Multi Link [HOT]
A multi-link suspension is a type of vehicle suspension with one or more longitudinal arms.[citation needed] A wider definition can consider any independent suspensions having three control links or more multi-link suspensions. These arms do not have to be of equal length, and may be angled away from their "obvious" direction. It was first introduced in the late 1960s on the Mercedes-Benz C111[1] and later on their W201 and W124 series.[2][3]
Multi Link
Typically each arm has a spherical joint (ball joint) or rubber bushing at each end. Consequently, they react to loads along their own length, in tension and compression, but not in bending. Some multi-links do use a trailing arm, control arm or wishbone, which has two bushings at one end.
The solid axle multi-link system is another variation of the same concept, and offers some advantages over independent multi-link, as it is significantly cheaper and less complex to build, offering good mechanical resistance and excellent reliability with very similar benefits.
The arms have to transmit traction and braking loads, usually accomplished via a longitudinal link. They also have to control caster. Note that brake torques also have to be reacted - either by a second longitudinal link, or by rotating the hub, which forces the lateral arms out of plane, so allowing them to react 'spin' forces, or by rigidly fixing the longitudinal link to the hub.
Advantages also extend to off-road driving. A multi-link suspension allows the vehicle to flex more; this means simply that the suspension is able to move more easily to conform to the varying angles of off-road driving. Multi-link-equipped vehicles are ideally suited for sports such as desert racing.[citation needed] In desert racing, the use of a good sway bar is needed to counter body roll.
Multilink suspension is costly and complex. It is also difficult to tune the geometry without a full 3D computer aided design analysis. Compliance under load can have an important effect and must be checked using a multibody simulation software.
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A new feature of the medium access control (MAC) layer for the realization of EHT is multi-link operation. While physical-layer enhancements, including new bands, wider bandwidths, and higher spatial and modulation orders, are also to be adopted, as was done for prior generations, multi-link operation is a new approach being attempted for the first time in EHT. The idea is to enable a pair of devices to use multiple wireless links in different bands simultaneously for transmission and reception. The main benefit of this approach is the simultaneous exploitation of multiple bands at a lower hardware cost than that of a single multiband radio. This multi-link operation capability is different from the multiband support offered by the access points (APs) currently available on the market in that multi-link operation allows the concurrent use of multiple links at client devices and thus also enhances the throughput of a single data session (Multiband APs allow client devices to connect using only one band at a time).
The main challenges of multi-link operation are (1) the design of channel access schemes with and without the presence of radio frequency (RF) power leakage between links, and (2) the coexistence of devices using multi-link channel access schemes with legacy devices. A straightforward approach to channel access in multi-link operation is to run individual links with independent channel access processes, as in the conventional case; however, this is only possible when no RF power leakage exists between the links. If RF power leakage is present between the links due to imperfect RF shielding, filter artifacts, etc., then synchronous channel access is preferred. Moreover, multi-link devices using these channel access schemes must coexist well with legacy devices when occupying the shared medium.
There have been some research attempts toward multi-link aggregation in the unlicensed spectrum. Glia [10] is a software implementation combining orthogonal frequency channels of multiple center frequency bandwidths and can achieve a high data rate through semisynchronous channel access. E-MICE [11] exploits multiple Wi-Fi radio links in individual bands concurrently with the aid of either link-layer aggregation or the Multipath Transmission Control Protocol (MPTCP). It is equipped with an algorithm to dynamically turn on/off links and switch between them to minimize energy consumption with little performance degradation. Naribole et al. [12] have developed a constraint-aware aligned downlink ending protocol for simultaneous downlink transmissions over multiple channels for devices with no ability to simultaneously transmit and receive. Listen-before-talk schemes (LBT) for multicarrier aggregation of Long-Term Evolution with License-Assisted Access (LTE-LAA; LTE for unlicensed spectrum) [13,14] have been studied, and enhanced schemes for use in the presence of RF power leakage were proposed in [15,16,17]. As proposed by the above-mentioned works, multi-link operation requires a new channel access scheme to better exploit multi-link transmission opportunities. However, none of these prior research works focused on the issue of coexistence with legacy single-link Wi-Fi devices for multi-link channel access schemes. Exploring the coexistence problem and designing solutions within the framework of EHT will be essential to facilitate the wide and fast penetration of the new approach.
This paper aims to illustrate enhanced multi-link channel access schemes for IEEE 802.11be EHT, identify the associated coexistence challenge, and propose solutions applicable to cases both with and without the capability of simultaneous transmission and reception. First, we describe the concept, architecture, and asynchronous and synchronous channel access schemes of multi-link operation, along with the RF power leakage problem. Next, we describe the design variants of the synchronous channel access scheme and demonstrate the coexistence challenge of the enhanced scheme. Subsequently, we propose four features to mitigate this challenge by assigning penalties to multi-link devices: repicking a backoff count, doubling the contention window size, switching to another contention window set, and compensating the backoff count. Then, five coexistence solutions are derived from combinations of these features. Comparative evaluation results are provided and analyzed for dense single-spot and indoor random deployment scenarios, demonstrating that the throughput and latency gains of multi-link operation differ between schemes. We also investigate the coexistence performance of multi-link operation with and without the capability of simultaneous transmission and reception and demonstrate that the proposed solutions mitigate the coexistence problem to different degrees. In particular, compensating the backoff count achieves the highest coexistence performance among the proposed solutions, with a marginal throughput decrease of multi-link devices. A metric for evaluating both the throughput and latency gains and the coexistence performance of a multi-link channel access scheme using a single value is also proposed.
The motivation for MLO is to increase throughput by aggregating multiple bandwidths across bands at a low hardware cost. Ideally, the maximum achievable throughput of MLO is the sum of the achievable throughput for each link. For example, if each of n links can achieve identical throughput, their MLO should ideally achieve n throughput.
If an MLD is equipped with high-fidelity filters and shielding, this RF power leakage problem can be avoided, thus enabling STR. However, if the level of power leakage to the nontransmitting links of an MLD ruins either their clear channel assessment (CCA), signal reception, or both (We consider the case in which the power leakage of a transmitting link ruins both the CCA and reception of other links in an MLD), it does not have the STR capability. Depending on whether they possess the STR capability, MLDs can be classified into the following two types:
This Async scheme is ideal for STR MLDs. However, if it is applied to a non-STR MLD, as shown in Figure 4b, a transmission on a link will appear as interference beyond the ED threshold on other links, thus causing them to sense a busy medium and freeze their BOs until the transmission ends. Therefore, for a non-STR MLD, Async significantly degrades the ability to utilize multiple links.
Synchronous Operation (Sync). The synchronous multi-link channel access scheme of EHT is illustrated in Figure 5. In this scheme, individual BOs are performed on all links. If a link finishes its BO early, it waits until the other links have also finished their BOs. If the channel of a link that has been waiting after finishing its BO becomes busy, the link must rerun its BO (the contention window (CW) remains unchanged).
This operation scheme lessens the failure of link utilization in non-STR MLDs due to RF power leakage, thus enhancing their multi-link utilization. As illustrated in Figure 7, Sync-FT is applicable to both STR and non-STR MLDs but with a subtle difference in behavior.
Flowchart of the channel access procedure of a link under the Sync scheme with the proposed solutions (black-outlined boxes + black and red arrows: legacy operations, all boxes + black and blue arrows (excluding red ones): new operations). 041b061a72