Integrated Wireless-pon access Network Architectures Milos Milosavljevic

7.2.1 Radio Signal Generation 
The specification data for each generated WiMAX channel distinguishing between upstream 
and downstream transmission is given in Table 7-1. 
Table 7-1: Data sheet IEEE802.16-2005 WiMAX transceivers parameters [5]
 
Downstream 
Upstream 
FFT size 
1024 
Modulation 
64-QAM 
16-QAM 
Coding 
2/3 
3/4 
BW 
10MHz 
RCE 
-50dB 
-25.8dB 
Data rate 
25.2Mbit/s 
18.9Mbit/s 
To outline its basic performance characteristics, of relative importance to the experimental set-
up of this chapter, before being assigned to a subcarrier, the OFDMA symbols are coded and 
modulated. Channel coding is implemented by the application of convolutional codes. The 
encoded data is then passed to 64QAM and 16QAM modulators in downstream and upstream 
respectively. The modulated output in each case is then mapped and assigned to active 
subcarriers called sub-channels, formulating the OFDMA symbols. The latter are in the 
frequency domain and an inverse FFT is employed as a result to acquire their time domain 
equivalent.

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Chapter 7 Experimental Investigation of WiMAX Transmission over Multi-Wavelength PONs 
140 
The number of OFDMA subcarriers (corresponding to the FFT size) was selected according to 
standard deployments and was set to 1024. Consequently, for a 10MHz bandwidth, due to the 
limit of the Aeroflex RF output channels spacing, and 64-QAM modulation, the resulting 
aggregate data rate per sector for the downstream WiMAX link was specified at 25.2 Mbps. In 
view of 16-QAM modulation in upstream, the corresponding figure was defined at 18.9 Mbps. 
Important to stress out at this point, these data rates were calculated in view of guard and pilot 
subcarriers. These do not carry useful information but rather used for synchronisation and 
estimation purposes. A worst case scenario 25% cyclic prefix was also included. A standard 20 
MHz WiMAX bandwidth [4], feasible with readily available VSGs and additional subcarriers 
would have increased the aggregate data rates. The downstream WiMAX channel transmitter 
relative constellation error (RCE) was -50 dB, a figure higher than the minimum required for 
64-QAM modulation [4].
In addition, both TDM and FDM deployment options are available in a single service provider 
PHY. The FDM provision, adopted also by other 4G broadband wireless networks such as LTE 
[6], have a further capability of operating in either full or half-duplex modes.
According to the FDM mode, adopted for implementation in the experimental set-up, there are 
separate uplink and downlink sub-frames, which reside on different frequencies. Each frame 
begins with a preamble followed by a downlink transmission period and uplink transmission 
period. In each frame, the transmit transition gap (TTG) and receive transition gap (RTG) are 
inserted between the downlink and the uplink and at the end of the frame, respectively. This is 
done to allow the BS circuitry to change from transmit to receive mode. 

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