VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
Traditional MIMO schemes are designed mainly for the scenario of contiguous spectrum ranges.
We propose a scheme called VSMC MIMO (Variable numbers of Streams on Multiple Channels of MIMO system), which enables MIMO nodes to transmit variable numbers of streams in multiple discontinuous spectrum ranges. This scheme can largely improve the spectrum utilization and meanwhile maintain the same spatial multiplexing and diversity gains as traditional MIMO schemes.
A simple example of VSMC MIMO is illustrated in Figure 1. There are one transmitter S and three receivers R1, R2, and R3. The numbers of antennas of them are four, one, two and one. In cognitive radio network, the spectrum are occupied by primary users (PUs), the available channels at four nodes are different, as shown in Fig. 1. Traditionally, S can transmit four streams concurrently on channel one to R1, R2 and R3. In VSMC MIMO scheme, S can transmit four streams to R1, R2 and R3 on channel one, three streams to R1 and R2 on channel two and one stream to R1 on channel three concurrently, thus making full use of spectrum resources.
Fig. 1. VSMC scheme: variable numbers of streams on multiple channels
The arrow lines indicate the numbers of data streams between transmitters and receivers, and do not mean the actual signals at the antennas. All the figures adopt this approach for simpler illustration.
We adopt this scheme to implement cooperative MIMO relays in cognitive radio networks. We build a testbed by the Universal Software Radio Peripherals (USRPs) to evaluate the performances of the proposed scheme in practical networks. We use the NI USRP 2920, which is designed to operate in the 2.4 GHz or 5 GHz frequency range. We use MIMO cables to connect two USRPs as a pair to ensure that they share the same operating frequency. An external clock provides a 10 MHz clock and Pulse Per Second (PPS) signal (0-5 V, 1 Hz square-wave) to all the pairs to synchronize all the nodes of the testbed. We use a computer equipped with dual-core 3.20 GHz processors and 4 GB of memory to control USRPs. We implement the system setup in LabVIEW on the PCs.
Experiments of Two End Users
First, we design two setups of two end users in a cognitive radio network. The topology of the first setup is shown in Fig. 2(a). The numbers on the dashed lines indicate the numbers of common available channels between two nodes. The traffic demands for D and R are about 25 Kbps and 10 Kbps respectively. Each data stream can support about 10 Kbps data rate. In cooperative MIMO relay, R can receive four streams on the two available channels during the first time slot and retransmit packets to D in the next time slot, which can improve the throughput of D and the system. The MIMO relay transmission process is described in Fig. 2(b). The solid arrows denote the numbers of transmission channels without MIMO relay and the dotted arrows denote the added working channels in MIMO relay scheme. The gain of the throughput of the system can achieve 50% in theory.
In the next setup shown in Fig. 2(c), the numbers of common available channels between AP and R, R and D have changed. R receives five streams during the first time slot, then forwards three streams to D in the next time slot. Node D receives message from AP during all the transmission time. The MIMO relay transmission is shown in Fig. 2(d). The gain of the throughput of the system can achieve 75% in theory because this one employs more spectrum resources.
The experimental results of two secondary end users are shown in Fig. 3. In the first setup, the average throughput of the system increases 34.74%, compared to the traditional approach without MIMO relays. In the second setup, the average gain of the total throughput of the system achieves 55.02%.
Fig. 2. Experiment of two end users
Fig. 3. Total gain of two end users
Experiment of Three End Users
The topology of three end users is shown in Fig. 4(a). In this case, the AP and receiver R have two antennas as in the previous experiment. The node D1 and D2 have one antenna respectively. The traffic demands for D1, D2 and R are 15 Kbps, 15 Kbps and 0 Kbps respectively. According to our algorithm, node R receives packets of D1 and D2, and then forwards them to D1 and D2 as shown in Fig. 4(b). The theoretical gain of the system is 50%. Our experiment result of three secondary end users is shown in Fig. 5. The total gain of the system is increased by 34.11%.
Fig. 4. Experiment of three end users
Fig. 5. Total gain of three end users
Experiments result shows that VSMC MIMO can efficiently utilize the discontiguous spectrum and significantly improve the network throughput.