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Small Cells
Dec 21st, 2013 by Dan Lampie

Small cells are currently the big buzz word in the wireless industry.  Many of the major wireless operators in the US including AT&T, Verizon Wireless, and Sprint have committed to use small cells throughout their networks.  In fact AT&T has committed to deploy 40,000 small cells by the end of 2015, which reveals that wireless operators are serious about this technology.  This begs the question what are small cells and why are they needed?

Macro vs Small Cell in New York City

 

Small cells as its name suggests are smaller cellular base stations.  By smaller this includes physical size, RF coverage area, and cost.  Another term which is used in the same context as small cells is distrusted antenna systems or DAS.  A DAS is made up of a number of small antennas nodes which then connect back via fiber to a cellular base station.  With a small cell all the intelligence is housed within the device, while a DAS node is just a dumb transmitter and receiver and the intelligence is housed at remote location.  DAS technology has been in the marketplace for a couple of years and has been successfully deployed in both outdoor and indoor environments.  Small cells are brand new to the market, and they are gaining popularly as they should be cheaper and simpler to deploy than a DAS.

DAS Node

 

The need for small cells is being driven by the surge in mobile data consumption. The popularity of smartphones and tablets means that people are consuming large amounts of data on their mobile devices.  People are not just using their mobiles devices to surf the web, but they are streaming videos and uploading pictures using applications such as Netflix, YouTube, and Instagram.  While LTE was designed to support these mobile applications, the usage is growing quicker than improvements in wireless efficiency which is making networks congested.

Source: broadband.gov

 

To better understand the situation it is important to look at the capacity of an LTE base station which is called an eNB.  LTE is similar in technology to the 802.11N Wi-Fi standard.  Both use similar modulation schemes and data transmissions technologies.  Many LTE networks in the US use 10MHz LTE carriers using a technology called frequency division duplex, or FDD.  This means that 10MHz of spectrum is used in separate downlink and uplink channels.  This allows for full duplex communication and means the total amount of spectrum that is used is 20MHz.  Wi-Fi along with some forms of LTE use a technology called time division duplex or TDD.  With this technology the downlink and uplink data is interleaved in the time domain using the same channel.

Source: blog.3g4g.co.uk

 

The standard Wi-Fi channel is 20MHz wide which uses the same amount of spectrum as a 10MHz FDD LTE carrier.  While a normal Wi-Fi access point might only serve a few people, an LTE base station has to support hundreds of users in the same bandwidth.  An LTE base station is really a high tech Wi-Fi router with advanced resource and user scheduling technology.  If a hundred people tried streaming videos from the same Wi-Fi router the performance would be mediocre, and the same holds true with LTE.  To improve performance a simple solution is to decrease the number of people using the connection.  While this might seem obvious this is one way cellular operations ensure that their networks do not become overloaded and congested.

For the last twenty years the number of cell sites has been growing while the coverage area of each cell has been shrinking.  The concept is relatively simple and is known as cell splitting.  Instead of having one large cell site which serves an area, if two smaller cell sites are used which serve the same geographical area there will be close to double the capacity.  This concept has been successfully used for a long time, but today there is so much usage in major cities that a cell site is need on every block.  It is impractical due to cost and space requirements to put conventional cellular base stations on every block.  This is why small cells are being utilized.  They allow for a denser deployment as they can be mounted on light poles and sides of buildings instead on towers or rooftops.  Instead of having one conventional cell site every four blocks, now it is possible to have a small cell on every block greatly increasing capacity.

Source: http://www.research.att.com

 

Give that small cells are a new technology there are still many questions that still need to be answered.  Will small cells be economic viable?  Will small cells be reliable?  The big question remains whether small cells are the solution to the explosive mobile data growth that is occurring.  Regardless of the success of small cells, the increase in mobile data consumption will force wireless operators to come up with innovative ways to meet mobile data demands.

Making Sense of Sprint’s Network Vision
Apr 7th, 2012 by Dan Lampie

A little over six months ago, Sprint-Nextel laid out its strategy for revamping its wireless network and called the plan “Network Vision.”  If you have read any of my previous articles about Sprint, you would know that Sprint has not had any real network strategy since purchasing Nextel back in 2005.  Today Sprint still has numerous sites where they have yet to combine their iDEN, CDMA/EVDO, and Clearwire’s WiMax network which has resulted in poor coverage and high maintenance and real estate costs.  Well this is all about to change with Network Vision.  After seven years without any true network plan, Sprint-Nextel has something that actually makes sense.

Sprint Network Vision Tower (Alcatel-Lucent Equipment)

 

Here is a brief overview of what “Network Vision” entails.  The website, http://s4gru.com, has some excellent detailed information on what “Network Vision” really means from a technical perspective.

- Consolidate its cell sites, by removing sites that are not needed.  Sprint currently has 68,000 sites and will reduce this by 44% to eventually remain with 38,000 sites.

* AT&T claims they have 55,000 cell sites so once Network Vision is completed its nationwide coverage will still lag behind that of AT&T.


- Shutting down iDEN and reusing the spectrum to support at least one 800MHz CDMA 1X Advanced carrier.

* Deploying a 1X carrier in the 800MHz spectrum will greatly improve the voice performance along with coverage for Sprint, especially inside buildings.

Frequency plan for new 1X advanced carriers. Source: s4gru.com

 

- Deploying a LTE carrier in a 5x5MHz channel configuration in their 1900MHz (PCS) spectrum.

* LTE is the future and this will give Sprint the opportunity to have a nationwide LTE network.
* 5MHz channels will offer only half the data speeds of the 10MHz channel that Verizon Wireless uses.  Still it will be vastly faster than EVDO with 50ms latency.
* Deploying on the 1900MHz spectrum will mean that Sprint will not have as good coverage or indoor penetration as either Verizon Wireless or AT&T which are using 700MHz.

PCS Band Plan. Source: howardforums.com


- Using Remote Radio Heads (RRH) for their existing CDMA/EVDO network and upcoming LTE network
* RRH moves the base station amplifier from the bottom of the tower to the top of the tower.  This eliminates the attenuation of long runs of coax cable up the tower.  According to this spec sheet from Andrews, 100FT of 1 ¼ coax has a loss of 1.6dB at 1900MHz, or a power loss of 31% at the top of the tower.  Thus going with the RRH solution should yield 30%+ more power output along with a 30% increase in receive power over today’s coax solution.
* This should improve coverage and performance of Sprint’s existing CDMA/EVDO network.  The combination of the 800MHz spectrum and RRH should really help Sprint’s voice coverage.

Sprint is using three RRH per face (1 for 800MHz CDMA, 1 for PCS EVDO, 1 for PCS LTE)

 

At the end of the day Sprint will have a CDMA/EVDO/LTE network, just like Verizon Wireless.  By consolidating its cell sites and turning off iDEN, Sprint will save a ton of money on operating expenses.  It is interesting that Sprint is investing a lot of time and money upgrading CDMA/EVDO instead of just focusing on deploying LTE.  Additionally, with MetroPCS, AT&T, and Verizon Wireless all committing to VOLTE it is interesting that Sprint is planning on deploying CDMA 1X Advanced for voice calls.  Sprint must have believed that its CDMA/EVDO networks could be greatly improved with Network Vision and that both these technologies will be around for some time.  Sprint has been successful at finding ways to monetize its old networks, such as offering Boost Mobile prepaid service over its iDEN network.  As postpaid customers move to LTE, Sprint could offer competitively priced but slower data services overs its CDMA/EVDO network to maximize its investment.

The one element that was left out of Network Vision is Clearwire which Sprint owns 54% of the company.  If Clearwire partnered with Sprint, like Lightsquared attempted before all their GPS interference issues, Clearwire’s network consolidation could save a great deal of money for the small carrier.  Clearwire will be upgrading its network to LTE, but it will be based on TD-LTE technology instead of FDD-LTE that all the other US carriers are using.  Clearwire’s 2.5GHz spectrum limits its usefulness to urban areas and the high cell density needed for good coverage makes network expansion expensive.  Clearwire is hoping to sell extra LTE capacity to the major wireless carriers, but using a different LTE technology and a separate frequency band than everyone else will make this difficult. While Sprint’s issues with Clearwire remain, Network Vision is a huge step in the right direction for Sprint.  One complete it will offer much greater voice coverage, improved EVDO performance, and most importantly bring Sprint into the LTE game.

A single dual band antenna supports all three technologies

Verizon Wireless’ LTE supports IPv6
Apr 3rd, 2011 by Dan Lampie

When Verizon Wireless launched its 4G LTE network last year many were amazed by the speeds, but another important technological advance was forgotten, IPv6.  Verizon’s LTE network assigns a mobile or datacard a NAT’ed IPv4 address and a public IPv6 address.   Verizon Wireless is the first major ISP in the US to jump on the IPv6 bandwagon.  IPv6 has been one of those buzzwords for many years, but in the US the large allocation of IPv4 addresses meant that there was little urgency to move to IPv6.  Verizon Wireless has started the IPv6 revolution and while a limited number of sites IPv6 sites exist, this will soon change in the near future.

External Antennas for 4G LTE and WiMax
Mar 16th, 2011 by Dan Lampie

The easiest way to improve the throughput and reliability of mobile broadband connections is to use external antennas.  This concept has been used extensively in rural areas where the only form of high speed internet is usually a 3G cellular connection.  In the past this meant a directional antenna with potentially a low noise amplifier.  The same concept also applies to 4G technologies such as WiMax and LTE where having a strong receive signal and most importantly a high signal to noise ratio (SNR) is essential for high data throughputs.

LTE SNR vs Throughput. Source: Ericsson

 

While the same general concept external antenna exists for WiMax and LTE there is one major difference, MIMO.  MIMO uses two or more separate antennas to create virtual paths through the air instead of just one path with conventional 3G technologies.  MIMO is extremely important as it uses multipath to greatly increase the downlink throughput.  Currently only WiMax and LTE support MIMO in the downlink and most cellular operations are deploying a 2×2 MIMO solution which means two antennas are used for transmitting at the base station (cell site) and two antennas are used to receive at the mobile device (cell phone).  This means to maximize your WiMax or LTE datacard throughput you must use two antennas.  If you hook up only one external antenna even with the best RF conditions you will be limiting yourself to roughly half the downlink speed.  This means it is extremely important two use two separate external antennas to maximize performance with WiMax and LTE.

Downlink Throughput with 2 Antennas vs 1 Antenna

Signal to Noise Ratio with different antenna configurations

Additionally, MIMO works based on the correlation of the two different receive streams sent by the two different antennas at the base station.  If the receive streams are high correlated that means that the receiver in the datacard cannot differentiate the difference between both streams so there is no increase in data throughput.  The high correlation is one reason why hooking up a single antenna and using a splitter to divide the signal to each antenna port at the mobile device doesn’t achieve an increase in throughput.  The mobile device’s chipset will see that the signal coming into both antenna ports is roughly the same.  This is why it is important to use two separate antennas.  Additionally, to minimize correlation it is important to space the antennas minimally one wavelength (1.4ft for 700MHz) apart.  With 3G technologies you would point your external antenna in the direction of the base station and tweak the position until you received the highest receive power.  With 4G the general concept still applies but to reduce the MIMO correlation of each antenna you might have to point one antenna at a slightly different angle or use a different polarization to achieve maximum throughput.

Dual Yagi pointed directly at cell site. Good recieve signal but does't optimize MIMO correlation for my environment. Use $ software to see correlation or just run speed tests.

One Yagi pointed directly at cell site, the other pointed at a slight angle. Good recieve signal and optimizes MIMO correlation for best downlink throughput.

Both the Pantech UML290 and the LG VL600 LTE datacards on Verizon Wireless 4G LTE support external antennas.  This makes hooking up external antennas a breeze, unlike many of Clear’s and Sprint WiMax cards which require soldering.  Plug two pigtails into the external antenna jacks and you should be all set.  Only one antenna transmits, so if you plan to connect only one antenna make sure you try both ports, or you might not be able to connect to the LTE network.  To receive downlink throughputs of 20Mbps, it is important to have a signal to noise ratio of at least 15dB or higher.  This can be easily checked in the Verizon Wireless VZAccess Utility.

In rural areas achieving this high signal to noise ratio far from the cell will be hard to accomplish, but even at the edge of LTE coverage 5Mbps can be achieved.  It is also important to remember just not the improved data throughputs, but also the lower latency that LTE introduces.  Overall, pairing external antennas with 4G technologies such as LTE can result in a substantial throughput improvement and allow you to maintain a reliable connection even in fringe signal areas.


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