Multiplexing grows and WiMAX flourishes
01 April 2007
OFDM (orthogonal frequency division multiplexing) extends from the classic technique of modulation, in which information is transmitted on a radio channel through variations of the frequency, phase or magnitude of the carrier wave
It was expanded in 1965 with the fast fourier transform (FFT) algorithm. Today, it is in communication technologies from ADSL and WiFi to WiMAX, DVB, and UWB. The fundamental principle underlying OFDM
modulation is the division of the data rate (R bit/sec) among N parallel carriers, each with a data rate of R/N bit/sec.
These datastreams are transmitted on the channel using a series of sub-carriers (equal to the number of datastreams), differing in frequency (Äf) and organised to prevent interference.
Modulation types
In typical FDM transmissions, carriers are separated by guard intervals to filter at the receiver and the information recovered using classical demodulators. In OFDM, the subcarriers are oriented in an orthogonal
manner, allowing them to partially overlap without interfering with one another.
The OFDM sub-carriers are aligned so that the nulls of one sub-carrier’s spectrum coincide with frequency peaks of the adjacent sub-carriers, resulting in partial spectral overlap (see figure 2).
Non-linear distortion and phase noise are the factors that most often cause the loss of orthogonality, which then results in interference. The partial overlap of the subcarrier signals reduces the band occupied on the channel.
To create an OFDM signal, imagine that a bitstream must be transmitted, through a quadrature amplitude
modulation (QAM) scheme (each symbol = 2bits). To transmit this symbol on an OFDM sub-carrier, the
amplitude and phase of the sub-carrier are determined from the symbol itself. In general, the N symbols to be transmitted (S0 , S1 ... SN-1) generate N complex numbers (Z0 , Z1 ... ZN-1).
An inverse FFT is performed on these complex numbers, generating a series of time-domain samples, which are then transmitted on the channel using QAM. Given a generic symbol, z=a+jb, the
amplitude is:
while the phase is:
This process is repeated, resulting in symbol streaming by the same single carrier (the specific symbol/carrier association is typically specified by the protocol).
By extending the algorithm to a number of carriers, the process used to create a multicarrier OFDM signal becomes evident. The modulation scheme associated with the OFDM carriers can change dynamically
depending on the state of the channel.
Thus, it is possible to favour transmission velocity in locations lacking obstructions when, for example, there is visibility between the transmitter and receiver, i.e. line of sight. It is also possible to favour robust transmission when disturbances to the channel (e.g. fading) are present.
At the receiver, signal demodulation involves using a fast fourier transform (FFT) to extract the real and imaginary components of the symbol and thereby obtain the information.
Advantages of OFDM
The OFDM technique allows a channel that is prone to distortion to be divided into parallel stable sub-channels.
In the case of a multi-path environment, is the time delay between the symbol component that is obtained from the shortest path (e.g., line of sight) and the last component that is associated with the longest path. These differences occur because of reflections along the various paths between transmitter and receiver.
The coherence bandwidth (Bc ) is defined as:
If the signal bandwidth (B) is greater than the coherence bandwidth (B >> Bc ) there is potential for inter-symbol interference (ISI).
With OFDM, the signal bandwidth (B) is divided among a set of sub-signals with sub-bandwidth =B/N, such that each sub-bandwidth is smaller than the coherence bandwidth. This results in greater resistance to inter-symbol interference caused by multi-path.
OFDM and WiMAX
To analyse the physical layer of the protocol, it is useful to consider a WiMAX real/ideal signal originating from the Agilent MXG signal generator and evaluated using the MXA signal analyser and vector signal
analysis software (VSA 89601A).
WiMAX IEEE 802.16-2004
WiMAX (worldwide interoperability for microwave access) is a broadband wireless access technology based on IP protocol. It provides broadband access to areas lacking pre-existing cable and telephone networks.
As such, WiMAX must be able to provide coverage in areas in which there is lack of visibility between the user and the transmitter (NLOS) and at distances of up to 30km.
WiMAX is specified by the IEEE 802.16- 2004 standard for the 2.5 to 2.69GHz and the 3.4 to 3.6GHz ranges; the total occupied band can be between 1.25MHz and 20MHz.
The WiMAX standard provides for the use of 2048 or 256 carriers of three types: data, pilot and unused (null). In the case of 256 carriers, a certain number will function as guard intervals (56 unused carriers), and
only 200 will be effectively used. Among these 200 sub-carriers, 192 transport data and eight are pilots.
For the pilot carriers, BPSK modulation is always used. For the data carriers, the standard specifies the
modulations BPSK, QPSK, 16 QAM, or 64 QAM (using different amplitudes, the QAM constellations do not overlap), depending on the robustness of the channel.
The transmission initiates using the simplest modulation (BPSK). Through an adaptive process, the channel is evaluated and whenever possible, a higher order modulation is implemented, thereby increasing the channel’s throughput.
The analysis of the constellation performed by the vector analyser reveals all the modulation types present in the frame (the burst is simply the number of symbols that use a particular v modulation and coding).
Frame structure
The IEEE 802.16-2004 standard specifies two channel duplexing modes including timedivision duplex and
frequency-division duplex. In the case of time-division duplexing, the downlink burst is followed by one or more uplink bursts resulting in a frame with a total duration ranging from 2.5 to 20msecs (the standard
supports seven different frame durations).
A short guard interval, the transmit/receive gap is placed between the downlink and uplink bursts. Similarly, following the last uplink burst, there is another guard interval that precedes the subsequent frame, the receive/transmit transition gap.
The durations of both guard intervals are determined by the standard depending on the band occupied on the channel and on the duration of the OFDM symbol. The downlink frame begins with two OFDM symbols (QPSK modulation) that are used for receiver synchronisation and channel estimation. These two symbols constitute the long preamble.
The preamble is followed by the frame control header (FCH). Within the FCH, the downlink frame prefix specifies the type of modulation and the number of symbols associated with subsequent bursts.
The modulation and the coding used by the burst immediately following the FCH is specified, within the DLFP, by the Rate_ID.
Among the information contained in the header, one can find the code assigned to
the basestation, the downlink interval usage code. This refers to the profiles of the bursts following the first in the downlink frame and in the uplink frame.
The data that follow the section dedicated to the FCH can vary between 12 and 108Byte, depending on the modulation type and coding used.
In the first symbol of the downlink preamble, the standard does not provide for the use of all 200 sub-carriers. Rather, a subset of 50 OFDM sub-carriers is used without occupying the central frequency. The frequency response of the downlink preamble, obtained by extracting a segment of the corresponding samples in the time domain, is shown in figure 1.
The downlink preamble is transmitted with 3dByte more power with respect to the data in order to facilitate reception, demodulation, and ultimately, decoding. Similarly, the uplink frame initiates with an OFDM signal that is used by the basestation for synchronisation with the transmitter. This symbol is called the short preamble.
Due to the variable modulation types and flexible bandwidths, it is difficult to accurately quantify the channel throughput possible with this technology which has already evolved to mobile WiMAX (802.16e).
ROBERTO SACCHI is an application engineer, Agilent
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