Protocols

In [69] Nabar et al. analyse the ergodic channel capacity and the outage prob- ability of several cooperative protocols for AF and DF relaying considering the single relay channel model given in Section 3.1.3. These protocols realize different degrees of broadcasting and receive collision. Due to the half-duplex constraint, transmission is divided into two timeslots. The degree of broadcast- ing is equal to the number of listeners during the same time-slot and the degree of receive collision is given by the number of terminals transmitting during the same time-slot. The protocols are summarized in Table 3.1. In detail [69]:

• Protocol I In the first time-slot, the source message is received by the relay and the destination. During the second time-slot, the source and the relay transmit to the destination simultaneously. Thus, this protocol implements the maximum degree of broadcasting during time-slot 1 and maximum degree of receive collision during time-slot 2.

• Protocol II In the first time-slot the source message is received by the re- lay and the destination. During the second time-slot only the relay trans- mits to the destination. Thus, this protocol implements the maximum degree of broadcasting during time-slot 1 but not the maximum degree of receive collision.

• Protocol III In the first time-slot, the source message is received only by the relay. During the second time-slot, the source and the relay transmit to the destination simultaneously. Thus, this protocol does not implement the maximum degree of broadcasting but the maximum degree of receive collision during time-slot 2.

Assuming the flat Rayleigh fading channel model, it is shown in [69] that proto- col I outperforms protocol II and III in terms of the ergodic channel capacity9 and outage capacity, considering both AF and DF relaying. Protocol I and III are also called non-orthogonal cooperative protocols since the source and the relay are allowed to transmit simultaneously. On the contrary, protocol II is an orthogonal protocol since either the source or the relay transmit within a timeslot.

Laneman and Wornell proposed in [13, 14] several cooperative protocols of type II for AF and DF relaying considering multiple sources. To this end, the sources share the available bandwidth using orthogonal FDMA (OFDMA). Fig- ure 3.7a illustrates the non-cooperative OFDMA technique, where the three sources transmit simultaneously over block length N, each source in its own frequency band.

In contrast, in Figure 3.7b the block length is partitioned between the sources. In the first block, all sources transmit their own information to one another and to the destination. During the successive blocks, the sources act as relays for each other by repeating the received information, while only one relay is active per frequency band. For this channel model, Laneman et al. investigate three repetition based protocols in [14]:

• Fixed relaying This protocol is akin the type II protocol given in Table 3.1, while the sources transmit according to the transmission scheme given in Figure 3.7b. It is shown in [69, 14] that AF relaying, in comparison to DF relaying, allows to achieve full spatial diversity gain.

• Selection relaying A common assumption is that the receivers have knowledge about their channel coefficients and that the sources are able

9_{In the derivation for the ergodic channel capacity for the protocols I to III, coding was}

assumed to be performed over an infinite number of coherence intervals. Throughout this work, it is assumed that coding is performed over a single coherence interval, but in this case the ergodic channel capacity is zero.

Chapter 3: Channel Models and Cooperative Communication 30 F req u en cy 3 transmits 2 transmits 1 transmits N

(a)... without cooperation

F

req

u

en

cy

3 transmits 1 repeats 3 2 repeats 3 2 transmits 1 repeats 2 3 repeats 2 1 transmits 2 repeats 1 3 repeats 1

N

(b) ... with repetition based cooperation

F req u en cy 3 transmits D(3) relay 2 transmits D(2) relay 1 transmits _{D(1) relay} N

(c) ... with space-time coded cooperation

to sense activity in the frequency bands. Consider that the channel gain of the source 1 to source 2 link falls below a certain threshold. In this case, source 2 rejects to relay for source 1 in the second time-slot. Sensing the frequency bands, source 1 observes silence on its frequency band and re-transmits its information itself.

It is shown in [14] that this protocol enables to achieve full spatial diversity gain applying DF relaying.

• Incremental relaying A disadvantage of the fixed relaying protocol and the selection relaying protocol is the reduction of throughput. But if the channel between a source and the destination is good, relaying might not be necessary. Therefore, Laneman et al. introduce an incremental relaying protocol where the source transmits its message in the first time slot. In case that the destination is not able to decode the direct transmission successfully, it sends a negative-acknowledge signal. Only in this case, the corresponding relay retransmits the message in the next time slot. If the destination is still not able to decode the message successfully, it again sends the repeat request and the next relay retransmits the message. This protocol enables to exploit full spatial diversity gain applying AF relaying and outperforms the selection relaying protocol and the fixed relaying protocol in terms of throughput.

Laneman and Wornell propose in [13] a further orthogonal protocol for the DF relay based on unitary space-time codes investigated in [62]. In pertinent literature, this protocol is known as the Laneman-Wornell orthogonal proto- col. In comparison to selection relaying and incremental relaying, this protocol allows to achieve full spatial diversity gain with higher spectral efficiency.

The Laneman-Wornell orthogonal protocol is illustrated in Figure 3.7c. Let D(i) denote the set of relays that are able to decode the message transmitted from source i in the first time-slot. Furthermore, the columns of the employed STBC matrix are distributed over the relays. These relays re-encode the received message using the STBC and forward their columns to the destination in the frequency band of source i.

One drawback is that before transmission, the columns of the code matrix have to be assigned to the relays. In a group of relays, where the set of par-

Chapter 3: Channel Models and Cooperative Communication 32

ticipating relays is not constant, this might not be possible. In [72] it is shown that assigning the code matrix columns randomly to the relays allows to achieve full diversity gain for an infinite number of relays. But for a finite number of relays, the probability that all relays select the same column is finite and thus, diversity gain is achieved only if the SNR between source and relays is below a certain threshold.