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The system model in Java Build QR Code JIS X 0510 in Java The system model

The system model using barcode encoder for java control to generate, create qr code image in java applications. rfid We consider Java qrcode dynamic spectrum access networks where multiple secondary users are allowed to access the temporarily unused licensed spectrum bands on an opportunistic basis, without con icting or interfering with the primary spectrum holders usage. Such scenarios can be envisioned in many applications. Considering the fact [114] that heavy spectrum utilization often takes place in unlicensed bands while licensed bands often experience low (e.

g., TV bands) or medium (e.g.

, some cellular bands) utilization, IEEE 802.22 [196] considers reusing the fallow TV spectrum without causing any harmful interference to incumbents (e.g.

, the TV receivers). Moreover, with regard to more ef cient utilization of some cellular bands, [317] considers sharing the spectrum. 3.2 The system model between a ce qr-codes for Java llular communication system and wireless local-area network (WLAN) systems. In rural areas where there is little demand on the cellular communication system, the WLAN users can ef ciently increase their data rates by sharing the spectrum. In order to take advantage of the temporarily unused spectrum holes in the licensed band, without loss of generality we consider a snapshot of the above spectrum access networks shown in Figure 3.

1, where two secondary users and one primary user coexist, and the secondary users opportunistically utilize the spectrum holes in the licensed band. Note that the system diagram shown here serves only as an example model with which to gain more insight and the scenario with multiple secondary users will be studied in detail in the following section. The primary user, denoted by P, has a license to operate in the spectrum band.

The offered traf c for primary user P is modeled with two random processes. The service request is modeled as a Poisson process with rate P s 1 . The service duration (holding time) is negative-exponentially distributed with mean time 1/ P s, so the departure of user P s traf c is another Poisson process with rate P s 1 .

The secondary users are denoted by A and B, and set S is de ned as S = {A, B}. For each secondary user , where S, its service session is similarly characterized by two independent Poisson processes, with arrival rate s 1 and departure rate s 1 . They contend to access the spectrum when primary user P is not using the spectrum band.

Since the primary user has a license to operate in the spectrum band, its access should not be affected by the operation of any other secondary user, and priority to access the spectrum is given to primary user P. We assume that the secondary users equipped with cognitive radios are capable of detecting the primary user s activities, i.e.

, the appearance of the primary user in the spectrum band and its departure from the spectrum.. Base Station User B Primary user Secondary users User A User P Throughput Time The system m odel (upper: system diagram; lower: throughput vs. time)..

Markov models for dynamic spectrum allocation Furthermore, the secondary users access is assumed to be controlled by a secondary management point so that they can distinguish whether the spectrum is occupied by the primary user or secondary users. Therefore, when primary user P appears, the secondary users should adjust their transmission parameters, for instance reduce the transmit power or vacate the channels and try to transfer their communications to other available bands. The interference-temperature model proposed by the FCC in [115] allows secondary users to transmit in licensed bands with carefully adjusted power, provided that secondary users transmission does not raise the interference temperature for that frequency band over the interference-temperature limit.

Although it can provide better service continuity for the secondary users to remain operating in the band with reduced power, the capacity they can achieve is very low [65] [66]. Therefore, in this chapter, we assume that, when primary user P appears, any secondary user should vacate the channel and the traf c currently being served is cut off. While primary user P is being served, any entry of the secondary user s traf c into the spectrum is denied until service of primary user P has nished.

At the bottom of Figure 3.1, we show an example of system throughput versus time for dynamic spectrum access. First, user A accesses the spectrum band, followed by user B.

During service of B, user A accesses the band again and shares the spectrum band with user B, which may result in less throughput to both user A and user B due to their mutual interference. After service of user A has nished for a while, primary user P accesses the band, and user B s service is interrupted. After user P has vacated the band, service of user B continues until its service duration ends.

Afterwards, user A accesses the band, and its service is ceased when primary user P appears and resumed when service of P has nished in the same way as for user B. For any secondary user that operates in the spectrum band alone, its maximal data rate [79] can be represented by r1 = W log2 1 +.
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