The articles about the future of Wi-Fi in last month’s IEEE Communications Magazine have me thinking about my experience with the history of Wi-Fi.
802.11(b) - This was the standard when I started doing wireless projects in the early 00s. (b) uses the 2.4 GHz band. Maximum raw data rate is 11Mbps.
802.11(g) came along in the mid 00s. It used OFDM and higher-order modulation to achieve 54Mbps data rate with same bandwidth (20 MHz) as (b).
When my colleague and mentor Jim Weikert and I tested the first cards, we spent days trying to work out why there were so many packets re-sent because the receiver did not acknowledge the packet. We later realized that those higher-order modulations require a very good signal-to-noise ratio (SNR) to maintain a low bit error rate (BER). The fallback algorithm would go to a rate that had a 10% packet error rate (PER) We were used to 0% PER with 802.11(b). The reason is your overall throughput is great at, say, 36 Mbps data rate with a 10% PER than it would be if it fell back to 24Mbps or 18Mbps to attain a lower PER. Complicating matters, OFDM requires excellent amplifier linearity, so most transmitters must reduce their output power to send the higher data rates.
802.11(a) came along around the same time. It’s just like (g) but using the 5-6 GHz ISM bands.
802.11(n) became the commonly-used standard around 2010. It supports 40 MHz wide bandwidths, slightly more than doubling the number of OFDM subcarriers, making it like (g) but times two. It also supported multi-stream MIMO. MIMO is the most amazing technology I have worked with. MIMO allows different streams of data to be sent from different antennas on the same frequency at the same time. The receiving side has the number-crunching power to detangle the cacophony of interfering signals coming in on its antennas using the model of the channel function between each transmitter and receiver.
In my experience the 40 MHz channels were an instant leap forward, doubling throughput, at the price of only 3 dB of link margin. Multi-stream MIMO was a technical marvel but of less practical benefit. It works flawlessly in the ideal case of connecting each transmitter and receiver with separate cables and attenuators. In a practical scenario, it works best if the antennas a separated at least a few percent of the distance between the transmitter and receiver and/or have different polarization angles. If the paths between the array of antennas are too similar, multi-stream MIMO won’t work or it may work but only at low data rates. Working out whether to do single-stream at a high data rate or multistream at a lower rate is not trivial.
802.11(ac) supports 80 MHz and 160 MHz channel and more MIMO options. One of the papers in IEEE Communications discussed MIMO techniques in (ac) and future Wi-Fi standards. I told the authors of my experience with MIMO being difficult to use in practical scenarios. I asked if it was practical for handheld devices, for which it’s hard to separate the antennas. Dr. Joonsuk Kim explained that we are beginning to see portable devices using MIMO. It has improved since the first (n) cards a few years ago. There is such demand for high-throughput, throughputs hard to attain simply by increasing the number of subcarriers. I mentioned problems I had seen using MIMO in a multi-path rich environment with large delay spreads. Dr. Inkyu Lee said we could always decrease the symbol rate and increase the number of subcarriers, although that may never be part of the Wi-Fi standard. The guard interval between Wi-Fi symbols is 800ns with an optional 400ns for slightly higher throughput.
Below is a graph showing how much throughput is possible with 802.11(ac) for various numbers of streams. It all depends, of course, on the antennas being separated well and the channel functions between each TX and RX antenna being different enough for the receiver to recover the individual streams.
I now accept the 10% retry rate that Jim Weikert and I first observed 10 years ago with 802.11(g) as a normal fact of Wi-Fi. The challenge for Wi-Fi products now is to choose when to use more carriers (wider channels) and when to use MIMO. The challenge of picking the right modulation coding scheme (MCS) and switching dynamically as channel conditions change will become a fact of life like a 10%-15% retry rate has been for the past decade.