What’s The Difference Between 802.11n And 802.11ac?

This article explains the difference between the currently popular IEEE wireless local-area network (WLAN) standard 802.11n and the forthcoming 802.11ac, which defines a faster version

The 802.11 standard was first available in the late 1990s. It was not an immediate success. When the 11b version arrived in 1999, it facilitated the first widespread implementation of WLAN technology. The 802.11b standard is often considered the first generation, 802.11a is the second generation, 802.11g is the third generation, and 802.11n is the fourth generation. When 802.11ac becomes available this year, it will represent the fifth generation.

Today, the most widely implemented form is 802.11n, which is used in access points, routers, laptops, smart phones, tablets, and other mobile devices. Devices using 11n are all backward compatible with older 802.11a/g equipment.

The 802.11 standard defines both a physical layer (PHY) and media access control (MAC) layer in the networking scheme. While the MAC tends to remain mostly the same, the PHY changes to include the most recent wireless technology for greater speed and link reliability.

802.11n
The 11n version of 802.11 is the outgrowth of progressive changes to the standard over the years. The first generation, 11b, used direct sequence spread spectrum (DSSS) to achieve data rates to 11 Mbits/s in a 20-MHz channel in the 2.4-GHz industrial-scientific-medical (ISM) radio band.

When the 802.11b standard was ratified in 1999, the speed of wireless at 11 Mbits/s was quite useful and faster than almost any other similar technology. But with the growth of the Internet, that speed soon became a disadvantage. A faster version was sought, bringing about major changes in the PHY.

The second-generation 802.11a came along in 1999. It was the first to use the 5-GHz ISM band and orthogonal frequency division multiplexing (OFDM) with 64 subcarriers spaced at 312.5 kHz. Channel bandwidth was 20 MHz, and modulation types of binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), 16-phase quadrature amplitude modulation (16QAM), and 64-phase quadrature amplitude modulation (64QAM) were defined. This permitted the data rate to increase to a maximum of 54 Mbits/s.

While the 802.11a version was more robust because of the OFDM characteristics that mitigated multipath reflections and the 5-GHz assignment meant less interference, the higher frequency still limited the range. This version was never too popular despite its advantages. Fewer 5-GHz 11a chips were made.

The big breakthrough came when the 802.11g standard was approved in 2003. Essentially, 11g is the same as 11a but operating in the 2.4-GHz band. Using the same OFDM and modulation options, it too could deliver up to 54-Mbit/s data rates. This version was immediately popular because the many IC companies competing for the business brought chip prices very low.

The current 11n standard is a further improvement over 11a/g. It adds 40-MHz channels and multiple-input multiple-output (MIMO) features to the OFDM, allowing data rates to as much as 600 Mbits/s.

MIMO is a technique of using multiple receivers, transmitters, and antennas to achieve spatial division multiplexing (SDM). SDM transmits fast multiple data streams concurrently within the same 20- or 40-MHz channel bandwidth. Pre-coding and post-decoding as well as unique path characteristics are used to distinguish the data streams. This allows the data rate to be multiplied by a factor roughly equal to the number of data streams.

The 11n standard permits up to four transmit and four receive channels (4x4), although 1x2, 2x2, and 3x3 versions are more widely used. The 600-Mbit/s data rate is achieved using 4x4 MIMO with 64QAM in a 40-MHz channel.

What makes 11n so popular is its ability to carry video, enabling wireless connectivity in TV sets, DVD players, and other video equipment. Using compressed video (H.264, MPEG4, etc.) full HD signals can be transmitted reliably with the higher levels of MIMO.

The WLAN space is dominated by 11n Wi-Fi, as it is commonly available in all smart phones, tablets, and laptops. It also is the wireless technology of most hotspots and access points, including millions of home wireless routers. Increasingly, it’s being embedded in consumer electronic equipment. It’s backward compatible with previous standards as well, allowing 802.11a/g equipment to be used. Broadcom, Celeno, Marvell, Qualcomm Atheros, Quantenna, Redpine, and Texas Instruments supply 11n ICs.

Celeno’s CLR250 802.11n is a good example of a recent product (Fig. 1). This 3x3 MIMO chip can operate in the 2.4- and 5-GHz bands. It also can achieve a data rate to 450 Mbits/s. And, this chip features a USB port. Another version of the chip, the CLR260, has a PCI Express port.

Figure 2 shows a recent 802.11n reference design that uses Quantenna’s QHS710 chip. It also employs 4x4 MIMO to achieve a maximum of 600 Mbits/s. The QHS710 is used in police surveillance and consumer TV products.

802.11ac
The IEEE task group has not yet finalized the 802.11ac version of the standard, but ratification is expected by the fall of 2012 if not earlier. A working draft is available, and semiconductor vendors have made chips available in advance of the final approval.

Specified for the 5-GHz ISM band only for minimum interference and maximum available bandwidth, 11ac is a further evolutionary version of 11n. It continues the use of MIMO and OFDM. However, some key changes boost theoretical data rate in excess of 3 Gbits/s depending on modulation, channel bandwidth, and MIMO configuration.

The primary changes are 80- and 160-MHz wide channels in addition to the usual 40-MHz channel. As the bandwidth increases, so do the number of OFDM subcarriers to a maximum of 512 at 160-MHz bandwidth. OFDM also adds 256-phase quadrature amplitude modulation (256QAM), which further boosts data rate.

Finally, 802.11ac defines a greater number of MIMO versions with a maximum of an 8x8 configuration. A multi-user version, MU-MIMO, is also defined. The standard supports coexistence and compatibility with previous 11a and 11n devices as well. And, transmit beamforming is an option to extend range and ensure link reliability.

Already, multiple vendors are announcing chips for this standard including the companies mentioned above. Broadcom, one of the first with products, is offering 5G Wi-Fi solutions (Fig. 3). Its BCM4360 implements a three-stream 11ac capable of 1.3 Mbits/s. It also has a PCI Express interface.

The BCM4352 (PCI Express interface) and BCM43526 (USB) are two-stream devices with speeds up to 867 Mbits/s. The single-stream BCM43516 can reach 433 Mbits/s and has a USB interface. Similarly, the Redpine Signals Quali-Fi 802.11ac ultra-low-power chips target embedded mobile devices.

Agilent Technologies has full testing capability available for 802.11ac development. This includes signal analysis software for the company’s 89600 vector signal analyzer, signal creation software with Signal Studio, and 160-MHz analysis bandwidth on its PXA signal analyzer. National Instruments also offers 802.11ac testing support in its PXI analyzer modules and related software.

The Wi-Fi Alliance is currently developing its usual testing and certification program for 11ac products. It will be ready this fall. After that, you’ll begin to see end products like routers and access points as well as embedded versions in consumer video products, tablets, laptops, and eventually smart phones. It is expected that 11ac will be widely adopted, perhaps even more quickly than 11n, because its huge boost in data rates makes video transport even better.