Wimax standards and Security The Wimax

FIGURE 7.2
Maximum transmission distance versus frequency domains from 2 to 66 GHz in OFDM with different modulation schemes.
Solving Equation 7.12 for maximum transmission distance d denoted as d_max, we obtain

c
d_max = d₀ × 10 exp P_t + G_t + G_r − 20 log ^4πd0f
— X_σ − C_f − C_H − L − SNR_min − N₀ ,10ρ (7.13)
Figure 7.2, derived from Equation 7.13, shows the relation of the frequency and the distance between two isotropic antennas with different modulation schemes when the modulation is 16-QAM and 64-QAM and the required SNR_min is 18.2 dB and 22.4 dB, respectively.

4. 
   Mobility Prediction
   

In this section, we will discuss the mobility of the MSS in detail. To prevent the out-of-service effect of MSSs due to mobility, we investigate a location prediction scheme to add to the PHS for channel migration. The IEEE 802.16e



FIGURE 7.3
An illustration of mobility.

standard [16] recommends that the BS has to broadcast a REP-REQ mes- sage to all MSSs for channel measurements within 10 s to check whether the MSS is still in the service set. Therefore, the BS can get the SNR value by the replied REP-RSP message from each MSS to estimate the distance periodically.

Thus, as shown in Figure 7.3, the movement distance between time t_a and
t_b of MSS_i denoted as ∆d_i(∆t) can be calculated by using cosine theorem as

i,t_a
∆d_i(∆t) = ,d²



+ d
2
i,t_b
— 2d_i,ta d_i,tb cos θ_tb (7.14)

= = −
where θ_tb can be estimated by using smart antenna systems [17,21] that employ antenna arrays coupled with adaptive signal-processing techniques at the BS. From Equation 7.14, the average velocity v_i of the MSS_i is given by v_i ∆d_i(∆t)/∆t ∆d_i(∆t)/(t_b t_a).

= +

i,t_b

(7.15)
To predict the maximum distance between the MSS_i and the BS in time t_c denoted as t_c^r , where t_c^r t_b ∆t, we have to obtain the φ_ta . According to the cosine theorem, φ_ta is obtained by

i,t_a

φ_t

= cos⁻
1 _d2

+ [∆d_i(∆t)]² − d²

− = =
We simply suppose that each MSS moves forward directly. Then the mov- ing distance can be estimated as ∆d^r(t_c t_b) ∆d(∆t) v_i∆t. Therefore, the estimated distance at time t₃^r will be
d_i,t^r = ,d² + [∆d_i(∆t) + v_i∆t]²


_c i,t_a
— ^,2d_i,ta [∆d_i(∆t) + v_i∆t] cos φ_ta (7.16)

Substituting Equation 7.14 in Equation 7.16 we have

c

i, _b

i,t_a

i,t_b
d_i,t^r = d²_t + 2v_i∆t,d² + d² − 2d_i,ta d_i,tb cos θ_tb

i

i,t_a

i,t_a

i,t_b

t_b

+ d

−
2v ∆td (d − d cos θ )



1/2

2
i,t_a
2
i,t_b
— 2d_i,ta d_i,tb cos θ_tb )^1/2

Once the d_i,tc^r ≥ wj, the BS will notice the MSS_i to migrate to a new chan- nel in Ak(k = [d_i,tc^r /w]) with the message (P_t^r , c_n^r ). For example, the MSS might
exceed the boundary of A_j or d_i,tc^r ≤ w(j − 1). Therefore, by using the pre-
diction to prevent the out-of-service effect, the performance of the BWA system can be maintained well. Besides, the overhead of prediction will not be heavy since we only use the routine procedure of channel measurement, which is specified in the IEEE 802.16 standard, to get the information for estimation.

4. 
   The Predicted Handover Scheme
   

Whenever an MSS in roaming between BSs, only two BSs need to be deal- ing with the handover. Consequently, the MSS should be informed for a crucial message from the only TBS so it can perform a fast handover with the TBS. Based on the above concept, we assume SBS will be allocated for one available channel to MSS_i in area A_SBS. A_SBS10 is the microcell from one of the fragment of a six-piecewise divided macrocell, forming h con- centric hexagonal cells with an equal width w. To prevent the out-of-VoIP service effect of MSS_i performing handover, we investigate a PHS devel- oped on a BS-centralized control mechanism to deal with the problem of handover. The SBS controls the location, distance, and direction of the MSS. According to these parameters, the BS will periodically compute the tim- ing of handover (T_HO) which is independent of the current moving speed of the MSS. The SBS will always periodically change the T_HO on receiv- ing a report response (REP-RSP) message from each MSS. According to the direction of MSS, the SBS will easily select the only TBS. Therefore, SBS will actively coordinate with TBS for the handover of MSS over the backbone.
When the SBS receives all handover-related information of the TBS, it may simultaneously convey to MSS. However, the MSS is required to only transmit the CDMA ranging code at the TBS, as a result, the MSS does not have to wait for the RNG-RSP message from TBS. By using the PHS, the SBS will handle all handover processes of the MSS and allows the MSS to easily use its service and also share a large loading amount of MSS. In the sequence diagram of PHS, steps (f) through (i) are defined in the IEEE 802.16e standard