IEEE 82.11n Standard

The key improvement in 802.11n is the capability of at least 100 Mbps MAC throughput and a maximum data rate of 600 Mbps. Achieving this requires PHY and MAC enhancements. There are two approaches to increase the PHY data rates in 802.11n, namely MIMO and 40 MHz bandwidth channels.

Basic Specification for IEEE 802.11n

Maximum data rate	600 Mbps
RF band	2.4 or 5 GHz
Modulation	DSSS or OFDM
Number of spatial streams	1, 2, 3, or 4
Channel width	20 or 40

Increasing from a single spatial stream to four spatial streams and four antenna increases the data rate by a factor of four. However, because increasing the number of antenna increases cost, modes that use three and four spatial streams are optional. In order to allow for handheld devices, the two spatial streams mode is only mandatory in an access point (AP).

40 MHz bandwidth channel operation is optional due to concern regarding interoperability between 20 and 40 MHz devices, the permission to use 40 MHz bandwidth channels in various regulatory domains, and spectral efficiency. However, the 40 MHz bandwidth channel mode has become a core feature due to the low cost of doubling the data rate from doubling the bandwidth. Almost all 802.11n products feature a 40 MHz operation mode.

Modifications were made to 802.11a/g waveform to increase the data rate. The highest encoder rate in 802.11 is 3/4, and was increased to 5/6 for 802.11n, resulting in 11% increase in data rate.

With improvement in RF technology, two extra frequency subcarriers could be squeezed into the guard band on each side of the spectral waveform and still meet the transmit spectral mask — this increases data rate by 8%.

The waveform in 802.11a/g and mandatory operation in 802.11n contains an 800 ns guard interval between each OFDM symbol. An optional mode was defined with a 400 ns guard interval between each OFDM symbol to increase the data rates by another 11%.

Interoperability between 802.11a/g and 802.11n at the physical layer is achieved by defining a waveform that is backward compatible with 802.11a and OFDM modes of 802.11g. To ensure backward compatibility between 20 MHz bandwidth channel devices and 40 MHz bandwidth channel devices, the preamble of the 40 MHz waveform is identical to the 20 MHz waveform and is repeated on the two adjacent 20 MHz bandwidth channels that form the 40 MHz bandwidth channels.

Unfortunately, MIMO training and backward compatibility increases the overhead, thus reducing efficiency. In an environment that is free from legacy devices (termed greenfield), backward compatibility is not required. 802.11n includes an optional greenfield format that eliminates the components of the preamble that support backward compatibility. The greenfield format preamble is 12 μs shorter than the mixed format preamble — the improved efficiency becomes pronounced for short packets, e.g., for VoIP traffic.

The Physical Layer

The 802.11n PHY operates in three modes:

1. Legacy mode. Frames are transmitted in the legacy 802.11a/g OFDM format.
   
2. Mixed mode. Packets are transmitted with a preamble compatible with the legacy 802.11a/g. The Legacy Short Training Field (L-STF), the Legacy Long Training Field (L-LTF), and the legacy signal description (L-SIG) are transmitted so they can be decoded by legacy 802.11a/g devices. The rest of the packet has a new MIMO training sequence format.
   
3. Greenfield mode. High throughput packets are transmitted without a legacy-compatible part.
   

The mixed mode and greenfield mode are known as the High Throughput (HT) modes.

The fundamental issue regarding MAC inefficiency is overhead, i.e., headers (MAC header, frame check sequence (FCS) and PHY header), interframe spaces (IFs), backoff time and ACKs. Frame aggregation was introduced to 802.11b MAC to address this inefficiency.

As data rate increases, the time on air of the data portion of the packet decreases while the PHY and MAC overhead remains the same. Frame aggregation increases the length of the data portion to increase overall efficiency. There are two types of aggregation:

1. MAC service data unit aggregation (A-MSDU) resides at the top of the MAC and aggregates multiple MSDUs into one MPDU. Each MSDU is prepended with a subframe header consisting of the destination address, source address and a length field specifying the length of the SDU in bytes. This is then padded with 0 - 3 bytes to round the subframe to a 32-bit word boundary. Multiple subframes are concatenated together to form one MPDU. An advantage of A-MSDU is it can be implemented in software.
2. MAC protocol data unit aggregation (A-MPDU) resides at the bottom of the MAC and aggregates multiple MPDUs. Each MPDU is prepended with a header consisting of a length field, 8-bit CRC and 8-bit signature field. These subframes are padded to 32-bit word boundaries. Each subframe is concatenated together. An advantage of A-MPDU is that if an individual MPDU is corrupt, the receiver can scan forward to the next MPDU by detecting the signature field in the header of the next MPDU. With A-MSDU, any bit error causes all aggregates to fail.

Source:

• IEEE 802.11: Wireless LANs
• Y. Xiao, "IEEE 802.11n: Enhancements for Higher Throughput in Wireless LANs", IEEE Wireless Communications, Volume 12 Issue 6, 2005
• E. Perahia, "IEEE 802.11n Development: History, Process, and Technology", IEEE Communications Magazine, Volume 46 Issue 7, 2008