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Signal Chain Basics #81: OFDM Basics

As more high-speed amplifier applications deal with communication systems, wired or wireless, a brief introduction and overview of orthogonal frequency division multiplex (OFDM) is necessary. With a basic understanding of system fundamentals where high-speed amplifiers are used, engineers can choose the right amplifier for their systems and optimize overall design.

OFDM
OFDM is a form of multicarrier modulation that divides the available allocated spectrum (channel) into multiple subcarriers (or subchannels). These subcarriers are narrowband, overlapping, orthogonal, and digitally modulated (e.g., QPSK or QAM). The advantage is that they make for a very spectral-efficient method of communication versus typical TDM and FDM methods. In time-domain multiplexing (TDM) one carrier is used to place multiple data streams staggered in time. Frequency-division multiplexing (FDM) is a multicarrier approach that uses multiple subcarriers, each with its own frequency allocation. For transmission on one main channel, OFDM saves spectral efficiency by allowing for close, overlapping subcarriers transmitting higher data rates with lower bandwidth allocation (Figure 1).

Figure 1

OFDM provides higher spectral efficiency versus other multicarrier modulation.

OFDM provides higher spectral efficiency versus other multicarrier modulation.

In most communications systems where close carriers can corrupt the signal of interest being received, filter banks typically are needed. Within these systems, the hardware must use filtering, channel separation, and guard-banding for better signal fidelity (Figure 2).

Figure 2

In a typical communication system, filtering, channel separation, and guard-band needs increase system complexity.

In a typical communication system, filtering, channel separation, and guard-band needs increase system complexity.

Within an OFDM signal the subcarriers can be closely spaced, even overlapping without the need for guard-banding and filtering. There is limited interference from the close carrier spacing that you would expect with other techniques. This occurs because the signals (subcarriers) are orthogonal to one another as the carrier spacing is equal to the reciprocal of the symbol period. A very basic visual example of OFDM is in Figure 3. Subcarriers are spaced 25 kHz apart, and the full OFDM channel is 10 MHz wide.

Figure 3

Basic example of OFDM in a 10 MHz bandwidth utilizing 25 kHz subcarrier spacing.
For a larger version of this image, click here.

Basic example of OFDM in a 10 MHz bandwidth utilizing 25 kHz subcarrier spacing.
For a larger version of this image, click here.

Following each symbol, a cyclic prefix (CP), or guard interval, is inserted and associated with the OFDM signal. This helps to prevent inter-symbol (ISI) and inter-channel (ICI) interference but incurs a loss in data rate and power. This tradeoff is typically considered when defining CP for a communication standard using OFDM.

OFDM in use
More and more technologies are adopting OFDM. The need for efficiently transmitting high data rates has increased OFDM’s popularity.

Wired communications that employ OFDM include DSL, digital video broadcasting, European standard (DVB-C2), power line communications, and multimedia over coax (MoCA). Wireless technologies and mobile standards have adopted OFDM in a variety of segments. Digital audio broadcast (DAB) utilizes OFDM for mobile reception of high-quality digital audio signals, whereas WLAN uses OFDM to support high-bit requirements in WiFi (802.11 standard). Additionally, DVB-H, a mobile TV system, and LTE standard (3GPP Long Term Evolution) utilize OFDM with LTE using OFDMA in the downlink path. OFDMA is based on OFDM. The difference is that OFDMA has the ability to dynamically assign a subset of those subcarriers to individual users, making this the multi-user version of OFDM.

The push for high quality and higher-speed data rates lends itself to the adoption of OFMD.

Advantages and disadvantages
Compared to single-carrier modulation schemes, OFDM wins hands-down in its ability to cope with severe channel conditions. This includes narrowband interference, frequency-related channel fading, and long-wired applications (for example: non-optimal conditions through copper). Simply put, OFDM minimizes the impact of these channel impairments because OFDM can be received as many slowly modulated narrowband signals, versus one wideband signal.

The biggest challenge when implementing OFDM is the peak to average power ratio (PAPR) associated with this type of modulation. This effect impacts both the receive and transmit paths of the system. It places high linearity requirements on the amplifiers and limits full-scale utilization of the DAC and ADC. This is due to the back-off allocation needed to avoid performance degradation, or even damage at the peak levels.

As you can see, despite the PAPR drawback, OFDM has many application benefits, and its implementation will only continue to grow as communications systems demand higher data rates.

For information about implementing OFDM in power line communication (PLC) solutions, visit www.ti.com/smartgrid-ca. For information on how to implement OFDM in femto base stations, visit www.ti.com/femto-ca.

Join us next month when we will discuss crosstalk between outputs in clocking devices.

References:

  1. Jean Armstrong, Senior Member, IEEE, OFDM for Optical Communications, Journal of Lightwave Technology, Vol. 27, No. 3, February 1, 2009.
  2. S. Srikanth and P. A. Murugesa Pandian, Anna University; Xavier Fernando, Ryerson University, OFDMA in Wimax and LTE: a comparison IEEE Communications Magazine, September 2012, pp 153 – 161.

— Carissa Sipp is a systems and applications engineer for TI’s High-Speed group. She received her BS degree from The Ohio State University. Carissa can be reached at .

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2 comments on “Signal Chain Basics #81: OFDM Basics

  1. blueplanet
    September 12, 2013

    I really enjoyed reading this article.  It provided a very clear and easy-to-understand basic description of OFDM.  I look forward to the next in the series from this author!

  2. Brad_Albing
    September 22, 2013

    Carissa – very nice article – you did a good job explaining this for those of us who don't have a good understanding of how OFDM works (e.g., me).

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