Spatial Diversity In Wireless Communications

Spatial diversity is one of those fundamental technologies used in wireless communications
(cellular networks, Wi-Fi, satellite communications, and broadcasting) that does not get much exposure. The technology is used to combat fading and improve signal quality, enabling reliable communication links, especially in challenging environments characterized by obstacles, interference, or long propagation distances. Let’s take an introductory look. 

Spatial diversity exploits the spatial dimension of wireless channels by deploying multiple antennas at either the transmitter or receiver, or both. By leveraging spatial separation between antennas, spatial diversity techniques minimize the effects of fading, which result from signal attenuation, reflections, and scattering in multipath propagation environments. Through the simultaneous reception of multiple independent copies of a transmitted signal, spatial diversity enhances the likelihood of receiving at least one strong signal, thus improving the overall reliability of communication links.

 

There are three key methods involved - Selection Diversity, Maximal Ratio  Combining (MRC) and Equal Gain Combining (EGC).

 

Selection Diversity: In selection diversity, multiple antennas are strategically placed to receive the same signal, and the antenna with the highest received signal strength is chosen for further processing. This technique is relatively simple to implement and offers improved diversity gain, particularly in scenarios with moderate to severe fading.

 

Maximal Ratio Combining (MRC): MRC combines signals from multiple antennas with different complex weights, determined based on the channel conditions. By weighting each received signal based on its signal-to-noise ratio (SNR) and combining them coherently, MRC maximizes the received signal power, thereby enhancing the overall signal quality and reliability.

 

Equal Gain Combining (EGC): EGC employs a simpler approach by combining signals from multiple antennas with equal weights. While less complex than MRC, EGC provides diversity gain by mitigating the impact of fading through signal averaging.

 

Spatial diversity offers an effective mechanism to combat fading and enhance signal reliability. Through the strategic deployment of multiple antennas and the application of diverse combining techniques, the technology improves data transmission across a wide range of environments and applications.

FCC Proposed New 6 GHz Wifi Spectrum

On October 2, the FCC proposed WiFi access to the 6 GHz region (5.925-7.125 GHz) in addition to the currently accessible 2.4 GHz and 5 GHz bands of frequencies. 

Currently most WiFi access points operate at either one of those 2.4 GHz of 5 GHz bands. Problems arise as more and more devices are connecting via WiFi. Due to longer wavelengths, 2.4 GHz band signals travel further but with the growing plethora of wireless devices, often suffer from congestion and interference. The 5 GHz band typically operates at higher speeds but does not travel as far due to the shorter wavelengths. 

Opening up the 6 GHz region will provide close to three times what is available in the 2.4 and 5 GHz regions - great for locations where lots of people are connecting at the same time (think college campuses, airport terminals, etc). 

The 6 Ghz frequency region is currently used for point-to-point microwave links and earth-to-space communications along with other data links and there will likely be some opposition. There is a current FCC public commenting period and there will be another vote once the commenting period is over. For details you can read the full FCC Notice of Proposed Rulemaking for ET Docket No. 18-295; GN Docket No. 17-183 linked here.

Calculating Wavelength If Frequency Is Known

I get this question a lot. It’s not exactly phrased this way though. Typically it’s along the lines of “What’s the wavelength of the WiFi signals in my home or office?”

Good question and a pretty simple calculation! I do realize with a quick Google search you can look the value up but….. that takes the fun out of it J

First, let’s define wavelength. Electromagnetic radiation is sinusoidal in nature and wavelength, represented by the Greek letter lambda (λ), is a distance measurement usually expressed in meters. Wavelength is defined as the distance in meters of one sinusoidal cycle as illustrated in the figure below.

Most WiFi signals run at around 2.4 Giga Hertz (GHz) or 2.4 Billion cycles per second!

Now, in you home or office, you’ve likely got a lot of other wireless devices (microwave, ovens, cordless phones, baby monitors, etc) operating in this same 2.4 GHz frequency range. In the WiFi world, the 2.4 GHz WiFi signal range is divided into 11 channels and channels can be selected when setting up a wireless network to avoid other devices transmitting in the same frequency range.

Ok – back to our question – what’s the wavelength? Here’s how we do the calculation:

12.5 cm is approximately 4.92 inches and...... that's your wavelength.

Open WiFi Networks and (Lack Of) Security

I get asked about open WiFi hotspots and if they are secure lots these days. Examples would be certain hotels, restaurants, etc. My short answer - these days many are not secure and.... regardless.... you should always avoid using them. Here's why. 

Most public WiFi hotspots do not encrypt information going back and forth in the air and are not secure. There's lots of free hacking tools that just about anybody can quickly learn to use to get any information you send back and forth when connected to these networks. Here's some good guidelines originally published by the Federal Trade Commission:

Use these tips to tell if a Wi-Fi network is secure:

• If a hotspot doesn’t require a password, it’s not secure.
• If a hotspot asks for a password through the browser simply to grant access, or asks for a password for WEP (wired equivalent privacy) encryption, it’s best to proceed as if it were unsecured.
• A hotspot is secure only if it asks the user to provide a WPA (wifi protected access) password. WPA2 is even more secure than WPA.

Use these tips for a safer Wi-Fi experience:

• When using a Wi-Fi hotspot, only log in or send personal information to websites that you know are fully encrypted. The entire visit to each site should be encrypted – from log in until log out. 
• To determine if a website is encrypted, look for https at the beginning of the web address (the “s” is for secure), and a lock icon at the top or bottom of the browser window. Some websites use encryption only on the sign-in page, but if any part of the session isn’t encrypted, the entire account could be vulnerable. Look for https and the lock icon throughout the site, not just at sign in.
• If you think you’re logged in to an encrypted site but find yourself on an unencrypted page, log out right away.
• Don’t stay permanently signed in to accounts. After using an account, log out.
• Do not use the same password on different websites. It could give someone who gains access to one account access to many accounts.

As a general rule of thumb, an encrypted website protects only the information sent to and from that site. A secure wireless network encrypts all the information sent over it. 

How do you get around the connectivity problem? I recommend using a personal WiFi hotspot with security implemented. You can get yourself a dedicated device like the one I have or most smartphones can be used as a hotspot if you pay an additional monthly fee. Here's more information from AT&T on different personal WiFi hotspot options.

Podcast: The Need for Speed:802.11ac – 5th Generation Gigabit WiFi

Today, Mike Qaissaunee and I recorded an 18 minute and 40 second podcast titled The Need for Speed:802.11ac – 5th Generation Gigabit WiFi.

Rumors are that Apple is planning on incorporating support for the new faster 802.11ac Wi-Fi specification into products this year. In this podcast we discuss the 802.11ac and other wireless specs.

We discuss the following questions:

• So, what’s the deal with this 802.11ac?
• These 802 dot whatever standards - where do they come from?
• So this 802.11ac is considered non-finalized. what does that mean?
• I seem to get interference from things like wireless home phones. I know spectrum is involved.
• So if I set my access point to run at 5GHz, will all my devices work? What do i need to understand to make it work?
• What about range? You mentioned range limitations at 5 GHz.
• Are there any ways to extend the range? I’ve heard about something called MIMO.
• You mentioned 802.11a which is pretty old. Is the use of 5 GHz new?
• When will we see 802.11ac products on the market?
• What kinds of products from Apple? What are people saying?
• What do you mean when you say potentially for the mobile devices?

Along with the Superbowl!

You can listen in the player here:

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If you have iTunes installed you can listen to and subscribe to our podcasts by clicking here.

Reader Question: Is Someone Jamming My WiFi?

I recently received the following email message from a reader:

Hello,

I read some of the information you provided regarding Internet signals getting jammed intentionally and otherwise. Perhaps you can shed light on an issue. When our neighbors are NOT home, I can use a PC with wireless internet (set up in a room of my home facing their home) without ever getting knocked off the internet. When they ARE home, the signal repeatedly is lost. However, A laptop in another part of the home is rarely affected.

So I set up the laptop in the PC room and lost signals (when neighbor was home) on both machines (Provider rep. suggested I do this). Neighbor walks dog or otherwise is not on their property and there is no issue with signals. AND it is random. Usually neighbor comes in for lunch break and the Inet signal is lost.

Home from work and it is lost. I unplug wireless and it comes back. On and off. This is a new development (maybe two months. After the local police informed said neighbors to stop calling 911 to report bogus complaints on us, the signals began to drop. So we believe after police warned them to stop wasting 911 resources, they got a jammer and jam our signals at every opportunity to harass us. (Honestly, this is our first and hopefully last neighbor war. We don't know why they hate us so much but have been informed they hate everyone so we try not to feel too special.)

Question 1 - How can we test or otherwise determine the signals are being jammed (we are sure they are but need proof) and pin point the source? Prove or show great reason why the source is illegal.

Question 2 - How can we protect the signal from getting jammed?

Thanks for your insight.

I've written here in the past about the jamming of cell phone, GPS and Wi-Fi signals. Here's some ideas and possible answers to the reader's two questions.

Question 1 - How can we test or otherwise determine the signals are being jammed (we are sure they are but need proof) and pin point the source? Prove or show great reason why the source is illegal.

The best way to confirm someone is jamming is to use something called a spectrum analyzer. Wireless frequency spectrum analyzers are commonly used measure signals and interference. You could spend thousands of dollars on a full blown analyzer from a company like Agilent or use a 2.4 GHz USB spectrum analyzer from a company like MetaGeek. The company sells a 2.4 GHz analyzer for $99 that comes with software that will run on both PCs and Macs. According to MetaGeek, this analyzer will track all radio activity from any 2.4GHz device including WiFi, cordless phones, microwave ovens, Zigbee and Bluetooth. The software that comes with the device also graphically shows which channels to use and which ones to avoid. Here's more of when you would want to use a device like this from the MetaGeek website:

• If you install, maintain, or troubleshoot access points, find the open channel and minimize the interference.
• If you work with consumers, avoid a revisit by using a Wi-Spy in case they own a microwave or cordless phone.
• If you experience WiFi interference on a regular basis, discover competing access points.
• Conduct site surveys.

You could purchase one of these and, attached to your laptop running on battery, walk around your home looking for jamming/interference signals. If you want to get up unto the higher frequencies where the 802.11n devices have the option of operating (802.11n can use both 2.4 GHz and 5 GHz frequencies.), it will cost you quite a bit more money to measure interference. MetaGeek sells something called the Wi-Spy DBx, a 5GHz analyzer, for $599 that also comes with software.

You may also want to first try KisMac or iStumbler on an Apple machine or NetStumbler on a PC. These applications run on the computer and give you access point information including channels being used. Sometimes just swapping a channel can fix interference problems. For example, if your neighbor is using channel 6 you may want to change your access point to use channel 11.

Question 2 - How can we protect the signal from getting jammed?

If your neighbors are jamming your signal with a well designed jamming device, determining and using an open channel on your wireless access point won't work. If the jamming has been going on for a while chances are the jammer they are using functions only at 2.4 GHz. I'm I think the best thing to try (if you are currently running a 802.11g network) initially would be to switch over to an 802.11n access point and upgrade to 802.11n on your computers. If you have newer computers that may have 802.11n support built in.

You could run the 802.11n network at the higher 5GHz frequency which would be immune to the lower 2.4GHz jamming signals. This would be an inexpensive attempt that would also give you the bonus of much better network bandwidth and immunity from other interference sources (e.g some cordless phones, microwave ovens, etc) in you home.

I'm looking forward to hearing if this works.

FCC 700 MHz Auction Update

Yesterday the Federal Communications Commission (FCC) published a list of 96 accepted and 170 incomplete applications for the upcoming 700 MHz spectrum auction. Google is by far the biggest name on the accepted list. Both AT&T and Verizon currently have a status of Incomplete but are expected to bid. Also currently on the incomplete list are cable company Cox Communications, cell phone technology company Qualcomm and oil company Chevron.

Google is probably the most interesting bidder - a couple of years ago I wrote about Google purchasing $100M of dark fiber here. At the time I was speculating on the company setting up a free national WiFi network.

Recently, I also wrote about the Google Phone.......

Let's think about this...... fiber, WiFi, phones....... Many think, if Google wins the auction, they will sub network construction out to a company experienced in building out wireless networks - examples would be a Verizon or AT&T. I'm have second thoughts and am wondering if we will be seeing telco service trucks driving around with ladders on the roof and a Google decal on the side.

All applications must be completed by January fourth to participate in the auction. A mock auction will be held on January 22 and the real auction held on January 24. Stay tuned.

Open WiFi Access Point Concerns

Yesterday the US House of Representatives passed, in a 409 to 2 vote, the Securing Adolescents From Exploitation-Online Act of 2007

If you currently have an open WiFi access point at home, own a business that provides open access, work at a college offering open wireless access, work at a library, etc you should be aware of this Bill. Here's the Bill Summary from the Library of Congress website:

Securing Adolescents From Exploitation-Online Act of 2007 or the SAFE Act of 2007 - Amends the federal criminal code to expand the reporting requirements of electronic communication and remote computing service providers (service providers) with respect to violations of child sexual exploitation and pornography laws. Requires such service providers, in reporting violations of such laws to the CyberTipline of the National Center for Missing and Exploited Children (Center) to provide:

(1) information on the Internet identity of a suspected sex offender, including the electronic mail address, website address, uniform resource locator, or other identifying information;

(2) the time child pornography was uploaded or discovered;

(3) geographic location information for the offender; and

(4) images of such child pornography.

Requires the Center to forward each report which it receives from a service provider to a designated law enforcement agency.
Requires service providers to preserve images of child pornography for evidentiary purposes.
Authorizes the Center to provide images of child pornography reported to its CyberTipline to service providers to enable such providers to stop further transmissions of of pornographic images.
Grants service providers and the Center immunity from civil claims or criminal charges for complying the requirements of this Act, except for certain intentional or reckless misconduct.

If somebody you know or don't know uses your open network to do something illegal you could be responsible. And..... it's not just WiFi providers that need to be concerned - email account providers, Website hosts, etc will also need to comply.

Initial fines could go as high as $150,000 with additional fines up to $300,000 for repeat offenders. The Bill now goes to the Senate.

WiFi Jamming and Defense

The most popular WiFi standards today, 802.11b and 802.11g, both operate in the 2.4 GHz (Giga Hertz) frequency band and are susceptible to interference from other products operating in the 2.4 GHz band including microwave ovens, Bluetooth devices, baby monitors and cordless telephones. 802.11n, currently in Draft 2.0 standard (with projected standard approval in the fall of 2008), can use both 2.4 GHz and 5 GHz frequencies.

WiFi signal jamming can be intentional using jamming or unintentional using devices that use the same frequency band. For example, if you live in an apartment or condominium with neighbors close by you could be unintentionally jamming their WiFi network every time you use your 2.4 GHz wireless phone.

Dedicated WiFi Jammers are extremely easy to find on the Internet and also relatively easy for someone with electronics and soldering experience to make. One of the more popular home-made jammers is called the Wave Bubble, a self-tuning, wide-bandwidth portable RF (Radio Frequency) jammer that is small enough to fit inside a pack of cigarettes. The Wave Bubble (also refered to as Wavebubble) is tunable and can be used to jam two-band systems such as cell and single-band systems like cordless phones, GPS, WiFi and Bluetooth devices.

Wave Bubble jamming frequency tuning is done by plugging the device into the USB port on a computer and entering the frequencies that are to be blocked. The user can even enter more than one frequency range and the device will advance to the next frequency in memory each time it is powered off and on. Wave Bubble output power is relatively low at .1 Watt for the high frequency bands and .3 Watts for the low frequency bands. Even at these low power levels, with a properly tuned external antenna, the effective range is about a 20 foot radius. With just the internal antenna range is significantly less.

Some of you are probably asking why I'm writing about this stuff - in my opinion it is no secret (try Googling "wireless jammers")....... and....... I have always believed that in any situation, in order to properly defend against an attack, it is important to understand as much as possible about the method of attack.

Now - regarding defense - the Daily Cup of Tech has a good list of tips to help defend against someone jamming your WiFi network. Here it is:

1. Always wire your access points. This way, you cannot be jammed from the server.
2. Wire access points directly back to the central switch and try to avoid bridging your connections. This will help prevent a cascade effect.
3. Do periodic audits of your “air space”.
4. Don’t broadcast that you are using wireless technology. Turn off your SSID on your wireless systems.
5. Do not place server rooms on an external wall. This could allow someone to jam your wireless link without even entering your office.
6. Catalog and label all server room hardware. Periodically audit your hardware lists.
7. Tightly restrict physical access to your server rooms.
8. Train all employees to immediately questions unaccompanied strangers in the office.

Once again - remember - these devices are illegal in the United States.

FCC Rejects 2155-2175MHz WiFi Proposal

On Friday the Federal Communications Commission released an order dismissing a couple of WiFi applications and petitions from M2Z Networks and NetFreeUS. M2Z's FCC petition is linked here, NetFreeUS's is linked here and the FCC rejection is linked here. The companies had proposed building a network using the 2155-2175MHz frequency band.

M2Z's petition seemed to get more press - let's take a look at it. M2Z proposed ad-supported "free" wireless Internet access at 384 Kbps downstream and 128 Kbps upstream.

[Most consumer Internet services provide more downstream (coming to you) bandwidth because the majority of traffic is coming downstream to you. Think about the way you "surf" - a short address typed in browser menu bar goes upstream to server and the then server sends an entire page of website content to you downstream. For this reason these kinds of services are referred to as "asymmetrical" - in fact the "A" in ADSL is short for "Asymmetrical".]

If you wanted more bandwidth or did not want the filters, M2Z proposed an upgrade to a 3 Mbps premium service for an unspecified cost. In return for use of the spectrum, both companies had proposed giving a percentage of revenue to the U.S. government. These petitions had been sitting at the FCC for a while with M2Z's at the FCC for over 16 months.

The FCC rejection document is interesting - it is good to see the level of attention and detail in it from the FCC. According to News.com:

The FCC said it wasn't persuaded that allowing a single company to control the slice of spectrum without first seeking broader comment on how the band should be used would serve the public interest. The regulators concluded that it's preferable to conduct their usual rule-making process to set parameters for the spectrum's use--a move that would begin "shortly," they said.

"Many have suggested that we should auction this spectrum, while still others suggest that due to the high demand for this spectrum we should consider unlicensed use of the band," FCC Chairman Kevin Martin said in a statement. "Each of these proposals has merit, and consideration of either would be inappropriately foreclosed by granting forbearance in this instance."

Regulators commented that the proposed bandwidth was relatively "slow" [I agree - a lot has changed in 16 months and.... it continues to rapidly change] and a consolidation of public interest groups, calling themselves the Media Access Project, had come out very strongly against the M2Z and NetFreeUS petitions. The group especially had First Amendment concerns with regards to content filtering [I agree with this concern also]. Here's a link to Media Access Project's position PDF.

It is unclear what the FCC will do with this spectrum - it could be auctioned or left unlicensed. The rejected companies do have the option of appealing the FCC decision.

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Read Show Notes and listen to Mike Q and my latest Podcast titled Enterprise 2.0 linked here.
Podcasts also free on iTunes.
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Skype Everywhere

There have been a couple of interesting Skype product upgrades/releases over the past few days.

The first is SoonR Talk, an AJAX enabled application that allows Skype to run on the iPhone and other mobile devices.

The second is the release of Skype on the Nokia N800 Internet tablet. The small hand-held device connects to available Wi-Fi networks that we're all finding just about everywhere these days.

Here's a Yahoo News quote from Gartner analyst Elroy Jopling:

"We will see more Skype and similar free Wi-Fi phone services moving into mobile devices in the U.S. and Europe, he said, although Europe could adopt it more quickly. However, he said he expects to see "mobile operators put up as many roadblocks as they can" in both places".

Both of these products allow free Skype voice calls from anywhere to anywhere with Wi-Fi access.

I'll be finishing my Day 2/3 NCTT Conference blog tomorrow - the Conference was EXCELLENT!

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Listen to Mike Q and my latest podcast "One Week with the iPhone" linked here.

Hands-On iPhone Impressions

I was in Providence, RI on the fourth visiting my daughter and we ended up heading over to the Mall downtown for some lunch. There's an Apple store in the mall and - of course - we just had to get some hands-on time with the iPhone. They had a bunch of them out for people to play with and the four of us (12 year old, 15 year old, my wife and I) each ended up with one for about an hour. Here's our impressions:

Overall User Interface - Excellent. Extremely intuitive.I enjoyed the pinch zoom ans stretch features and I love the way the screen re-orients itself as it is rotated back and forth between portrait and landscape. I also love flicking the pages around - much easier than my Mobile PC device scroll bars.

Size - Perfect. Small enough to fit in a pocket and big enough so I can see everything on the screen.

Display - WOW - Incredible. Fingerprints - not an issue.

Multitouch Keyboard - Excellent interface. I've heard some negatives about it but, in our opinion, it is easier to use than a Blackberry keyboard.

Voice Quality - Incredible. You could actually make voice calls right from the demos on the floor. Both ends crystal clear.

Comfort - when held to the ear like a regular phone - excellent. And.... it was cool to the touch on my ear (the heat of my current phone drives me crazy)!

Email - Very Good basic email features and functions. You cannot BCC right now. The intelligent keyboard using predictive techniques is excellent. SSL and encrypted authentication are also supported.

Safari - Very good. I'm a Firefox fan - can't give it an excellent! Seriously, I was very impressed. On the down side - at this time Safari does not support Flash, Java, or QuickTime plug-ins.

iPod - this is a new interface, it is different than the iPod! In portrait orientation there are four buttons at the bottom of the screen that you can "shortcut" with your favorite functions. When you rient the device into landscape you get a "Cover Flow" view showing album covers that you flick as you move through them.

Speed - it starts and shuts down fast. I could not test the AT&T EDGE network because we were on a Wi-Fi connection in the store.

Widgets - these are what Apple calls the applications currently on the iPhone. The 2 Megapixel camera is a step up from any phone I have ever had. Mike has a pic he took with his posted here. There is a weather widget, a stock widget and - as you all konw - a uTube widget. All very easy to use.

It was very interesting to watch people come up and pick out up for the first time. Amazing to watch the looks on their faces and see them focus on figuring out how to use it. It is incredible intuitive - no instructions, nobody showing people how to start using it. Old timers, kids - doesn't matter - everyone I watched picked it up and immediately started using it. It was even more interesting to watch people try and put it down and, once they did, have them come back for more in 5 or 10 minutes.

One father with a couple of young children (I'm guessing around 10 years old) was dragged in by his kids. The kids each picked up one of the demos with him rolling his eyes. Within two minutes he had one in his hands and all I kept hearing was "Wow!" and "Look at this!".

After I put it down for the last time I pulled out my Razor and I could feel myself technologically taking a massive leap backwards...... as I called Cingular in an attempt to break my current phone contract......