Fine-grained frequencies for simultaneous communication
Abstract:
Simultaneous intra-group communication is one of the possible ways to handle the scaling issues in large decentralized systems. FDMA is an elegant strategy to carry out intra-group activities in parallel. However, due to increasing congestion in the licence free ISM bands, especially in the urban area, its hard to get enough number of good standard channels for multi-group setting in decentralized systems like IoT/WSN. In this work, we leverage concurrent transmission for efficient simultaneous intra-group one-to-all dissemination. Concurrent transmissions based strategies schedule the packet-transmissions in a special way so that the overlapping packets, instead of colliding with each other, result in constructive- interference or capture-effect. We propose a simple strategy to exploit this feature under concurrent transmission to achieve fruitful simultaneous intra-group communication in overlapping channels. Through extensive evaluation in testbeds for WSN/IoT under 2.4 GHz ISM band we show that, the proposed strategy can efficiently support simultaneous intra-group one-to-all dissemination even with allocation of channels having Center Frequency Distance (CFD) 1 MHz in most of the common group divisions. Experiments with a wide variety of group divisions also reveal that under concurrent transmission a CFD of 3 MHz and above can tolerate strong inter-group interference when the groups are quite mixed with each other which can be quite common specially when the nodes are mobile.
Key idea and results:
Use of synchronous transmission strategy
In this work, we exploit the special abilities under concurrent transmission, e.g., constructive interference and capture-effect to combat the inter-group interference under continuous assignment of fine-grained frequencies for simultaneous intra-group communication. When more than one packets arrive at a certain receiver node at the same time, based on the timing of their arrivals and the difference in their signal strengths, it may result in the following possibilities depending on the transmission.
Under traditional asynchronous transmission (usual):
Collision may happen in an arbitrary way.
Reduces the scope of capture-effect.
Under synchronous transmission:
Collision can be forced to happen with high chance during the preamble part.
Enhances the scope of successful capture effect and hence proper reception.
Inter-Group separation using fine-grained frequencies
Fine-grained frequency assignment
Allocation of orthogonal channels for different groups fully removes the possibility of inter-group interference. When two channels are separated by 1 MHz (i.e., CFD = 1 MHz), the inter-channel interference is the maximum. Two channels are free of any inter-channel interference when the CFD goes above 9 MHz. The purpose of the study in this work is to investigate how much inter-channel interference due to overlapping channels can be tolerated under the proposed concurrent transmission based framework. To carry out a systematic study, we define 5 different strategies collectively denoted as Gi. Under two-group settings, Gi indicates that the defined two groups are separated by channels having CFD i MHz. We explicitly study the performance of G1, G2, G3, G4, and G5. In the case of more than two groups, we first find out which pair of groups are adjacent to each other. A group i is called adjacent to another group j if and only if group i has at least one node that has at least one neighbor belonging to group j. In the offline channel allocation, we ensure that the adjacent groups are allocated with channels that have CFD of i MHz under strategy Gi with the minimum total number of channels.
Results
Comparison of average reliability for various group divisions in three different testbeds for IoT/WSN.
It has been shown that in 2.4 GHz ISM band, concurrent transmission under 802.15.4 can enable a quite fruitful in-parallel intra-group dissemination even when the groups are separated by channels having CFD of 1 MHz for most of the common group divisions. We further demonstrate that under the proposed framework a 3 MHz CFD based channel allocation can be quite successful to support in-parallel intra-group dissemination even when the groups are very interspersed with each other, e.g., when the nodes have the ability to move.