as a breakthrough for wireless systems of revolutionary importance. This modulation format has been shown to be spectrally compliant, achieve higher data rates and higher spectral efficiencies, allow for longer range, and support backwards-compatibility to existing OFDM standards. MIMO Definition.

This article will explain how MIMO's and its OFDM modulation technique is well suited for the next generation of cellular technology deployment. But first, let's take a quick look at MIMO technology.

MIMO basics

It is often useful to provide a short definition of a technology that sums up its salient characteristics. For MIMO, this would be: Two or more data signals transmitted in the same radio channel at the same time.

MIMO is a revolutionary multi-dimensional approach that transmits and receives two or more unique data streams through a single radio channel whereby the system delivers two or more times the data rate per channel.

As shown in Figure 1, more than one coherent radio up-converter and antenna are used to transmit the multiple signals, and more than one coherent radio down-converter and antenna are used to receive the multiple signals.

With MIMO, the maximum data rate per channel grows linearly with the number of different data streams that are transmitted in the same channel.

Click here for Figure 1

Figure 1:MIMO transmits signals simultaneously on the same channel.

MIMO's approach is to transmit and receive two or more unique data streams through a single radio channel. This means the system can deliver two or more times the data rate per channel. By allowing for the simultaneous transmission of multiple data streams, as shown in Figure 1, MIMO multiplies wireless data capacity without using additional frequency spectrum.

Peak throughput

Peak throughput in MIMO systems increases by a factor that is equal to the number of signal streams transmitted in the radio channel. Because there are multiple signals each being transmitted from a different radio and antenna, MIMO signals are sometimes called "multi-dimensional" signals. Conventional radio signals are referred to as "one-dimensional signals" because they only transmit one data stream over the radio channel even if multiple antennas are used.

The appetite for higher data rate continues as consumer demand for bandwidth hungry applications like Internet and email access grows to encompass gaming, streaming audio and video.

Advancement in handset processors and further integration of technologies like higher mega-pixel cameras, videos, etc into handsets, will only exacerbate the challenge of enabling more bandwidth consuming applications at longer ranges and more efficient utilization of the limited spectrum available to network operators.

Research, innovation, development and application of MIMO to OFDM in the WLAN space by Airgo Networks and subsequent adoption of MIMO by 802.11n standard body has proven the capability of MIMO to not only provide significantly higher data-rates but also extend the range of the Wi-Fi to reach more mobile customer segments. Seeing the effectiveness and superior capability of MIMO in enhancing data rate and extending range, other standards organizations realized that it can do wonders for other technologies, both fixed mobile and cellular. Standard bodies like 3G, WiBro, WiMAX, 802.20 and 4G have been exploring the use of MIMO and some have already adopted it in their respective technology areas.

Path to 4G cellular

Since its inception, CDMA has been a revolutionary technology. Due to CDMA’s superiority over other standards like GSM, Qualcomm not only drove the standards for 3GPP2 but also penetrated into the 3GPP WCDMA standards. CDMA’s huge success has helped Qualcomm extract significant royalties from handset OEM’s.

But, recently, OFDM has been gaining ground and emerging as the new disruptive technology of the future. OFDM has achieved this distinction by its ability to handle the common radio frequency distortions without the need for complex equalization algorithms and its ability to easily scale in the spectrum domain.

During last few years, OFDM technology has been successfully applied to number of wireless applications like WLAN, Broadcasting (DVB), and WiBro/WiMAX. Recently, a startup, Flarion, proved the promise of OFDM for cellular applications with burst rates for downlink up to 3.2Mbps.

Realizing the potential of OFDM as the technology for 4G standards, Qualcomm acquired Flarion. The acquisition indicates that OFDM is the technology of the next generation.

MIMO technology is at various stages of adoption or deployment in third generation cellular systems, broadband fixed wireless systems, high speed WLAN and mobile ad-hoc networks.

Based on Airgo’s pioneering efforts, economical MIMO OFDM based WLAN systems became a reality in 2003 and this set a new course for the wireless industry. MIMO-OFDM will be the basic modulation format for 802.11n Wireless LAN (WLAN) since every proposal submitted for this standard uses MIMO-OFDM as the primary method to achieve significantly greater range and throughput over 802.11a or 802.11g.

Widespread acceptance

Over the past few years, MIMO technology has penetrated a variety of other important commercial wireless markets besides WLAN as shown in Figure 2.