Wireless Systems Design

Eliminate IFFT From Baseband Processing

Using A 65-MHz DAC Board And A 16-Channel Digital Upconverter, It’s Possible To Generate A 12-Carrier Simulated OFDM Signal.

Orthogonal Frequency Division Multiplexing (OFDM) has been proposed as a transmission method to support high-speed data transmission over wireless links in multipath environments. In OFDM systems, information streams are split into smaller blocks. These blocks are then transmitted using orthogonal carriers.

Traditionally, the implementation of OFDM has focused on baseband implementation of Inverse Fast Fourier Transform (IFFT) at the transmitter, which generates the orthogonal carriers. Using a 65-MHz digital-to-analog-converter (DAC) board and a 16-channel digital upconverter (DUC), however, OFDM signals can be generated extremely easily. This approach eliminates the implementation of complex IFFT in baseband processing.

In wireless environments, transmitted signals follow several propagation paths. Many of these paths, having reflected from surrounding objects, reach the receiver with different propagation delays. This multipath causes delay spread, intersymbol interference (ISI), fading, and random phase distortion (FIG. 1).

Specifically, the delayed copies of the transmitted signal interfere with subsequent signals. The result is intersymbol interference. The transmitted symbol rate is therefore limited by the delay spread of the channel.

Due to the increasing popularity of new data services and multimedia applications, the demand for high data rates has increased. Running high-speed data applications with traditional wireless technologies, such as FDMA and TDMA, is not efficient because of increased overheads. A great deal of research is underway to find new technologies that efficiently support high-speed data rates. Many proposals are now based on OFDM. In addition, many new standards have chosen OFDM. Examples include the European radio (DAB) and TV (DVB-T) standards, as well as the IEEE 802.11 wireless-LAN standard.

In OFDM, the high-speed data stream is split into many substreams, thereby minimizing ISI. Each substream is modulated separately and transmitted on a different carrier. The frequency spacing between these carriers is kept equal. As a result, these carriers become orthogonal to each other.

Because of this orthogonality, carrier frequency bands can overlap. In addition, receivers can demodulate the overlapped signals. In this way, ISI is minimized and any overheads associated with FDMA are reduced (FIG. 2).

To generate the orthogonal carriers, OFDM requires multiple modulators and associated circuits. The use of conventional modulators increases the hardware complexity and cost. Software radio, however, provides a very simple method to generate the orthogonal carriers easily and without the implementation of IFFT. A 65-MHz DAC PCI card (ICS-660) and a 16-channel digital upconverter (DC-60-M1) have been used to demonstrate this concept. Together, they can generate 12 orthogonal carriers.

The ICS-660 is a 14-b, 65-MHz DAC. It can be fitted with the 16-channel DC-60-M1 modulator, which can support individual channel bandwidth of 1.6 MHz. The next-generation versions of these cards support sampling up to 105 MHz and channel bandwidths up to 5 MHz. This DUC-DAC combination offers a convenient way to implement and test an OFDM system. An arbitrary frequency distribution and modulation scheme (e.g., BPSK, QPSK, QAM, etc.) can be programmed either in MATLAB or C. In addition, multiple boards can be used to support more than a 16-carrier OFDM system. To implement a 52-carrier OFDM system—as specified in IEEE 802.11—four ICS-660 (with the DC-60-M1) boards are therefore needed.

The OFDM implementation that uses ICS-660 and DC-60-M1 is shown in Figure 3. The parameters used are as follows:

  • Input data rate = 3.125 Mbps
  • Number of carriers: N = 12
  • Number of carriers for data transmission = 10
  • Number of carriers for pilots = 2
  • Coded data rate = 6.25 Mbps (using 1/2-rate convolutional encoder)
  • Data rate per carrier = 312.5 ksymbols/s

Twelve sub-carriers are used as an example with 312.5-kHz spacing. The sub-carrier falling at the center of the band (k = 93) is not used. Two pilots have been inserted at k = 90 and k = 96. Figure 4 shows the frequency-allocation scheme.

The data, which is first generated using a random-number generator, is QPSK modulated. The pseudo-random pilot sequence is BPSK modulated. Pilot signals are used to make the coherent detection robust against frequency offsets and phase noise\[1\]. Data and pilots are transmitted in blocks called OFDM symbols. Each OFDM symbol consists of 10 coded data bits (5 information bits) and two random-sequence pilot bits.

The output spectrum is shown in Figure 5. To expand to 52 sub-carriers, four 65-MHz DAC PCI boards may be used. They can implement the 52-carrier OFDM system as specified in the IEEE 802.11 WLAN standard (FIG. 6).

This article has demonstrated an alternative approach for generating OFDM signals using high-speed DAC and DUC boards. OFDM signals may be very easily generated without the need for a complex IFFT implementation in baseband processing. Such an approach is practical for a system with a small number of sub-carriers. In addition, multi-board synchronization allows additional sub-carriers to be generated. This limitation is usually acceptable in a lot of applications, as the bandwidth utilization is sufficiently high in orthogonal frequency division multiplexing. As a result, large-data-rate services may be offered with small numbers of sub-carriers.

References:

  1. http://standards.ieee.org/reading/ieee/std/lanman/802.11a-1999.pdf
  2. Ap-Note: AN-SR-6 "Arbitrary Generation of Radio Waveforms" (www.ics-ltd.com/white_papers.htm)
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