In the IEEE 802.15.6 Wireless Body Area Networks (WBAN) standard , an optimization for low-power in-body/on-body nodes is aimed to serve a variety of medical and non-medical applications .
The coexistence of many BANs in the near vicinity of each other (elevator for example) can lead to interference between these BANs because of the large number of sensors each piconet can have and unpredictable movement of these sensors. In addition, no proper global coordination scheme exists as there is no natural choice of coordinator between piconets . The previous factors cause a considerable degradation in the performance for each interfering piconet in the near vicinity. Generally, co-channel interference between the different piconets in a WBAN, can be mitigated by using multi-access schemes like the direct sequence ultra wideband (DS-UWB) scheme.
Since these nodes are employed for different applications, it imposes the occurrence of different traffic patterns and/or quality of services (QoSs). This leads to the coexistence of a mixture of these nodes with different requirements in the same WBAN . In other words, the network consists of multi nodes with various priority in this WBAN systems . The IEEE 802.15.6 standard introduced a priority-based scheme in order to fulfill the expected differences in QoS in order to grantee fairness to all nodes.
The new standard defines three physical (PHY) protocols, namely, NarrowBand (NB), Ultra WideBand (UWB), and Human Body Communications (HBC). As Medium Access Control (MAC) protocols, the standard defines also three MAC protocols, namely, scheduled access and scheduled-polling access, improvised access and unscheduled access, and random access which applied either via CSMA/CA or slotted Aloha. We assume in this paper that a system uses UWB over slotted Aloha MAC.
In this paper and for all scenarios, we assume that a coexistence between nodes, belonging to different orientations, try to contend to the same time slot in the contention access period (CAP). In the conventional systems that employ slotted Aloha  as its CAP scheme, if two or more packets overlapped, even partially, a collision is considered to have occurred and a retransmission request for all the packets will be issued.
In a previous paper , we proposed to employ spreading slotted Aloha scheme in the new IEEE 802.15.6 standard as a configurable system applied to multiple priority services. Spreading slotted Aloha has been used previously [7, 8] for satellite and other applications but not for WBAN. As another contention access-based MAC protocol, CSMA/CA for IEEE 802.15.6 has been analyzed in different literatures [9, 10]. During these schemes, the throughput, delay and energy efficiency are the major metrics to evaluate the performance.
The novelty of spreading slotted Aloha is the combination between spreading technique and the slotted Aloha MAC protocol proposed in the standard in order to allow multiple packets to be transmitted and overlapped in the same time slot, without being considered as a collision. The spreading slotted Aloha scheme relays on the characteristics of the spreading techniques to extract each user’s packets correctly.
In the previous work, we show the improvement in the probability of correctly retrieving the transmitted packets (throughput) under the proposed spreading slotted Aloha applied for IEEE 802.15.6 standard. We conclude that, in general, the proposed scheme achieved a better throughput comparing to the conventional slotted Aloha proposed in the standard. But the delay resulted due to the spreading code usage is always a tradeoff factor against the achieved throughput in the spreading schemes generally.
As we expected, the spreading code length assigned to the different nodes contending to the medium, plays an important role in the overall system’s performance.
Another important observation was that the contention probability (CP) value assigned to every node, according to its priority, has a considerable effect on the system’s performance. For nodes with high contention priority (usually medical nodes), it was shown that these nodes achieved much higher throughput combined with much lower delay comparing to the throughput and delay achieved by low priority nodes which employed for entertainment purposes.
Because the entertainment devices usually serve an audio or video streaming applications with continuous traffic, its generated data rate is higher than medical devices which serve a discrete transmission traffic. This combination of different data rate and the different nodes’ priorities is another reason for the results we get in the previous work.
Moreover, the initial CP value (the maximum value, CPmax according to the current standard) assigned to the low priority nodes is relatively small which increases the period as each packet stays in the system in order to be transmitted.
Finally, the reason we focus on in this paper is the dynamism of lowering the CP value in case of failure in transmission. The current dynamism is going as halving the CP value every two transmission failures which adds more delay to the packet in process and to the total delay of the node especially if the node’s data generation rate is high which increases the node’s queue length or increase the overflow dropped packets.
For the previous reasons, we propose a novel modification for the dynamism of lowering CP which takes into account the node’s current queue length as an effective factor for choosing the next CP value in case of failure to transmit the current packet.
The remaining sections of this paper is organized as follow: Section 2 discuss some previously related research. In Section 3, we give a brief description of the current dynamism of changing the CP value in the IEEE 802.15.6 standard and introduce and state the proposed dynamism. Simulation scenario, parameters and numerical results are presented in Section 4. The conclusion and open problem is drawn in Section 5.