Wimax standards and Security The Wimax

TABLE 7.2
Parameters Used in Disruption Time
Parameter Value

Spectrum (GHz) (for 7 BSs) 2.40–2.46

The distance between two BSs (m) 1000
Bandwidth (MHz) (BW) 10
FFT size (N_FFT) 1024
DL/UL ratio 3:2
OFDMA frame length (ms) 5
Sampling frequency (MHz) 11.42
Subcarrier spacing (kHz) 11.16
Useful symbol time (µs) 89.64
Guard time (µs) 11.2
OFDMA symbol time (µs) 100.84
Number of subchannels 30
Number of OFDMA symbol per frame 49
CW_min (opportunities) 32
CW_max (opportunities) 1024
CW request oppurtunity per frame (OFDMA symbols) 12
Maximum number of CW request retry 10
Ranging opportunity per frame (OFDMA symbols) 12
Maximum number of ranging retry 10
Average time of contention ranging (ms) T_CR 120
Average time of reauthorization (ms) T_RA 175
Rendezvous time (ms) T_R 50
Average time of reregistration (ms) T_RR(2frames) 35
Average time of Internet delay (ms) T_ID 50
Average time of RNG-REQ (ms) T_RNG 25
Average time of frame synchronize (ms) T_SYN 5
MSS«SBS (ms) (1frame*2way) 10
SBS«nBS (ms) (1frame*2way) 10
SBS«TBS (ms) (1frame*2way) 10
MSS←SBS (ms) (1frame) 5



×
The simulation environment is built by one serving BS with 40 MSSs and 6 nBSs concurrently within a 1500 1500 m square. All MSSs are randomly developed around the BS. All MSSs execute the ranging request process by adopting QPSK 1/2 encoding rate.
Figure 7.5 illustrates the average service DT during handover process under a given number of MSSs with a fixed speed of 100 h/km. The DT parameters of the IEEE 802.16e standard we used are shown in Table 7.2. The service DT of L₀ and L₁ are much larger than that of L₂ and PHS, because of the long reauthorization and reregistration process. As shown in the figure, the mini- mum disruption time of PHS reaches 11 ms when M reaches 40. The reason why PHS outperforms L₀, L₁, and L₂ is that PHS considers a BS centralized control mechanism to predict T_HO and to deal with the problem of handover beforehand. As a result, PHS will accommodate more MSSs and thus get less DT as the number of MSSs increases. On the contrary, L₀, L₁, and L₂ only reach their minimum disruption time at 335, 300, and 70 ms due to long process.


600
500

Disruption time (ms)
400
300
200
100
0
40 50 60 70 80 90 100
Number of MSS