High-Density and De-Densified Smart Campus Communications. Daniel MinoliЧитать онлайн книгу.

2 Traditional WLAN Technologies

High‐Density Communication (HDC) technologies include the extension of traditional technologies (e.g. Multi‐User Multiple‐Input[s] And Multiple‐Output[s] [MU‐MIMO] approaches, also sometimes known as Massive MIMO), as well as new or evolving technologies.¹ In this basic overview chapter, some of the underlying technologies and principles that are applicable to HDC in local environments are surveyed,² but the discussion is not focused per se on the high‐density specifics, which are covered in the chapters that follow.

2.1 OVERVIEW

The Electrical and Electronics Engineers (IEEE) developed Wireless LAN (WLAN) standards starting in the late 1990s; IEEE 802.11b was ratified in July 1999. The 802.11b Wi‐Fi standard had a maximum link speed of 11 Mbps; the 2003 802.11a/g revision increased the speed to 54 Mbps with the introduction of Orthogonal Frequency Division Multiplexing (OFDM) technology. The 2009 802.11n allowed a single stream link to operate up to 150 Mbps. The 2013 802.11ac revision of the standard offered the possibility of link speeds around 866 Mbps on a single spatial stream with wider channels (on 160 MHz) and higher modulation orders (256‐QAM) (433 Mbps on smaller channels). While using the maximum number of spatial streams allowed in the standard, this system could support a speed of 6.97 Gbps (these maximum speeds are only achievable in the controlled Radio Frequency [RF] lab environments³). 802.11ax, also called High‐Efficiency Wireless (HEW), aims at improving the average throughput per user fourfold in dense user environments; additionally, this standard implements multiple mechanisms to support more users with consistent and reliable data throughput in crowded spectrum environments.

802.11‐based WLANs include an infrastructure Basic Service Set (BSS). The BSS provides the basic building‐block of the environment and typically includes an Access Point (AP) and one or more associated stations (STAs) – also known as Wireless Nodes (WNs) (or also known as User Equipment [UE] in some other contexts). The AP is a station configured to control and coordinate functions of the BSS. Obviously, a physical environment can include a large number of BSSs. The stations can transmit data to the AP, and in some environments, they can also transmit information to and receive information from another station. A station such as a personal computer or cellular phone may operate as an AP, such as when a cellular phone is configured to operate as a wireless hotspot. The AP can transmit information to a single station, selected from the plurality of stations in the BSS using a single frame, or it can simultaneously transmit information to two or more stations in the BSS using either a single OFDM broadcast frame, a single OFDM MU‐MIMO transmission frame, or a single Orthogonal Frequency Division Multiple Access (OFDMA) frame [2]. An AP is a particular type of station; however, for clarity, the term STA is generally used to refer to non‐AP stations.

A WLAN device incorporates a Medium Access Control (MAC) and a Physical Layer (PHY) protocol stack at the lower layers and a set of Internet protocols (e.g. as promulgated by the Internet Engineering Task Force [IETF]) at the upper layers. See Figure 2.1. An AP may include a WLAN router function, or it can be a stand‐alone AP, a WLAN bridge, or a Light‐Weight Access Point managed by a WLAN controller. See Table 2.1 for some preliminary system details. In addition to the infrastructure mode, relatively new “ad hoc” modes (including a form known as Direct Wi‐Fi) have emerged.