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.
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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