Abstract
This chapter is about the beamforming approach in wireless 5G networks, which involves communication between multiple source-destination pairs. The relays can be multiple-input multiple-output (MIMO) and/or distributed single-input single-output (SISO), and full channel state information of source-relays and relay-destinations are assumed to be available. Our design consists of a two-step amplify-and-forward (AF) protocol. The first step includes signal transmission from the sources to the relays, and the second step contains transmitting a version of the linear precoded signal to the destinations. Beamforming is investigated only in relay nodes to reduce end user’s hardware complexity. Accordingly, the optimization problem is defined to find the relay beamforming coefficients that minimize the total relay transmit power by keeping the signal-to-interference-plus-noise ratio (SINR) of all destinations above a certain threshold value. It is shown that this optimization problem is a non-convex, and can be solved efficiently.
Keywords
- beamforming
- 5G wireless networks
- MIMO
- optimization
1. Introduction
Recently, cooperative communication has become one of the appealing techniques that can be used in 5G wireless relay networks to achieve spatial diversity and multiplexing, which overcomes the channel impairments caused by several fading effects and destructive interference. Though various cooperative communication schemes exist [1, 2], the AF scheme is more attractive due to its simplicity since the relays simply forward the amplitude phase-adjusted version of received signals to destinations. In Ref. [2], a distributed beamforming relay system with a single transmitter-receiver pair, and several relaying nodes have been proposed. The authors assumed that perfect channel state information (CSI) is available at all relay nodes. Although the same scenario is investigated in Ref. [3], the second-order statistics of all channel coefficients are assumed to be available at the relays. Furthermore, the beamforming weights are obtained in order to maximize the signal-to-noise ratio (SNR) at destination subject to holding the relay power above a certain threshold value.
In the past three decades, code-division-multiple-access (CDMA) systems have been extensively investigated as the one of the important candidates for transmitting data over single channels while sharing a fixed bandwidth among a large number of users [4]. The design of receivers to increase the number of supported users, in these systems, has been explored in Ref. [5, 6]. In Ref. [6], joint channel estimation and data detection based on an expectation-maximization (EM) algorithm [7] is proposed. The authors have shown that the proposed receiver achieves a near-optimum performance with modest complexity. Furthermore, the authors in Ref. [5] designed a double stage linear-detection receiver to increase the number of supported users on the system. This design requires complex processing at the receiver’s side instead of using a precoding scheme at the transmitter where more hardware complexity is tolerable. Therefore, the authors in Ref. [8] studied a MIMO CDMA system implementing zero-forcing beamforming (ZFBF) as an efficient precoding technique.
Though various complex multiuser detection techniques that can be used in CDMA systems [9], the unconventional matched filter receiver is chosen at destination nodes due to the intractability of the precoding design when other forms of detectors are used. In this article, we have focused on the optimization of the beamforming weights applied to the outputs of matched filter banks to minimize the total relay transmit power subject to a target SINR of all destinations. Our proposed distributed CDMA-relay network can easily overcome the other multiplexing schemes such as space division-multiple access (SDMA), time division-multiple access (TDMA) or frequency division-multiple access (FDMA). The SDMA schemes [10] in which sources, destinations and relays are distributed in the space, have two disadvantages. First, these schemes should have a significant number of relays in proportion to their users to be able to overcome channel impairments at destinations. Although the SDMA scheme with the limited number of relays cannot compensate the interference power, our CDMA schemes can easily satisfy the network QoS due to their ability to decrease the interference effect at destinations. So, the second disadvantage of SDMA is the inefficient use of hardware communication resources. In the SDMA scheme, if the number of users increases, the network data rate can significantly decrease. Therefore, the number of relays should be considerably increased to be able to satisfy the QoS constraints, which is costly for the network operator.
2. 5G wireless system and equations
Consider a wireless relay network with
3. MIMO relay networks
In this section, a peer-to-peer MIMO-relay network with
Let
where
where
The received signal in MIMO relay has been processed by the beamforming weights, that is,
The
where
The three last terms of Eq. (5) are the desired received signal, interference and noise at the
where
First, using Eq. (3), the total relay transmit power can be calculated as
where
For any conforming matrices
Therefore, Eq. (7) can be rewritten as the following quadratic form:
where
Using Eq. (5), the desired signal power at the
where
Also, using Eq. (5), the received noise power at
where
Finally, the power of the received interference at the
where
In this case, the instantaneous SINR for
Since
In this problem, if all the matrices
This optimization problem is non-convex, because the Rank(
The optimal value of the relaxed problem is a lower bound of the optimal value of SDP problem (Eq. 16).Well-known semi-definite problem solvers such as SeDuMi or CVX can solve the above problem in polynomial time using interior point methods. If the optimal value of Eq. (17), that is,
Then, solving problem Eq. (16) from
In the above algorithm, the acceptable scaling factors are those that satisfy
Consequently, the approximate solution of problem (Eq. 16) is
For examination, we assumed that channel state information is known at a processing center and the beamforming weights are optimized and spreaded to the nodes from this processing Center [17]. In each simulation snapshot, the channel coefficients
Figures 2 and 3 show the minimum MIMO-relay transmit power
In Figures 4 and 5, we examine the network performance by changing the number of MIMO-relay antennas and number of source-destination pairs. As expected, more power saving will be obtained by increasing the number of MIMO antennas and/or decreasing the number of user nodes.
4. MIMO-CDMA relay networks
In the last section, we obtained the optimal beamforming weights for a MIMO relay network. Here, in addition to the multiple antenna technique, CDMA is applied to the network to increase the order of multiuser multiplexing. CDMA systems can share a fixed bandwidth among a large number of users without the need of frequency division or time division between nodes. CDMA introduces a diverse range of trade-off between receiver complexity and system performance.
As shown in Figure 6, a two-step AF protocol is used for this MIMO-relay network. In the first step, each source user broadcasts its precoded signal (i.e.
Let
The vector form of Eq. (20) can be written as:
where
By denoting the cross correlation between
where
where
The output of the matched filter in each relay has been processed by the beamforming weights
Another filter bank is applied to the output of each MIMO antenna, which generates
The
where
where
The object of the network beamforming is to minimize the total relay transmit power subject to maintaining every destination SINR above a pre-defined threshold value
First, using Eq. (27), the total MIMO-relay transmit power can be obtained as:
where
For simplicity, Q can be represented by the following quadratic formml:
The kernel of the above form can be expressed as a Kronecker products as follows:
where
where
Also, the instantaneous desired signal power at the
By defining
where
Also, the received noise power at
where
Finally, the power of the received interference at the
The instantaneous SINR for
By defining
We solve this optimization problem in a same way as the previous section. The first simulation scenario was carried out to consider the total MIMO-relay transmit power versus destination SINR threshold value, for different values of users’ correlation factors. The averaged results are shown in Figure 7. The network consists of two source-destination pairs and four MIMO-relay antennas. Figure 7 shows that the total MIMO-relay transmit power in all cases increases by raising
When
Figures 8 and 9 display the minimum relay transmit power versus
5. Distributed relay networks
In this section, we considered a distributed relays network, instead of MIMO-relay. The optimization problem is defined to find the relay beamforming coefficients that minimize the total relay transmit power by keeping the SINR of all destinations above a certain threshold value.
Consider a wireless relay network with
where
where the following definitions have been used:
The output of the matched filter in each relay has been processed by the beamforming weights
Another filter bank is applied to the output of each relay, which generates
The vector forms of Eq. (47) can be written as
The
where
where:
The three last terms of Eq. (50) are the desired received signal, interference and noise at the
The object of the network beamforming is to minimize the total relay transmit power subject to maintaining every destination SINR above a pre-defined threshold value
where
Note that using Eqs. (48) and (44),
Using Eq. (50), the desired signal power at the
where
where
The relay noises are assumed to be zero-mean and independent with the equal noise power. So, we have
Finally, the power of the received interference at the
where
In this case, the instantaneous SINR for
Since
In this problem, if all the matrices
The problem is non-convex, because the Rank(
This optimization problem has been solved in a same way as the previous sections. Figure 11 shows the total relay transmit power versus destination SINR threshold value, for different values of users’ correlation factors. The network consists of two source-destination pairs and four relays. Figure 11 shows that the total relay transmit power in all cases increases by raising
Figure 12 displays the minimum relay transmit power versus
Figure 13 shows the minimum relay transmit power versus the network data rate (
Also, it can be seen from Figure 13 that the minimum relay transmitted power increases with the increase of
6. Computational complexity
Since the CDMA relay systems have a heavy computational complexity, the aim of this section is to analyze the computational form of related algorithms used in practice [20]. Here, the computational complexity of a standard SDP is introduced and extended to our case. The standard SDP problem with equality constraint is given as:
where
where
It is shown in Ref. [22] that small update interior point methods (IPMs) are restricted to unacceptably slow progress, while large-update IPMs are more efficient for faster. Also, large update IPMs perform much more efficiently in practice, however, they often have somewhat worse complexity bounds. The complexity order of solving standard SDP problem is polynomial time.
For evaluating the complexity of our SDP problem with inequality constraints, we have to calculate the dimension parameter
Similarly, we can obtain the above conclusion for
Next, a new variable
As a result, the following standard form will be attained.
where
As a result of the above representation form,
Also, we can use the same procedure to calculate
Therefore, the complexity for problems (16), (42) and (63) for MIMO, MIMO-CDMA, and distributed-relay networks are as follows:
while a SDMA relay network has the complexity order of
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