Controller Based WLANs

Improve Wireless Capacity and Reliability

How to improve wireless capacity and reliability?

If you've used Wi-Fi, you know that the performance drops as you move further away from the Access Point.  802.11n and 802.11ac use many techniques to improve the capacity and reliability of Wi-Fi.

MAC Aggregation: One basic technique to improve capacity is to reduce the overhead of packet transmissions by aggregating multiple packets and by reducing the number of acknowledgement packets. This is called MAC aggregation and is supported by all 802.11n and 802.11ac devices.

Channel width: Another way to increase the capacity is to increase the width of the RF channel used for communication. 802.11 a/b/g used approximately 20 MHz wide channels. 802.11n supports 40 MHz wide channels. 802.11ac will support 80 MHz and optionally 160 MHz channels.

Modulation and coding: Yet another way to improve the capacity of a wireless channel is to increase the number of bits coded per Hz. In other words, improve the modulation and coding techniques so that more bits can be sent with the same channel width. Both 802.11n and 802.11ac support additional modulation and coding techniques.

MIMO techniques: The term MIMO, which stands for Multiple-Input-Multiple-Output, refers to a set of radio techniques that are used to improve the capacity and reliability of wireless networks using multiple transmitters and receivers. Both 802.11n and 802.11ac support multiple MIMO techniques. 802.11ac will include MU-MIMO, which will allow multiple clients with multiple antennas transmitting and receiving independent data streams simultaneously, as well as “Space Division Multiple Access” (SDMA): streams separated spatially, not by frequency.

 

MIMO Techniques:

Before we talk about the MIMO techniques, it is important to understand some basic concepts.

Multipath:In a typical indoor environment, the transmitted RF signals encounter various objects such as walls, cubicles, doors and other equipment. Some of these objects may absorb the RF signals while others may reflect the RF signals. Due to these reflections, multiple copies of the RF signal from a transmitter to a receiver may take multiple paths, creating what is called a “multipath effect”.

Traditionally, multipath has been considered bad for Wi-Fi.  Reflected signals take longer to reach the destination than a signal using a direct path and reflected signals may undergo amplitude and phase changes. Therefore, when multiple copies of the signals arrive at the receiver in a multipath environment, they may have different delay, amplitude and phase. This would make it difficult for the receiver to reliably decode the signals if the variation in one or more of these parameters is high. As we will see shortly, MIMO actually makes use of this multipath.

Chains:A chain is a part of the radio used to transmit or receive.  In most implementations, transmit and receive chains are integrated into a single unit as a transceiver. For example, an 802.11 a/b/g radio typically has a single transmit and receive chain. MIMO radios require more than one chain. Sometimes the terms “chain” and “antennas” are used interchangeably in the context of MIMO.

Let’s now look at the MIMO techniques in detail. As we discussed earlier, MIMO techniques require more than one chain. Some MIMO techniques are applied to the transmitters or receivers only, while others are applied to both the transmitter and receiver. Some of the key MIMO techniques include Spatial Multiplexing, Diversity Techniques, Transmit beam-forming and Maximal Ratio Combining.

Spatial Multiplexing (SM):The term multiplexing refers to the ability to transmit and receive multiple data streams over a shared medium. For example, Time Division Multiplexing shares the medium by transmitting independent data streams at different times or timeslots. Similarly, Frequency Division Multiplexing, shares the medium by transmitting independent data streams at different frequencies or frequency ranges.

Spatial Multiplexing on the other hand shares the same space by transmitting independent data streams using spatially separated antennas. Each such independent stream is called a spatial stream. Spatial multiplexing makes use of the differences in multipath signatures of different antennas. The signal processing at the receivers are able to separate these different signals transmitted through different antennas.

The capacity of the MIMO system increases with the addition of each spatial stream, though the mere addition of an additional stream does not guarantee additional capacity. The implementation of spatial multiplexing requires support at both the transmitter and receiver. 802.11n and 802.11ac support spatial multiplexing with up to 4 and 8 spatial streams respectively.

Diversity Techniques:Apart from spatial multiplexing, multiple chains can also be used to implement different forms of diversity techniques. The goal of diversity techniques is to improve the robustness or reliability of connections in the presence of multi-path and other fading. Let’s look at a couple of techniques that are part of 802.11n and 802.11ac - Space-Time Block Coding (STBC) and Cyclic Shift Diversity (CSD), sometimes referred to as Cyclic Delay Diversity (CDD).

Space time Block Coding (STBC): STBC is a method where multiple redundant copies of data encoded in blocks are transmitted through multiple antennas. The basic idea is that some of the copies will arrive at the receiver better than others. A receiver that supports STBC can process the received copies and decode the data stream. Similar to Spatial Multiplexing, STBC requires support at both the transmitter and receiver.

Cyclic Shift Diversity (CSD): Similar to STBC, CDD technique also transmits multiple copies of data through multiple antennas. However, unlike STBC, no block coding is used in CDD and instead a specific cyclic shift is applied to each antenna. The advantage of this scheme is that this technique can be implemented at the transmitter with multiple antennas without having to change the receiver.

Transmit beam-forming (TxBF): Transmit Beam-forming improves the selectivity using multiple antennas, by digitally manipulating the transmitted signal such that the SNR at an intended recipient is maximized. The signals are transmitted through multiple antennas such that they add up constructively at the intended recipient. An accurate estimate of the channel state is required for TxBF to work properly. This requires coordination between the transmitter and receiver, where the receiver estimates the channel state and communicates this to the transmitter so that the transmitter can use this information to beam-form towards the client.

It is possible to implement TxBF without the support from the receiver by estimating the channel state as seen by the transmitter in the reverse direction. However, this approach has many limitations due to the differences in channel state as seen by the transmitter and receiver. Even if channel state information is available from the receiver, the TxBF may improve the performance only in certain circumstances as it can adversely interact with spatial multiplexing.

Maximal Ratio Combining (MRC):MRC is a technique used in the receiver to linearly combine the received signals from multiple antennas to maximize the SNR of the received signal. When multiple antennas receive the copies of same signal with different multipath fading, MRC processes the signals to decode the strongest signal. This signal processing technique works at the subcarrier level to construct the strongest signal from multiple antennas.

Comparison of MIMO techniques:

Let’s compare various MIMO techniques discussed. It is also important to understand how some of these techniques may not work well together:

 

MIMO Technique

Implemented at the Transmitter?

Implemented at the Receiver?

Notes

Spatial Multiplexing

YES

YES

Both transmitter and receiver must be able to encode and decode multiple spatial streams

STBC

YES

YES

Block coding requires the receiver to implement STBC

CDD/CSD

YES

NO

Receiver is agnostic to CSD since the there are no changes to the contents of data streams. They will appear to the receiver as if they had gone through multipath fading.

TxBF

YES

YES for good performance

Receiver implementation is required for accurate channel estimation

MRC

NO

YES

Transmitter is not involved in MRC. However, the MRC gain at the receiver varies depending on the Tx techniques implemented.

 

3x3:2, 4x4:4, 2x3:2….What do all these numbers mean?

We discussed how multiple antennas or chains are required to implement various MIMO techniques. Now let us look at how the number of transmit and receive chains as well as the number of spatial streams are specified. The commonly accepted terminology is T x R: S [Show this on a slide], where the first number (T) indicates the number of transmit chains, the second number (R) indicates the number of receive chains and the third number (S) indicates the number of spatial streams. The number of spatial streams (S) is less than or equal to the lessor of the number of transmit and receive chains. The number of spatial streams determines the maximum possible capacity. If the number of transmit chains and/or the number of receive chains is higher than the number of spatial streams, the extra chains may be used to improve reliability and diversity.

Sometimes the last number, i.e., the number of spatial streams, is not included in the specification of a MIMO AP. However, this can cause confusion as the maximum capacity is determined by this number, not the number of chains. In 802.11n, single spatial steam systems support up to 150Mbps while two spatial stream systems support up to 300 Mbps. Similarly 3 spatial stream systems support 350 Mbps and 4 spatial stream systems can theoretically support 600 Mbps, though it is not easy to achieve such multiplexing gain in a typical environment and AP design. 802.11ac systems can support much higher data rates than the 802.11n systems for the same number of streams.

Adding an extra chain produces diminishing returns as the number of spatial streams increase. For example, adding a chain to a two stream 2 x 2 system is less beneficial than adding a chain to a single stream 1x1 system. For higher number of spatial streams, the addition of a chain may not be worth the cost and complexity.

 

Summary:In summary, the term MIMO includes many technologies that improve the capacity and reliability of links. Spatial multiplexing is a fundamental technology in 802.11n and 802.11ac that increases the available capacity. All the other techniques defined in these standards, STBC, CSD, TxBF and MRC improve the reliability of reception in one way or the other.

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‎09-26-2014 03:36 AM
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