This model shows part of the downlink physical layer of a wireless communication system according to the cdma2000 specification. In particular, the example focuses on Radio Configuration 3 of a forward fundamental channel of a 1x (that is, spreading rate 1) forward link between a base station and a mobile station.
cdma2000 is a terrestrial radio interface for the third generation of wireless communications developed within the framework of the International Mobile Telecommunications (IMT)-2000 standard, as defined by the International Telecommunication Union (ITU). The specifications of the cdma2000 system are being developed by the Third Generation Partnership Project 2 (3GPP2). For more information, see http://www.3gpp2.org.
The cdma2000 air interface is a direct spread technology. This means that it spreads encoded user data at a relatively low rate over a much wider bandwidth (1.23 MHz for the 1x case), using a sequence of pseudorandom units called chips at a much higher rate (1.2288 Mcps). By assigning a unique code to each user, the receiver, which has knowledge of the code of the intended user, can successfully separate the desired signal from the received waveform.
Note: cdma2000 is a registered certification mark of the Telecommunications Industry Association (TIA). cdma2000 is not used herein to indicate TIA certification of this Proof of Concept.
The key components of the physical layer are the transmitting base station, channel, and mobile receiver. The transmitting base station in turn includes the encoder and transmitter, while the mobile receiver includes the decoder and receiver subsystem.
Parameters in the Model
A configuration block labeled "Model Parameters" enables you to set parameters such as data rate, insertion rate of the power control subchannel, spreading code index, QOF (quasiorthogonal function) index, and the channel model. For example, you can switch to an AWGN channel by setting the Channel Model parameter in the dialog box to AWGN Channel.
Encoder. The encoder subsystem performs these tasks:
Insertion of frame quality indicator
Appending of tail bits before coding
Transmitter. The transmitter spreads and modulates the signal. More specifically, it performs these tasks:
Long code scrambling
Power control insertion
Signal point mapping
Spreading by Walsh code and a QOF (quasiorthogonal function) mask
Walsh code rotation
Quadrature scrambling by a PN (pseudonoise) sequence
Transmit filtering by an oversampled square root raised cosine filter
Channel. The default channel model includes the effects of both multipath Rayleigh fading and additive white Gaussian noise. You can vary the channel using parameters in the Model Parameters block in the corner of the model.
Receiver. The most important part of the receiver subsystem is the rake receiver and channel estimator. To look at the details of the rake receiver, follow these steps:
1. Double-click the icon labeled Receiver at the top level of the model to open the receiver subsystem.
2. Select the Rake Receiver block in the receiver subsystem.
3. Choose Look Under Mask from the window's Edit menu.
To look at the details of the channel estimator, which is inside the rake receiver, follow the procedure above and then these additional steps:
1. Select any of the Rake Finger blocks in the rake receiver subsystem.
2. Choose Look Under Mask from the window's Edit menu.
3. Select the Channel Estimation block in the rake finger subsystem.
4. Choose Look Under Mask from the window's Edit menu.
Besides the rake receiver, other operations in the receiver are straightforward inverses of the operations in the transmitter subsystem.
Decoder. The decoder subsystem inverts the operations performed in the encoder subsystem.
The following sections of the model calculate various error rates:
The BER Results: Raw BER section computes and shows the bit error rate of the data between the transmitter input and the receiver output. This computation excludes the effects of interleaving, coding, and power control bit insertion.
The BER Results: Channel Bits section computes and shows the bit error rate between the transmitting base station and the mobile receiver.
To view data graphically, open the scopes by double-clicking the Open Scopes icon while the simulation is running. The scopes show the following information:
The From Channel scatter plot shows the output of the channel.
The After Derotation scatter plot shows the data after the receiver subsystem has compensated for the phase rotation caused by the channel.
The After Rake scatter plot shows the output of the rake receiver after the rake receiver has compensated for the attenuation caused by the channel. The output from the rake receiver is later sent to the deinterleaver.
The After Pulse Shaping spectrum plot shows the power spectrum of the signal before the channel.
The From Channel spectrum plot shows the power spectrum of the signal after the channel.
In addition to illustrating a cdma2000 application, this example also illustrates several techniques for modeling in Simulink. In particular, this example shows how you can
1. Use the Communications System Toolbox™ extensively to implement wireless systems.
2. Represent the architecture of the design using subsystems.
3. Reuse and share custom-built blocks using a library. To view the library for this example, double-click the cdma2000 Library icon in the top right corner of model.
4. Control the parameters of the simulation using a configuration dialog box.
5. Incorporate customized C code using the S-Function Builder. In this model, several identical S-Function Builder blocks use custom C code to implement an extended PN sequence. To view the code, follow these steps:
Double-click the icon labeled Transmitter at the top level of the model to open the transmitter subsystem.
Select the Spreading Non-TD Mode block in the transmitter subsystem.
Choose Look Under Mask from the window's Edit menu.
Double-click one of the cdma2000_expandPNSeq blocks to open the S-Function Builder.
Click the Outputs tab to view the C code.
These other examples are related to wireless communications:
 Tiedemann, Edward G., Jr., "cdma2000 1X: New Capabilities for CDMA Networks," IEEE Vehicular Technology Society News, Vol. 48, No. 4, November 2001, pp. 4-12.
 TIA/EIA/IS-2000.2-A, Physical Layer Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, Arlington, VA, March 2000.