Impact of adjacent channel suppression / interference on 802.11 WLAN

Impact of adjacent channel suppression / interference on 802.11 WLAN

With the rapid promotion and application of wireless networking technology and other wireless technologies in the same spectrum range without license restrictions, the radio frequency (RF) interference suffered by Wi-Fi (802.11) products is increasing day by day, which seriously affects the data throughput of the wireless local area network (WLAN) performance. At the same time, for new WLAN applications such as multimedia audio and video, streaming media, WLAN voice, and other applications that require quality of service (QoS) functions and lower packet error rates, the market requires higher data throughput rates. Due to the increasing in-band and adjacent-band interference to WLAN equipment in the environment, the design of radio frequency and digital filtering is critical. This paper analyzes the source of adjacent channel interference (ACI) and radio frequency design practice. Through this practice, the adjacent channel suppression (ACR) of WLAN can be improved and its performance can be comprehensively improved.

Overview

In the 2.4GHz and 5.xGHx unlicensed frequency bands, ACI issues and the need to improve the performance of Wi-Fi and WLAN technology of RF receivers have been greatly influenced by manufacturers, system designers, integrators and the Federal Communications Commission (FCC) )s concern. In fact, when the FCC released an additional 250MHz spectrum for 802.11 WLAN (starting at 5.4GHz), it noted that it would soon adjust regulations for WLAN crowded spectrum bands. The FCC may recently issue a “NOI” to collect information about the possibility of establishing government standards for designing RF receivers in this spectrum.

What is stake?

Before the interference problem is solved, the future development of the WLAN market will be greatly affected. Currently, WLAN access point devices (APs) or client base stations will be interfered by other neighboring WLAN APs and base stations and non-802.11 devices operating in the same unlicensed frequency band. This situation is similar to the problem faced by the mobile phone industry, which uses a channel frequency reuse solution to solve this problem. With the development of the 802.11 market and the increasing use density of WLAN technology, this problem will become worse in the following applications:

● Company / Enterprise deployment

● Intensive business hotspot deployment (commercial street, etc.)

● Residential apartment building deployment

● High-density city deployment

Many interference sources can adversely affect WLAN performance, including the following non-802.11 devices:

● Cordless phone (2.4 or 5.xGHz)

● Bluetooth personal area networking equipment (2.4GHz)

● Bluetooth wireless headset is a special case

● Pulse radar (the United States is studying the use of 5.4GHz band for pulse radar)

● Microwave oven (50% busyness and idleness in the 2.4GHz band will generate pulse interference)

● Low energy RF light source (2.4GHz)

● Fake signal RF noise in integrated devices, handheld terminals, and PDAs using multiple wireless technologies including cellular, Bluetooth, and WLAN

● Broadband 5GHz equipment to meet the emerging "full frequency band" requirements

Interference may also originate from adjacent channels. In this case, the design of the RF subsystem and digital filtering of the 802.11 system can also greatly affect the performance of the AP or base station. In addition, the physical design of the WLAN network can eliminate many reflections of in-band interference. The performance of the WLAN is usually determined by the signal-to-interference ratio (S / I or SIR), which is defined as the ratio of the data signal to the interference signal. For WLAN performance, SIR is usually more important than signal-to-noise ratio (SNR). Figure 1 below explains this concept.

Obviously, the signals generated by commercial wireless devices are not perfect. Indeed, the signal from the 802.11 radio generates some energy beyond its licensed frequency band, which is called sideband transmission. This situation also occurs in other wireless devices, such as Bluetooth, cordless phones, and other devices that occupy the same frequency band as 802.11. Although filtering can minimize RF interference from adjacent channels, this interference also generates sidelobe energy, which falls within the passband of the 802.11 WLAN signal. If the ACI is stronger than the 802.11 signal, the sideband energy from the ACI will dominate the noise layer of the channel. as shown in picture 2.

The WLANRF receiver can be designed with an effective ACR, which can transmit a narrowband signal with a bandwidth of about 0.10 of the 802.11 signal. These narrowband signals include cordless phones and Bluetooth signals. However, wideband ACI can generate a large amount of sideband energy that enters the passband of the 802.11 receiver. Under these conditions, the number of link margins or the size of the SIR will have a decisive impact on the data throughput of the WLAN.

It is the development trend of the wireless industry to provide a 5.xGHz RF architecture that can span all frequencies in the world's unlicensed and restricted frequency bands. The figure below (Figure 3) shows how these so-called "full-band" radios operate from 5.150 GHz to 5.875 GHz. If the radio frequency in this band contains the Japanese distribution that will come into effect in 2007, the range can be extended from 4.9 GHz to 5.875 GHz. Assuming that there are some high-power interference sources in this band, such as radar and navigation systems, then full-band RF also requires some level of channel selective filtering to avoid any performance degradation caused by these high-power interference sources.

With the above content as the background, the rest of this white paper will mainly introduce the following:

● RF receiver design that can provide adjacent channel suppression (ACR) for interference;

● ACR filtering technology, which can be implemented in embedded applications where Bluetooth and 802.11 technologies coexist on the same product platform. Special emphasis on the problems encountered in wireless headphones;

● Interference caused by adjacent 802.11 cells in dense user environments.

Provide ACR receiver design

The ability of an RF system to suppress interference from adjacent channels depends mainly on the architecture of the receiver. Although several receiver architectures are currently available, because direct conversion (DC) and dual-channel conversion or super-het (super-het) architectures are commonly used in WLAN systems, this white paper only analyzes these two architectures.

In order to incorporate effective ACR functions in the design of WLAN receivers, two points must be considered in the receiver link. As follows:

● Low noise amplifier (LNA) and IP3 input signal saturation;

● The current signal level of the analog-to-digital converter (A / D) in the system's signal baseband processor.

In 802.11 systems, the input signal level of most LNAs is saturated between -20 and -30 dBm. If a strong input signal exceeds this level, the LNA will stop providing gain and will actually suppress the nonlinear distortion of the signal. The carefully designed LNA can operate with input levels up to -10 to -15dBm. When the input signal exceeds -10 to -15dBm, some systems can bypass the LNA. Thus, the input signal can be as high as + 4dBm, but the compromise result is lower receiver sensitivity.

At the other end of the LNA's RF processing link will be input to the system's A / D converter. These converters have a limited dynamic range. Therefore, the ACI cannot be filtered out, causing the digital noise layer to dominate the received signal. Assuming that the WLAN radio frequency is designed with at least 20dB of digital filtering, then the ACI noise and 802.11 signal on the A / D signal power should be the same (equal power point).

Table 1 shows examples of interference sources in the 2.4GHz band. The effective interference figures in this table (column 5) explain why the saturation point of the LNA is so important.

Most of the interference sources in Table 1 are narrow-band devices, such as cordless phones or Bluetooth products. In many cases, such products can be operated within a meter or in a WLAN client device. Even with propagation losses, these interference sources can still provide up to 0 dBm for the LNA located at the end of the 802.11 receiver link.

802.11 receiver architecture

Figure 4 compares the difference between the superheterodyne receiver architecture and the DC receiver architecture. This example assumes that the adjacent narrowband strong interference originating from the cordless phone is -15dBm, and the target of the received WLAN signal level is -80dBm. In other words, the difference in received power between interference and WLAN signals is nearly 65 dBm. This situation can easily happen, for example, a user may work on a portable computer connected to a local WLAN while chatting on a cordless phone.

Figure 4 shows that the filtering design of the superheterodyne receiver architecture can reduce the ACI to an acceptable level. With at least 20dB digital adjacent channel filtering, the superheterodyne receiver can receive 11 megabit (Mbps) CCK or 22Mbps PBCC802.11 Wi-Fi signals per second without increasing the packet error rate.

If the DC architecture is used, the surface acoustic wave (SAW) filter on the intermediate frequency (IF) is removed, resulting in the interference signal on the A / D converter in the receiver link being 40dB, which is higher than acceptable. Using A / D oversampling and recursive decimation filtering (recursivedecimaTIonfiltering) can still recover 802.11 signals. For example, a GSM receiver uses a DC architecture and provides an ACR of approximately 80 dB by oversampling a GSM signal of approximately 300 KHz at approximately 26 MHz. Unfortunately, due to technical limitations and the low power consumption requirements of battery-powered products, almost 100% of the signals used for oversampling are narrow-band signals like GSM signals, and cannot be broadband signals like 802.11 signals.

Figure 5 below shows the effect of strong ACI on the A / D converter. The high-level ACI results in a noise layer that dominates the SIR of the 802.11 channel, which weakens the strength of the WLAN signal due to atmospheric noise and quantization.

For a WLAN that has implemented an OFDM modulation scheme, the fast Fourier transform (FFT) in the receiver link has been lost during the round-trip transmission from one frequency receiver to another. This results in an average out-of-band suppression layer of approximately 25dB. Figure 6 explains the SinX / X response of each FFT receiver.

Receiver

Although it has exceeded the scope of this white paper, it is worth mentioning that ACR filtering in the 802.11 receiver link can reduce power consumption because the sampling rate of the A / D in the baseband processor will decrease. In order to meet the requirements of anti-aliasing, it will increase the burden of other analog filtering instead of sampling at a higher rate. In the so-called full-band radio frequency of the 5 GHz band, this anti-aliasing problem is particularly critical because the front end of these radio frequencies is a signal with a bandwidth of nearly 1 GHz. This means providing a spectrum of hundreds of megahertz for the A / D converter in the receiver link. Included in this signal may be a high-power pulsed radar signal, which will dominate the receiver link.

So far, convergence has become a major trend in the electronics field. In the mobile phone and PDA markets, this means converged handheld terminals, smart phones, wireless PDAs, and multimedia devices, including three wireless technologies: cellular technology, 802.11 Wi-Fi WLAN, and Bluetooth. Many experts predict that the cost-effective aggregation equipment will come out in 2004. This new type of mobile handheld terminal will focus on multimedia applications such as MP3 music and video streaming. To provide a compelling user experience, these new devices must be able to take full advantage of the higher data rates and high-speed WLAN connections provided by the new generation of cellular protocols and infrastructure. Wireless Bluetooth headsets and other types of peripherals will add a lot to the convenience and ease of use of these devices.

Problems with Bluetooth and WLAN coexistence

Figure 7 explains how to use such devices in WLAN hotspots. In this case, the user can communicate over a Voice over IP (VoIP) connection via WLAN or can download MP3 or video streams through the device's 802.11 modem. In addition, the converged devices can also be connected to Bluetooth headsets for dedicated monitoring.

The use case depicted in Figure 7 will soon appear in the market, but users need a coexisting solution to take full advantage of all wireless technologies in this application. Since the Bluetooth and WLAN modems in converged cellular phones / PDA devices operate in the same unlicensed frequency band, they will interfere with each other. In addition, other 802.11 client devices in the area will also compete to access the same WLAN access point that aggregates cellular phones / PDAs.

The only coexistence solution specified in the current Bluetooth standard version 1.0 requires the Bluetooth and WLAN sharing system's Media Access Controller (MAC) function so that other technologies will remain idle during the WLAN or Bluetooth transmission process. After monopolizing the MAC for a predefined period of time, Bluetooth or WLAN will be controlled by other technologies.

In an environment where the traffic on the WLAN is small and there is minimal QoS activation, this MAC time sharing arrangement can not only avoid the problem of coexistence interference between WLAN and Bluetooth, but also provide acceptable performance. In this environment, WLAN access points can implement proactive automatic request protocols to retransmit lost or delayed packets. Unfortunately, with the deployment of advanced energy-saving technologies and the surge in demand for QoS services, the performance in WLAN access point (AP) units will be rapidly reduced.

For example, the coexistence of WLAN and Bluetooth is becoming more and more serious, resulting in 802.11AP unable to sense whether related clients are suffering from non-WLAN interference from Bluetooth devices or cordless phones. Using queuing algorithms or scheduling routines to program APs for applications that require QoS features does not alleviate the problem of in-band interference. Because APs are not aware of interference, they cannot schedule around interference at all.

Even if the AP has the 802.11 automatic response queue (ARQ) function, the fault tolerance of the link can only reach 5%. As we approach and exceed this percentage point, we must increase the packet queue size on the AP so that they can store and reassemble sporadic packets. Multimedia applications that usually require QoS functions (such as high-quality audio or MPEG2 video) quickly deviate from the definition of QoS in the 802.11 standard. As an alternative, ARQ will be removed from the link that requires QoS. In this case, the voice performance will be slightly improved, with an acceptable packet error rate of less than 2%, but the performance of any kind of media stream Are unacceptable.

Remember that in the transmission mode, the WLAN client only uses a small part of the bandwidth of 802.11 WLAN. According to typical rules of thumb, 80% of the client's active WLAN time is used for reception, and only 20% of the time is used for transmission. When transmitting, the client usually sends a short confirmation packet to the AP. The exception to this rule is the file transfer from the client, but these files are always divided into packets of no more than 1,500 bytes during the transfer process and are transferred at the "available bit rate" (ABR).

By applying this information and other characteristics of 802.11 operation to the example of a converged WLAN / Bluetooth PDA listed in Figure 7, it is concluded that WLAN and Bluetooth operations need to be performed simultaneously in an environment where WLANAP is properly loaded. The specific analysis of this state is as follows.

The Bluetooth headset connected to the wireless PDA listed in FIG. 7 has a link bandwidth of at most 700Kbps, and does not carry protocol overhead. If a PDA user plays an MP3 audio stream file from a server on the Internet, then this application will require a Bluetooth bandwidth of about 128Kbps, and the total Bluetooth bandwidth is 700Kbps. The time for Bluetooth signal transmission in the air accounts for 18%. Compared with this, the same application only uses 128Kbps PDAWLAN bandwidth, and the total bandwidth is 11Mbps. In addition, 802.11 operation will involve acknowledged transmission (ACK) while receiving MP3 streams. The number of these ACKs is equivalent to 1/16 of the WLAN bandwidth. In other words, it takes less than 0.1% of the time for the client to perform 802.11 transmission.

If WLAN and Bluetooth transmission block or interfere with each other, then Bluetooth will cause 18% of the time interference to WLAN transmission, because Bluetooth needs to be transmitted in the air and the same length of time. In turn, WLAN transmission will cause less than 1% of the time interference with Bluetooth transmission. The result is that when loading an appropriate number of APs, Bluetooth transmission must be performed and WLAN signals must be received at the same time. In short, the Bluetooth and WLAN functions of the PDA must be running at the same time.

But the question ensues: In a convergence device that uses WLAN and Bluetooth technology, can WLAN continue to receive downloads from the AP, regardless of the operating mode of the device's Bluetooth subsystem? After careful design, planning, and deployment decisions on Bluetooth implementation, the answer is yes. First, designers must take advantage of Bluetooth 1.2's power control (category 3 device) function and Bluetooth's adaptive frequency hopping (AFH). Figure 8 below shows how AFH avoids direct in-band interference with WLAN operation.

If the system is to deploy power control technology, then the Bluetooth power on the LNA in the receiver link will be reduced proportionally so that the sideband energy level falls within the 2.4GHz band without having to consider ACR filtering. It is expected that the Bluetooth signal will reach a propagation loss of -40 to -50 dBm. Thus, the power transmitted by Bluetooth is in the range of -25dBm to -15dBm, so as to maintain a low error rate in the link. Figure 9 explains how power control techniques can reduce spectrum transmission in the Bluetooth channel.

Examining handheld terminal devices with Bluetooth and 802.11, as well as some other operating features, further illustrates the problem of coexistence. In this example, it is assumed that the handheld terminal device has a 0dBm Bluetooth transmitter and an 802.11 receiver, and has one of the following capabilities:

1) Power control technology can provide 20dB isolation between Bluetooth and WLAN.

2) There is 0dB isolation between Bluetooth and 802.11, but the system can disconnect the LNA in the RF receiver link. The system does not have a power control function.

For the sake of simplicity, the content discussed here will be limited to receiver designs using superheterodyne architectures. Figure 10 shows one of the situations in which the receiver can operate. In the first case above, there is 20dB isolation between the device's Bluetooth and WLAN, then the receiver must have at least 15dB digital filtering. In the second case, there is no isolation between Bluetooth and WLAN, so it must have 30dB filtering and digital gain. For the second case, you can also choose to limit the receiver to 802.11 signals greater than approximately -60dBm, which does not require any special filtering.

This example shows that the superheterodyne receiver can achieve 20dB isolation by using power control technology, thereby achieving continuous 802.11 and sorted Bluetooth (collatedBluetooth) operation. If MAC-level time coordination is added between Bluetooth and 802.11 in the system, the impact of WLAN transmission interference on the Bluetooth transmitter will be minimized. Thus, in fact, when there is any traffic load or coverage requirement on the WLAN unit, Bluetooth and WLAN can be operated almost seamlessly and synchronously.

In-band interference and link budget

This section discusses in-band interference and its impact on RF links that limit WLAN. To illustrate this problem, we briefly introduce the interference caused by two 802.11 access points, but the analysis is also applicable to in-band interference caused by Bluetooth, cordless phones, or microwave ovens.

The signal propagation loss of 802.11AP depends on the environment, but in general, the signal loss is a function of the distance between the AP and the user. Under ideal line-of-sight conditions, the signal loss is proportional to the square of the distance (R2). Generally in the actual environment, the signal loss can be expressed as the cube of distance (R3). Under adverse conditions, the signal loss is usually equal to the fourth power of the distance (R4).

In addition, the range of a particular 802.11AP is also a function of several other factors, including the AP's transmit power (typically 20dBm), antenna gain, and the sensitivity of the receiver used for a modulation. In this example, it is assumed that the antenna is a general omnidirectional antenna with a gain of 0dB. More complex modulation schemes require a higher signal-to-noise ratio (SNR) in order to be able to receive 802.11 signals at a certain bit error rate (BER). To achieve higher SNR, the receiver must have higher sensitivity and / or the range of the transmitted signal must be reduced proportionally.

Table 2 shows how the different modulation schemes of 802.11g and 802.11b affect SNR, receiver sensitivity, and signal range. Please note that 802.11b with CCK modulation has the same SNR as 802.11b with PBCC modulation.

It can be seen from the table that if the signal propagation loss is generally R3 in the actual setting, the corresponding range of the 11-Mbps AP using CCK modulation or the 22-Mbps AP using PBCC modulation is approximately 400 feet. Assuming that the general suburban range is about 200 feet, as the deployment of 802.11 becomes more and more dense, the probability of interference between APs in neighboring cells increases. The most unfavorable situation of a single living unit is that two APs in a side-by-side house may be separated by only 10 feet of space and two walls. In a complex apartment-like structure, the interval between two or more APs may be only a wall or a floor, which makes in-band interference face more serious challenges. The width of a general apartment is no more than 100 feet, which is only half the width of a suburban house.

It is worth mentioning that the average data throughput through an 802.11 battery with 22-Mbps AP (using the PBCC modulation scheme developed by TI) is very reasonable. Table 3 shows the average data throughput rate of different modulations under different levels of signal propagation loss. Assume that in most existing settings, the signal loss is usually R3. Most importantly, as can be seen from Table 3, the average data rate of PBCC is almost double that of CCK modulation over the entire battery. PBCC and CCK have the same sensitivity, and therefore have the same range. In addition, as can be seen from these average data rate graphs, when multiple modulation schemes are used in a battery, the throughput can be slightly increased by 5% to 10%. With the help of multiple modulation schemes, customers can be provided with the best data rate and range. PBCC modulated 802.11b has the same SNR.

In-band signal and interference analysis

Figure 11 illustrates how two adjacent APs can cause mutual interference. When two RF signal sources (such as two APs) are placed close together, thermal noise and path loss become the second most important factors to consider because in-band interference will have a major impact on the effective range and data rate of the AP. As shown in the figure, in-band RF interference will cause the AP to fail over most of its coverage area.

Table 4 quantitatively analyzes the in-band interference problems of the two APs shown in FIG. 11. This analysis assumes that no technology (such as power control) has been implemented to alleviate some problems. The data in Table 4 comes from the general urban deployment of two 802.11APs. The transmit power of these two APs is 20dBm, the distance between them is 25 meters (about 75 feet), and their signal propagation loss is R3. The SIR is analyzed according to the distance from each AP to the midpoint of the two. Table 4 shows the SIR at different distances and the data rates supported by each SIR level.

This analysis points to the fatal effects of in-band interference. For example, an AP using PBCC modulation usually has an effective range of more than 135 meters, but in-band interference will reduce its effective range to only 7.5 meters. Moreover, the 802.11gAP with 54Mbps OFDM modulation should have an effective range of nearly 40 meters, but due to the impact of in-band interference, its coverage is limited to 2.5 meters.

Nowadays, because 802.11 WLAN applications are relatively few, and most applications require a small WLAN bandwidth, and can quickly correct errors in the transmission process, so rarely notice the in-band RF from one AP to another interference. However, as WLAN technology becomes more and more popular, more and more high-bandwidth applications that require QoS capability increase in-band interference. In fact, in-band interference caused by 802.11 technology will become more and more serious in high-density offices and residences such as urban residences, tenants' shared apartments and apartments.

Effect of power control on in-band interference

In the past, advanced power control technology was required in mobile devices to reduce power consumption and extend battery life. Now, another advantage of power control stands out. In systems or devices that use 802.11, power control can reduce in-band interference. For example, assuming that the accuracy of the open-loop power supply control is 1 dB, on the same RF channel, the average interference between two APs that are close to each other can be reduced by 6 dB. In smaller 802.11 batteries, power control can further reduce interference.

Table 5 shows the influence of power control technology on the SIR of APs at different distances and the corresponding modulation functions supported by each SIR level.

Even if the signal is still limited by in-band interference, power control technology can reduce the in-band interference by an average of 6dB, which can increase the range of the AP by 25%. In practical applications, as more and more WLANs are deployed, high-bandwidth QoS applications become more and more standardized, and may include power control, automatic frequency selection, and multi-band (2.4GHz and 5.xGHz). Several strategic measures to increase RF channel options.

Anticipated interference issues

In the next few years, as wireless local area networks become more common in residential and office environments, equipment manufacturers must carefully consider two potential issues when designing receivers. The two problems are:

1) The non-WLAN interference caused by the RF source due to the channel being close to the 802.11 frequency band without permission restrictions. This may come from a Bluetooth device, cordless phone, or microwave.

2) In-band interference caused by one 802.11AP or client to another 802.11AP or client. With the widespread application of WLAN technology and its increasing density, this problem will inevitably become more serious.

By following well-thought-out design practices, 802.11 receivers can be developed with appropriate adjacent channel suppression (ACR) functions to overcome the large number of adjacent channel interference (ACI) problems encountered in WLAN deployments. In addition, power control and other strategies can also be used in the design of WLAN receivers and transmitters to greatly improve the data throughput and range performance of APs and clients when in-band RF interference occurs.

In short, those 802.11WLAN equipment vendors that can provide a satisfying and compelling user experience will succeed in the market. Paying attention to the design quality of implementing WLAN chipsets in WLAN equipment will play an important role in ensuring user satisfaction.

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