6.3.1 System Model
Our aim is to provide spatial diversity through cooperation for an uncoded system without compromising the rate and spectral efficiency due to the half-duplex constraint, compared with direct transmission with no cooperation and without the need to allocate multiple relays per user as in [111], or complex iterative detection at the receivers as in [108][109][110]We achieve this goal using channel-aware adaptive cooperative system with limited one bit feedback. Our scheme restricts the relaying to the user experiencing the stronger channel to the BS. Furthermore, users switch to higher modulation orders during the cooperative mode to compensate for the half-duplex constraint and maintain the same spectral efficiency as direct non-cooperative mode.
We assume slow flat fading and that channel remains constant during a specified time period (frame). The duration of each frame is fixed and corresponds with channelβs coherent time. During a frame, each user will transmit a total of B bits regardless of whether cooperation takes place or not. Upholding the half duplex constraint, we will divide each frame into a number of time slots whereby only a single user is always to transmit at one time.
At the beginning of each frame, users will consecutively broadcast their training sequences to both the BS and their partners for channel estimation which will be used throughout the duration of the frame for both coherent detection of usersβ data at the relay and the destination as well as for usersβ scheduling, relay selection, and determining transmission mode (i.e. cooperative or direct transmission). It is worth noting that by rearranging the position of the training sequences in that way, we can employ them to facilitate scheduling and adaptive cooperation without adding any overhead compared with direct transmission.
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The BS uses these sequences to estimate the channels between it and the users, while the users will estimate inter-user channel between them. The BS will use its knowledge of usersβ channels to send a single bit feedback indicating which user is allowed to transmit at the first time slot. The BS always chooses the user with the weaker channel to the BS to transmit first to insure that the limited time and power available for cooperation is used efficiently. On the other hand, users utilise CSI about their inter-user channel to determine if they will cooperate or not based on a predefined threshold for cooperation. Since the inter-user channel is reciprocal, users can make this decision locally without the need to inform their partner.
Non-cooperative mode: In the non-cooperative transmission mode shown in figure 6.1.a,
users divide the reminder of the frame into two equal time slot and consecutively transmit their data encoded in a basic modulation set 2π1-QAM and using the same average power
(2P). To preserve power, the user experiencing the strong channel to the BS is turned off during the first time slot and no listening will take place.
Cooperative mode: In the cooperative mode shown in figure 6.1.b, users will transmit using
a modulation set 2π2-QAM, π
2 = 1.5π1. The reminder of the frame will be divided into
three equal time slots. In the first time slot the user with the weaker channel to the BS will transmit first with an average power of 1.5P, in the second time slot the user with the stronger channel to the BS will relay its partner data with an average power 1.5P, while it will use the third time slot to transmit its own data with an average power of 3P. The BS will combine the signals from the first two time slots using maximum ratio combining to detect the data from the weaker user.
158 TS1 TS2 F Uw Tx TSI Uw Tx F TS2 Us relay Uw Us Tx Us Tx Time Powe r Powe r Time
Figure 6.a non-cooperative mode
Figure 6.b cooperative mode
Figure 6- 1 Frame structure for direct and cooperative modes
Power allocation: It is worth noting that the average power per frame is the same for both
non-cooperative and cooperative modes. However, in the cooperative case, three quarters of the available power will be emitted from the user experiencing the stronger channel to the BS. In both modes, power is equally distributed between the two transmitted packets of both users, hence the data packet of the strong user is assigned double the power of that of the weak user because it is only transmitted once while that of the weak user is also relayed by the strong user.
Adaptive modulation: In the cooperative mode, users transmit with higher-order
modulation than that used for direct transmission. The new modulation level is chosen to increase the spectral efficiency to compensate for the extra time slot required to relay the weak userβs data by the user with the stronger channel. This will ensure that the total number of bits per frame remain unchanged compared with direct transmission. Even though increasing the modulation order leads to a reduction in the constellation minimum distance for the same energy per bit. This new higher modulation is accompanied by full diversity order which offsets this degradation leading to a net improvement in BER performance over direct transmission with lower order modulation. This is validated
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through MATALB simulation (Table 6.1) by comparing the SNR required to achieve a fixed BER between direct transmission where regular 2π-QAM is used and a cooperative
system using our proposed protocol where 21.5π-QAM is used in the cooperative mode. It
was shown from these results than the cooperative gain is reduced when high order modulation is used which usually corresponds to users close to the centre of the cell with good overall channels. However even in this scenario; a significant improvement in BER performance can be achieved without compromising bandwidth efficiency.
Table 6- 1: Net diversity gain (DG) at fixed BER for adaptive modulation cooperation where (D) refers to direct transmission and (C) refers to cooperative case. DG is defined as the reduction in SNR required for achieving the same BER in cooperative mode with higher order modulation as compared with that of direct transmission with lower MQAM modulation.
BER/DG 4-QAM(D) 8-QAM (C) 16- QAM (D) 64- QAM (C) 64- QAM (D) 512-QAM (C) 256- QAM (D) 4096-QAM(C) πππ- QAM (D) πππ- QAM (C) ππβπ 15 dB 13 dB 9 dB 7 dB 2.7 dB ππβπ 20 dB 18 dB 14 dB 12 dB 8 dB ππβπ 25 dB 23 dB 19 dB 17 dB 13 dB 6.3.2 Numerical example
We present a numerical example to illustrate the potential of the new scheme. For our simulations, the channels are Rayleigh fading with unit variance. We consider the case where two users employing QPSK modulation with an average transmit power of unity transmit in (a) direct non-cooperative way, (b) classical decode and forward cooperative transmission employing QPSK modulation, (c) adaptive decode and forward cooperative transmission employing QPSK modulation, (d) our proposed scheme employing QPSK for direct transmission and 8-QAM for cooperative mode. In all cooperative cases, we consider the inter-user channel between the two users (a) to have the mean channel gain equal to that of the channels between users and the BS (b) 10 dB higher.
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Figure 6-2 shows the BER performance of various cooperative schemes, it can be seen that while preserving the spectral efficiency of uncoded direct transmission, COOP-AM still manages to deliver a significant improvement in BER performance even when the inter-user channel has the same average channel gain as users due to its inherent adaptive nature. Furthermore the loss in performance due to the use of higher level modulation in the cooperative mode compared with adaptive decode and forward is relatively small accounting to 0.5 dB considering that COOP-AM have double the spectral efficiency of conventional adaptive decode and forward.
Figure 6- 2 BER Performance of cooperation with adaptive Modulation for two QPSK users