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Testing an OFDM-based MIMO system

Posted: 18 Feb 2009 ?? ?Print Version ?Bookmark and Share

Keywords:test MIMO? MIMO OFDM-based? MIMO?

The communications market and the underlying technology for voice and data services are constantly evolving. Customers are always expecting higher wireless bandwidths, and service providers want to sell high value services beyond voice. In addition, the cost of DSP technology required to deploy high-bandwidth broadband wireless systems has dropped to a point where it can now be used in the phones, PDAs and laptops on which we have become so dependant.

The technology of choice for wireless broadband is based on OFDM. OFDM technology is used in many SISO (single in, single out) radio standards and is also used in MIMO radio configurations. Standards using OFDM include WiMAX, LTE, WLAN and DVB. OFDM has many benefits over other radio technologies, including one key feature, which is overcoming multipath problems.

This article looks at a few key parameters and the tests needed to tell how an OFDM-based MIMO system is performing.

MIMO and OFDM 101
A typical MIMO configuration includes a 2 x 2 system with two transmitters and two receivers. A 4 x 4 system has four transmitters and four receivers, and so forth. MIMO transmitters transmit multiple OFDM signals using the same frequency channel. The challenge at the receiver end is to be able to get back to four independent radio signals. Hence, MIMO allows the same bandwidth to be used multiple times, allowing a higher spectral efficiency.

For example, if a MIMO transmitter transmits two signals on the same frequency, each signal takes a slightly different path to the receiver. Each signal's characteristics diverge from the originally transmitted signals, mainly due to multipath distortion. Two receivers receive a mixture of both the original signals.

OFDM is an efficient modulation scheme that is also very resilient to other RF interference, which is important as WLAN is deployed in the unregulated ISM band. OFDM also works very well in harsh multipath environments.

Most modern communications systems use a single carrier to transmit a single symbol at a time. Higher data throughput is achieved by increasing the symbol rate. However, the faster the symbol rate, the shorter the time the symbol "waits" for the received symbol. In environments where multiple signal paths exist, there comes a point where simply increasing the data rate is not an option, as any gains in "extra data" are lost through intersymbol interference (ISI). High symbol rates work well if there is a direct path, such as utilized in line of site microwave links, whereas reducing the symbol rate is necessary where multipath interference is a problem.

OFDM takes advantage of using much lower symbol rates per carrier to reduce multipath interference, but uses multiple carriers to increase the data rate.

Figure 1: By taking four independent OFDM carriers and placing them atop one another, MIMO transmission techniques allow up to 3.5 times as much information to be transmitted in the same bandwidth as a single carrier.

Instead of transmitting a single symbol at a time, OFDM transmits multiple symbols simultaneously on a number of carriers. The subcarriers are distributed in carefully chosen multiples of frequency such that they are "orthogonal," so that the closely adjacent subcarriers don't interfere with each other. Mathematically, orthogonality occurs when the frequency spacing between carriers is equal to the symbol rate.

As with any digital radio, the symbols are structured in time. With WiMAX (802.16e) and LTE, orthogonal frequency division multiple access (OFDMA) is used to allow a dynamically changing number of users to have access to the system. Each OFDMA time increment is referred to as a symbol and we could have, for example, a preamble symbol, then a down link map, some information we wish to receive, a transition gap, the information to transmit the uplink burst, then another transition gap before the cycle starts afresh. WiMAX can either use time division duplex or frequency division duplex between the up and the down link burst.

Thus, in every OFDMA symbol period, multiple symbols are transmitted in parallel. In fact, with WiMAX, we can transmit between 128 to 2,048 symbols per OFDM symbol period. The vertical axis is labeled "sub-channel number." The sub-channels are not actually physical channels, but groupings of our "parallel symbols" that are transmitted every OFDM symbol period. How the WiMAX signal is constructed and behaves over time is defined in the symbol map. The symbol map is essentially a two-by-two matrix of symbols, the vertical are the parallel symbols, and the horizontal represents how these symbols behave over time.

OFDM, a multicarrier modulation, splits a high-rate data stream into a number of low rate streams that are transmitted simultaneously over a number of sub-carriers. This scheme greatly reduces the data rate in the parallel sub-carriers, allowing an increase in symbol duration. This type of digital modulation scheme creates a time-domain waveform that looks very noise-like. Multi-carrier waveforms like those used in OFDM schemes have a high peak-to-average ratio (PAR).

OFDM uses the inverse FFT (IFFT) to break down a fast stream of data into many slower parallel data streams. This is then fed into an IFFT to have those slower streams distributed over the bandwidth as individual sub-carriers. The lower data rate of each of the sub-carriers allows a guard band to be added to the symbols being sent so that the guard band time is long enough to prevent most multipath reflections from causing any ISI. Fundamentally, the object is to slow down the symbol rate, while maintaining a high data rate by using an IFFT to facilitate the transmission of slow symbols in parallel rather than fast symbols in serial.

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