La incertidumbre y la predictibilidad

1. The PUs’ Channel-Usage Model

The PUs’ channel-usage model considered in this chapter is similar to that in Chap- ter II. In particular, we consider there are two non-cooperating types of users, namely PUs and SUs. The PUs, for example, TVs, cellular phones, or wireless microphones, are those to which an amount of wireless spectrum is licensed. On the other hand, the SUs are referred to those without pre-assigned wireless spectrum. However, the SUs equipped with the cognitive radios can transmit their own packets by seizing the opportunities that the PUs do not use the licensed wireless spectrum. In this chapter, the wireless spectrum accessible to the SUs is further divided into a number of channels, each with a fixed amount of frequency bandwidth.

Suppose that a spectrum licensed to the PUs consists of M channels, as depicted in Fig. 2 in Chapter II. We assume that for each channel, the channel usage pattern of the PUs follows independent and identically distributed (i.i.d.) ON/OFF random process, as shown in Fig. 3 in Chapter II. An ON-period represents that the channel is occupied by the PUs. An OFF-period represents that the channel is vacant and thus

can be opportunistically used by the SUs. Suppose that the ON- and OFF-periods on each channel are independent. Note that the average ON- and OFF-periods depend on the channel usage pattern of the PUs. In this chapter, we assume that the length of ON- and OFF-periods for i-th licensed channel follows exponential distribution with means equal to αi and βi, respectively. If we denote γi as the probability that

i-th channel is occupied by the PUs, then we have

γi =

αi

αi+ βi

, (3.1)

where 1 ≤ i ≤ M. Note that γi also represents the channel utilization of i-th channel

with respect to PUs.

2. The Protocol Description

Our proposed protocol does not employ a common control channel as the rendezvous where the SUs exchange the control packets for multi-channel resource reservation. Each SU has its own hopping sequence and switches across the channels following the hopping sequence. Each SU hops across all M licensed channels according to its own pseudo-random (PR) hopping sequence. SUs decide on their own PR hopping sequence based on their unique ID (e.g., their MAC address) and share the same hopping sequence generating algorithm. The hopping sequence is fixed for a given SU. As shown in Fig. 14, if a source SU, A, wants to send packets to its intended receiver, B, A follows the B’s hopping sequence to meet with it and exchange the negotiation packets if the channel is not used by the PUs.

Unlike the conventional channel hopping MAC protocol [89–91], at the beginning of each time slot, to guarantee that the SUs do not interfere with the PUs or the ongoing communication pairs of SUs, the SUs need to keep their transmitters quiet for a predefined period, called the quiet period. During the quiet period, if an SU

Ts 000000000000 000000000000 000000000000 000000000000 000000000000 111111111111 111111111111 111111111111 111111111111 111111111111 000000000000 000000000000 000000000000 000000000000 000000000000 000000000000 111111111111 111111111111 111111111111 111111111111 111111111111 111111111111 0000000000000 0000000000000 0000000000000 0000000000000 0000000000000 0000000000000 1111111111111 1111111111111 1111111111111 1111111111111 1111111111111 1111111111111 000000000000 000000000000 000000000000 000000000000 000000000000 000000000000 111111111111 111111111111 111111111111 111111111111 111111111111 111111111111 0000000000000 0000000000000 0000000000000 0000000000000 0000000000000 1111111111111 1111111111111 1111111111111 1111111111111 1111111111111 C t t t Channel 1 Channel 2 Channel 3 Channel 4 A C t B C A C B C AB A −> B A −> B Tq

Fig. 14. Illustrations of our proposed opportunistic MAC protocol. The shadowed rectangles represent the time slots during which the PUs are active. The empty rectangles represent the unused time slots. Let A, B, and C be three SUs. If A wants to send packets to B, A follows the B’s hopping sequence to meet with it and exchange the negotiation packets if the channel is unused by the PUs.

does not detect any signal on its current operating channel, it assumes that no PU is present in the current time slot and it is safe for SUs to use the channel. The channel operations for the SUs can be categorized as three types: advertising, negotiation, and data exchange. Note that in each time slot, all of these operations can be performed by the SUs only if there are no PUs’ signals detected during the quiet period.

The Advertising Operations: Each SU periodically broadcasts its own hopping sequence over an unused channel to let its neighbors know such that the neighbors can follow its hopping sequence to communicate with. When an SU is entering the network, it discovers its neighbors and adds them to its neighbors list table. The SU needs to send its ID so that its neighbors can know its home hopping sequence which is only decided by the SU’s unique ID. When an SU receives any packet, if the sender is a new neighbor, the SU records the sender’s home hopping signature consisting the time stamp and the unique ID of the sender. Whenever a new neighbor is discovered,

the receiver sends an extra packet with its own hopping sequence to accelerate the advertising process. The SUs can also ask their neighbors for a list of their known neighbors.

The Negotiation Operations: Before exchanging any data packets, the sender and receiver of SUs have to hop to the same channel at the same time slot to negotiate for the data transmission. In our proposed protocol, SUs can conduct the negotiations at different channels simultaneously. In other words, multiple pairs of senders and receivers can transmit or negotiate at the same time, which is different from the dedicated control channel based cognitive MAC protocols where only one pair of sender and receiver can conduct negotiation at the same time. In this sense, the proposed cognitive protocol can alleviate negotiation bottleneck.

Under our proposed protocol, each SU keeps one packet queue per destination to avoid head-of-line blocking. At the beginning of each time slot, if the SU has no data to send, it will follow its home hopping sequence and monitor on that channel. If the SU has packets to send, it allows itself to temporarily deviate from its home hopping sequence and transmit to a receiver on another channel. Otherwise, it simply stays on the current channel. The sender deviates from its home hopping sequence at the beginning of the time slot to monitor the receiver’s status. After the sender hops to the receiver’s channel, it needs to wait for the predefined quiet period. If during the quiet period, there are no signals detected, it sends a negotiation message to the receiver with a randomized delay. After receiving the acknowledgment from the receiver, the sender starts transmitting data packets, which means the negotiation is successful.

The Data Exchange: After a pair of SUs successfully negotiate for the channel, they start exchanging data packets. If they cannot finish the data packets in a time slot. They continue exchanging data over the same channel at the next time slot.

If the channel is occupied by PUs, then the pair of SUs wait on the same channel until the channel is vacant again. If the channel is occupied by PUs for consecutive 5 times, the pair of SUs gives up the data exchanging, which means that the data exchange fails.