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Abstract—IEEE 802.11ac is the fifth generation in Wi-Fi networking standards and will bring very high data rates to the wireless devices. Improvements in transmission speeds in the wireless networks will be dramatic. Entry-level IEEE 802.11ac products will provide a physical level data rate of 433 Mbps, which is at least three times faster than that of the most common devices using the current wireless standard, which is IEEE 802.11n. By 2015, virtually all new Wi-Fi products are expected to be based on IEEE 802.11ac technology. In this paper we compare the IEEE 802.11ac with its predecessor and analyze the results.
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I. INTRODUCTION (HEADING 1)
802.11ac is the name of an amendment to the IEEE 802.11 specification for Wireless Local Area Networks (WLANs). The main goal of the new 802.11ac amendment was to significantly increase the throughput within the Basic Service Set (BSS). The official target rates, as defined at the start of the project, are a maximum Multi-Station (Multi-STA) throughput of at least 1 Gbps and a maximum single link throughput of at least 500 Mbps. These higher rates are motivated by the continuing trend to transition devices and applications from fixed links to wireless links and by the emergence of new applications with ever higher throughput requirements.
Existing 802.11 technologies operate in the 2.4 GHz band (802.11b, 802.11g), the 5 GHz band (802.11a), or both (802.11n). 802.11ac operates strictly in the 5GHz band, but supports backwards compatibility with other 802.11 technologies operating in the same band (most notably 802.11n).
To achieve its goals, 802.11ac relies on a number of improvements in both the MAC and Physical Layer (PHY). The PHY improvements include:
•Increased bandwidth per channel
•Increased number of spatial streams
•Higher-order modulation – 256 Quadrature Amplitude Modulation (QAM)
•Multi-User Multiple Input Multiple Output (MU-MIMO)
In addition to these new PHY features, 802.11ac also supports a number of advanced digital communication concepts that were first introduced in 802.11n, such as space division multiplexing, Low-Density Parity Check (LDPC) coding, shortened guard interval (short GI), Space-Time Block Coding (STBC), and explicit feedback transmit beam forming (Tx BF).
The Media Access Control (MAC) layer includes many of the improvements that were first introduced with 802.11n. One notable enhancement is the larger maximum size of aggregate MAC Protocol Data Units (MPDUs).
TABLE III summarizes some of the advanced features used in 802.11ac over 802.11n. We’ll discuss them in more detail in the next section.
II. PHYSICAL AND DATA LINK LAYER IMPROVEMENTS
1. Physical Layer Improvements
The main aim of IEEE 802.11ac protocol is to attain throughput in the range Gigabits per second while allowing the co-existence of the legacy 802.11 devices. This is achieved by two methods. One is achieving high data rates in physical layer while other is reducing the overheads of the MAC Layer. Numerous new features are introduced in IEEE802.11n.
A. Channel Bandwidth
To increase throughput, 8/02.11ac introduces two new channel widths. All 802.11ac devices are required to support 80MHz channels, which doubles the size of the spectral channel over 802.11n. it further adds a 160MHz channel option for even higher speeds. Due to the limitations of finding contiguous 160MHz spectrum, the standard allows for a 160MHz channel to be either a single contiguous block or two non contiguous 80MHz channels.
Pilot carriers are a form of overhead used in OFDM, and they represent an overhead for the channel. In MIMO systems, a single pilot carrier can be more effective at assisting with the channel tuning operations. As a result, the pilot overhead in 802.11ac has less percentage of pilot subcarriers with the wider channels. TABLE II identifies the OFDM carrier numbering and pilot channel. The range of subcarriers defines the channel width itself. Each subcarrier has identical data-carrying capacity, and therefore, more is better. Pilot subcarriers are protocol overhead and are used to carry out important measurements of the channel. The table shows that as the channel size increases, the fraction of the channel devoted to pilot carriers decreases. As a result, the channel becomes more efficient as the width increases. The final column in the table depicts the throughput relative to the capacity of 20MHz channels in 802.11ac.
B. Modulation and Coding
Compared to prior 802.11n specifications, 802.11ac makes only evolutionary improvements to the MCS (Modulation and Coding Set). Improved modulation technology provides one of the major points where 802.11ac picks up speed. Using 256-QAM enhances user to send 8bits per symbol compared to the 6bits per symbol in 64-QAM used in 802.11n.
Selecting an MCS is much simpler in 802.11ac than in its predecessor. Rather than 70-plus operations offered by 802.11n, 802.11ac has only 10 as shown in TABLE III. The first seven are mandatory while the higher schemes are optional. Modulation describes how many bits are contained within one symbol. Higher modulations pack more data, but they require high signal-to-noise (SNR) ratios. 802.11ac uses the same error correcting codes as that of 802.11n. Modulation and coding are coupled together into a single number, the MCS index. Each value of the MCS index can lead to a wide range of transmission speeds depending on the bandwidth, spatial streams and guard interval.
One of the ways that 802.11ac simplifies the selection of MCS is that they are no longer tied to the channel bandwidth, as they were in 802.11n. To determine link speed, MCS index must be combined with the bandwidth of the channel.
The 256QAM constellation has 16 phase shifts and 16 amplitude levels. The large number of extra points in the 256QAM constellation plot has the potential to dramatically improve the transmission speed. This feature alone contributes to a 33% increase in the data rates compared to its nearest equivalent in 802.11n.
But in order to use 256QAM, the errors in the radio link must be much smaller than before.
C. Guard interval
802.11ac retains the ability to select a shortened OFDM guard interval. With 802.11ac, it has exactly the same effect as in 802.11n: the GI shrinks from 800ns to 400ns, providing a 10% increase in the data rates.
D. Spatial Streams and MU-MIMO
802.11ac allows support for up to 8 spatial streams – up from a maximum of 4 streams in 802.11n.
Support for more than one spatial stream is optional, however. The increased number of streams is most useful in combination with MU-MIMO.
MU-MIMO was added to 802.11ac to address the multi-STA throughput requirement. In MU-MIMO, the Access Point (AP) – or possibly another STA – transmits independent data streams to several STAs at the same time. Through preprocessing of the data streams at the transmitter (beam forming), the interference from streams that are not intended for a particular STA is eliminated at the receiver of each STA. Therefore, in theory, each STA receives its data free of interference from the transmissions that are simultaneously directed towards other STAs. In MU-MIMO, the spatial degrees of freedom are used to create independent transmissions to different STAs, while in single-user
MIMO,these spatial degrees of freedom are used to increase the throughput from AP to STA.
One drawback of MU-MIMO is that the amount of time that the medium is occupied is determined by the slowest link among all AP-STA pairs (or, more generally, the link that requires the most time to finalize its transmission). No new data can be sent to any of the STAs until all transmissions to STAs in the MU-group have ended. If there is too much difference in either the amount of data or throughput going to various STAs, this may lead to inefficient use of the wireless medium.
2. MAC Improvements
A. Packet Aggregation
Increased Aggregated MPDU (A-MPDU) size
The maximum size of an A-MPDU can optionally be increased to a maximum of 1,048,575 octets(compared to a maximum of 65,535 octets in 802.11n).
B. RIFS
Reduced Inter Frame Spacing (RIFS) is a deprecated feature of the 802.11n specification whose purpose was to increase MAC efficiency by reducing the gap between successive transmissions. RIFS can be applied between transmissions within the same burst. This mechanism was removed from 802.11ac, except for what is needed to maintain backward compatibility with 802.11n. It was felt that aggregation provided a more efficient way to increase MAC efficiency, and that the complexity of RIFS implementation did not outweigh its gains as a stand-alone mechanism.