Transmultiplexers

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3.4.3 Transmultiplexers

An application of multirate digital signal processing in communications is in the channelization

of data streams. The goal of these techniques is to convert a TDM signal, seen in

Figure 3.53, into a FDM signal, seen in Figure 3.54; conversely, an FDM signal can be

converted in a TDM signal.

Figure 3.54: FDM Example.

 

TDM to FDM Conversion

Figure 3.55 shows a block diagram of a system that performs a TDM to FDM conversion.

Note the similarity in system layout between that seen in Figure 3.55 and that seen in

Figure 3.48, the basic form of a DFT filter bank synthesizer. The underlying architecture is

remarkably similar even though a specific type of interpolator structure and modulator can

be used. The TDM to FDM conversion converts a single data stream into a channelized

version of the original signal. The IDFT process collects several narrowband signals and

generates a single wideband signal that contains all the information received.

[+] Enlarge Image

FDM to TDM Conversion

Figure 3.56 shows a block diagram of an FDMto TDMconversion system. The similarities

between this system layout and the basic form of a DFT filter bank analyzer as shown in

Figure 3.47 can be easily seen. Figure 3.47 may be considered as a specific case of the

more generalized system structure seen previously.

                             Figure 3.56: FDM to TDM.

Example: Watkins-Johnson’s Cellular Basestation Architecture

Wideband transceivers employing software radios can significantly reduce the cost and

complexity of basestations. Currently most of the deployed basestations utilize narrowband

transceiver technology in which each transceiver individually processes a separate

RF channel. In this implementation, the complexity of the system increases linearly with

the number of channels resulting in a fairly large and costly realization of the basestation.

On the contrary, a wideband transceiver processes the entire frequency band of interest

and uses low-cost digital signal processing instead of costly analog components to perform

individual transceiver functions on the signal at various stages.

 

A generic wideband receiver structure used in a basestation developed by Watkins-

Johnson, Inc.,¹⁰ is shown in Figure 3.57 [42]. In this implementation, the entire cellular

band of interest (e.g., North American cellular band A and B with extensions) is first downconverted

to baseband by a single RF tuner. The baseband signal with a bandwidth of approximately

14 MHz is then digitized by a single ADC, which is sampling at 30.72 MHz,

slightly above the Nyquist sampling rate. The output (real) of the ADC, which contains

all the channels in the cellular band, is digitally downconverted to individual I&Q complex

baseband channel signals by an oscillator tuned at the individual channel carriers (complex

modulation). The sampling rate of these individual complex signals is still that of the

ADC, i.e., 30.72 MHz. A decimation operation (lowpass filtering and downsampling) by a

factor of 384 is performed on these high-rate complex channel signals resulting in signals

with a sample rate of 80 kHz. The decimated channel signals are now ready for baseband

processing (demodulation). The baseband processing block requires an input sampling rate

that is an integer multiple of the baud (symbol) rate to facilitate symbol synchronization.

Usually the decimated signals’ sample rate is not an exact integer multiple of the baud rate.

So a further sample rate conversion is performed on the decimated signals before baseband

processing.

The part of the receiver that performs extraction of the individual radio channels from

the output of the ADC is called the channelizer. Reviewing Figure 3.57, it is clearly seen

that the structure of the channelizer is similar to the basic DFT filter bank analyzer explained

in Section 3.4.2. Alternately, according to the alternate implementation of the

DFT filter bank analyzer as in Section 3.4.2, the channelizer can be considered a bank of

complex-valued digital BPFs followed by mixers, downsamplers, and sample rate converters.

Figure 3.58 shows this representation of the channelizer where each complex BPF

H_k(ω) has a center frequency of , which corresponds to a

particular RF channel.

Theoretically, a filter bank channelizer can extract any channel in the band (−F_s/2, F_s /2)

where F_s is the sampling rate of the channelizer input (output of ADC). This implies that

the complexity of a filter bank channelizer remains constant, independent of the number

of channels. The impulse responses of the bandpass filters are defined by h_k(n) =

 where h₀(n) is a real causal LPF. It follows then that the frequency response

of the BPF H_k(ω) can be expressed as the modulated version of H₀(ω), i.e.,

Relevant NI products

Customers interested in this topic were also interested in the following NI products:

• RF and Communication Hardware and Software
  
• Other Modular Instruments (digital multimeters, digitizers, switching, etc...)
• LabVIEW Graphical Programming Environment

For the complete list of tutorials, return to the NI RF and Communications Fundamentals Main page.

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