In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, one parameter in providing good performance and capacity for a given communications protocol in a communications network is synchronization in time between network nodes, such as access points (APs) and wireless terminals, such as Stations (STAs) served by the network nodes.
Timing Advance is in cellular communications networks, such as Long Term Evolution (LTE) networks, used as a negative offset between the received subframe at the network node and the start of the subframe to be transmitted in the uplink (UL) from the wireless terminal. Timing Advance is used to ensure that the uplink subframes are synchronized at the network node and hence provide synchronization in time between the network nodes and the wireless terminals. The network node needs to estimate the misalignment in the uplink from each served wireless terminal to its own time reference for the subframes. In LTE the timing advance is estimated by the network node based on signalling from the wireless terminals on the Physical Random Access Channel (PRACH) in the uplink during initial access of the wireless terminal. The network node may use any reference signals in the uplink from the wireless terminals to estimate the timing advance. The network node then transmits the timing advance commands in a Random Access Response (RAR). This Timing Advance mechanism enables the wireless terminals to synchronize to the internal clock of the network node.
Unlike cellular communications networks, such as LTE, there is no common reference clock in non-cellular communications networks, such as wireless local area networks as standardized, for example, in IEEE 802.11 to use for synchronization in time. Because only one STA is transmitting at a time, the timing for transmission in the UL is just based on the timing for the reception in the downlink (DL). Effectively this means that from the AP's point of view, the delay between the end of the DL transmission and the start of the UL reception may vary depending on the distance from the AP to the STA. Specifically, the larger the distance between the AP and the STA, the larger the delay between transmission and reception will be. Thus, the above disclosed Timing Advance mechanisms are not currently needed, nor available, in IEEE 802.11. However, for the upcoming standard IEEE 802.11ax, orthogonal frequency-division multiple access (OFDMA) is considered as one component in the UL.
The operation of any IEEE 802.11 network is not dependent on strict synchronization between the AP and STAs. The STAs and APs are instead synchronized on a higher protocol level by the exchange of data/control frames. The smallest unit of time on the channel is one time slot (having a length of 9 μs in IEEE 802.11ac). The AP and the STAs are contending for accessing the channel by using Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). The level of timing accuracy in the contention is in the order of the propagation delay between the contending AP/STAs.
The introduction of uplink OFDMA or uplink multi-user multiple-input multiple-output (MU-MIMO) mechanisms in IEEE 802.11ax may generally require that the signals from the STAs arrive at the AP within the cyclic prefix (CP), as comprised in the OFDM symbols transmitted from the STAs to the AP. If this requirement is not fulfilled, then Inter Symbol Interference (ISI) will degrade the performance. There are currently no mechanisms available in IEEE 802.11 to synchronize the STAs with the AP on the physical (PHY) protocol layer to achieve the accuracy required by OFDMA in the uplink.
For the OFDM based IEEE 802.11 standards, the default length of the CP is 0.8 μs and will only suffice for shorter channels with small delay spread. At the AP, all the received signals from the STAs will therefore in at least some scenarios not arrive within the CP (if the propagation delays over the communications channel between the AP and the STAs are very different). One approach, as disclosed in US2013286959 A1, is to use a long guard interval In this respect, guard interval, guard period, and cyclic prefix, generally refer to the same technical feature. US2013286959 A1 discloses a method and apparatus to be configured to support coordinated orthogonal block-based resource allocation (COBRA) operations. An AP may be configured to indicate to a plurality of STAs that it may support COBRA. As particularly noted in US2013286959 A1, when the combination of timing difference due to uplink COBRA STAs and delay due to multi-path channel are larger than a guard interval of an OFDM system, the receiver (i.e., the AP) may have difficulty detecting the packets. Utilizing long guard intervals for uplink COBRA transmissions may be part of the solution. Moreover, the AP may estimate the round trip delay for one ore more STAs, and broadcast this information in an uplink COBRA announcement frame. The STAs may adjust the transmission time accordingly such that packets from all the uplink COBRA STAs may arrive within the guard interval. However, using such long guard intervals will at the same time increase the overhead and lower the system throughput.
Hence, there is still a need for an improved time synchronization of STAs in a wireless local area network.