3G Cell "breathing" and the need for 4G

[pm]

I thought this article was really interesting because it discusses something that I had no idea occurred which is that the size of a 3G CDMA cell changes in size depending on the number of users. This effect is called "breathing" because the size of a cell increases or decreases depending on users, and neighboring cells are also shifting in terms of geographic size. And while we here at Anandtech (myself included) tend to talk about CDMA as being that network that Verizon and Sprint use, and GSM as being the thing that everyone else uses, 3G "GSM" uses CDMA as well - just a different type.

My mental view of the cell phone network was that you have cell towers which distribute a signal in a fixed circle and then these circles overlapped to create a contiguous network. But apparently these circles of coverage change in size depending on users to avoid interference due to the way that CDMA works.

From this article: http://www.gizmodo.com.au/2011/01/giz-explains-why-your-call-dropped/

As one network engineer told me, sharing a cell tower is like sharing a room with a bunch of people that speak different languages. Different people can hold concurrent conversations, but everyone can understand what they need to – their brains block out the rest of the conversations, because to them, it’s all just gibberish anyway. Just like in this shared room, though, as a tower gets more crowded, the volume starts to rise. The more everyone speaks, the louder one has to talk to be understood. Likewise, the more people that are using a cell tower, the more power each phone needs to be “heard” by the tower. This actually results in a contraction of the cell’s coverage area.

So then theres this article - which I find a bit hard to read actually - talking about the problem of rolling out cell phone service in Bangkok:
http://www.telecomasia.net/blog/content/shrinking-3g-cells-and-need-4g

That when you have a lot of users in a small area, you need to put up more cell "towers" but this is expensive. And the long term solution is to switch everyone to LTE which doesn't have the CDMA interference problem - LTE uses OFDMA... which despite my electrical engineering degree, I totally fail to comprehend beyond the fact that it's not CDMA.

Anyway, the point of this post isn't really to make a point but to mention a phenomenon that I didn't know existed and which changed my view of how cell networks work when there are a lot of users present.

 

[jersiq]

Yeah, the uplink (cell phone to tower) has always been the most fragile piece. Heck, Base Stations transmit at 20-40 Watts full power (usually) without any issue. It's getting that tiny device with a tiny amplifier to reciprocate.
Usually what you do is give the higher frequency to the Base Station, and the lower frequency to the mobile as it's easier on the mobile. However, LTE upturned that in the 700 MhZ spectrum by inverting it for whatever reason.

Also, it's not really a circle. Each site is USUALLY broken up into 3 sectors, and each sector covers 120 degrees of a circle (called sectorization). It's much easier to control the RF of a site this way. What is done in CDMA is the addition of CDMA carriers (don't think companies, think bandwidth) Each CDMA carrier is ~1.25 Mhz wide and can accomodate a certain amount of users. So, if you have a site that reads high uplink noise as measured by the base station, you can add a carrier. So for example, if you had 100 users on a sector sharing 1 carrier and added a carrier, you'd get 50 per carrier. This generally reduces the noise in half, as the mobiles don't really transmit on a carrier they don't belong to.

LTE is pure OFDM in the downlink, however, uplink is SC-FDMA which has some CDMA to it. I think it's either PUSCH or PUCCH (channels for uplink communication) which is closer to CDMA, can't recall which channel it is.

So while cell breathing does exist, it's not as haphazard as the article may suggest. you don't have these incredibly vast fluctuations of 10dB say, which shrink coverage by three miles. There is uplink interference, but generally, most mobiles can overcome it unless it is a REALLY strong source.

 

[deputc26]

Yeah, the uplink (cell phone to tower) has always been the most fragile piece. Heck, Base Stations transmit at 20-40 Watts full power (usually) without any issue. It's getting that tiny device with a tiny amplifier to reciprocate.
Usually what you do is give the higher frequency to the Base Station, and the lower frequency to the mobile as it's easier on the mobile. However, LTE upturned that in the 700 MhZ spectrum by inverting it for whatever reason.

Also, it's not really a circle. Each site is USUALLY broken up into 3 sectors, and each sector covers 120 degrees of a circle (called sectorization). It's much easier to control the RF of a site this way. What is done in CDMA is the addition of CDMA carriers (don't think companies, think bandwidth) Each CDMA carrier is ~1.25 Mhz wide and can accomodate a certain amount of users. So, if you have a site that reads high uplink noise as measured by the base station, you can add a carrier. So for example, if you had 100 users on a sector sharing 1 carrier and added a carrier, you'd get 50 per carrier. This generally reduces the noise in half, as the mobiles don't really transmit on a carrier they don't belong to.

LTE is pure OFDM in the downlink, however, uplink is SC-FDMA which has some CDMA to it. I think it's either PUSCH or PUCCH (channels for uplink communication) which is closer to CDMA, can't recall which channel it is.

So while cell breathing does exist, it's not as haphazard as the article may suggest. you don't have these incredibly vast fluctuations of 10dB say, which shrink coverage by three miles. There is uplink interference, but generally, most mobiles can overcome it unless it is a REALLY strong source.

This is why I love AT forums.

 

[ChAoTiCpInOy]

This is amazing!

 

[corkyg]

It explains what happens when I visit Pasadena, CA, on block east of Orange Grove, and the signal strength varies from zero to 2 bars, and 3G downshifts to 1XRTT, and even that is a problem. I can go one block further east and get 3G at 4 bars. I have noticed this for the last few years with Sprint, VZF, and ATT.

 

[dagamer34]

Yeah, the uplink (cell phone to tower) has always been the most fragile piece. Heck, Base Stations transmit at 20-40 Watts full power (usually) without any issue. It's getting that tiny device with a tiny amplifier to reciprocate.
Usually what you do is give the higher frequency to the Base Station, and the lower frequency to the mobile as it's easier on the mobile. However, LTE upturned that in the 700 MhZ spectrum by inverting it for whatever reason.

Also, it's not really a circle. Each site is USUALLY broken up into 3 sectors, and each sector covers 120 degrees of a circle (called sectorization). It's much easier to control the RF of a site this way. What is done in CDMA is the addition of CDMA carriers (don't think companies, think bandwidth) Each CDMA carrier is ~1.25 Mhz wide and can accomodate a certain amount of users. So, if you have a site that reads high uplink noise as measured by the base station, you can add a carrier. So for example, if you had 100 users on a sector sharing 1 carrier and added a carrier, you'd get 50 per carrier. This generally reduces the noise in half, as the mobiles don't really transmit on a carrier they don't belong to.

LTE is pure OFDM in the downlink, however, uplink is SC-FDMA which has some CDMA to it. I think it's either PUSCH or PUCCH (channels for uplink communication) which is closer to CDMA, can't recall which channel it is.

So while cell breathing does exist, it's not as haphazard as the article may suggest. you don't have these incredibly vast fluctuations of 10dB say, which shrink coverage by three miles. There is uplink interference, but generally, most mobiles can overcome it unless it is a REALLY strong source.

Frequencies were inverted in the 700Mhz spectrum to avoid interference with Channel 51 I believe, so they gave the phone the higher frequency range instead of the lower one.

 

[soccerballtux]

It explains what happens when I visit Pasadena, CA, on block east of Orange Grove, and the signal strength varies from zero to 2 bars, and 3G downshifts to 1XRTT, and even that is a problem. I can go one block further east and get 3G at 4 bars. I have noticed this for the last few years with Sprint, VZF, and ATT.

that could just be a crappy PRL that they haven't optimized yet for that location.

 

[gsaldivar]

Yeah, the uplink (cell phone to tower) has always been the most fragile piece. Heck, Base Stations transmit at 20-40 Watts full power (usually) without any issue. It's getting that tiny device with a tiny amplifier to reciprocate.
Usually what you do is give the higher frequency to the Base Station, and the lower frequency to the mobile as it's easier on the mobile. However, LTE upturned that in the 700 MhZ spectrum by inverting it for whatever reason.

Also, it's not really a circle. Each site is USUALLY broken up into 3 sectors, and each sector covers 120 degrees of a circle (called sectorization). It's much easier to control the RF of a site this way. What is done in CDMA is the addition of CDMA carriers (don't think companies, think bandwidth) Each CDMA carrier is ~1.25 Mhz wide and can accomodate a certain amount of users. So, if you have a site that reads high uplink noise as measured by the base station, you can add a carrier. So for example, if you had 100 users on a sector sharing 1 carrier and added a carrier, you'd get 50 per carrier. This generally reduces the noise in half, as the mobiles don't really transmit on a carrier they don't belong to.

LTE is pure OFDM in the downlink, however, uplink is SC-FDMA which has some CDMA to it. I think it's either PUSCH or PUCCH (channels for uplink communication) which is closer to CDMA, can't recall which channel it is.

So while cell breathing does exist, it's not as haphazard as the article may suggest. you don't have these incredibly vast fluctuations of 10dB say, which shrink coverage by three miles. There is uplink interference, but generally, most mobiles can overcome it unless it is a REALLY strong source.

This person knows their stuff. I wish I could upvote this post.

 

[pm]

Yeah, the uplink (cell phone to tower) has always been the most fragile piece. Heck, Base Stations transmit at 20-40 Watts full power (usually) without any issue. It's getting that tiny device with a tiny amplifier to reciprocate.
Usually what you do is give the higher frequency to the Base Station, and the lower frequency to the mobile as it's easier on the mobile. However, LTE upturned that in the 700 MhZ spectrum by inverting it for whatever reason.

Also, it's not really a circle. Each site is USUALLY broken up into 3 sectors, and each sector covers 120 degrees of a circle (called sectorization). It's much easier to control the RF of a site this way. What is done in CDMA is the addition of CDMA carriers (don't think companies, think bandwidth) Each CDMA carrier is ~1.25 Mhz wide and can accomodate a certain amount of users. So, if you have a site that reads high uplink noise as measured by the base station, you can add a carrier. So for example, if you had 100 users on a sector sharing 1 carrier and added a carrier, you'd get 50 per carrier. This generally reduces the noise in half, as the mobiles don't really transmit on a carrier they don't belong to.

LTE is pure OFDM in the downlink, however, uplink is SC-FDMA which has some CDMA to it. I think it's either PUSCH or PUCCH (channels for uplink communication) which is closer to CDMA, can't recall which channel it is.

So while cell breathing does exist, it's not as haphazard as the article may suggest. you don't have these incredibly vast fluctuations of 10dB say, which shrink coverage by three miles. There is uplink interference, but generally, most mobiles can overcome it unless it is a REALLY strong source.

Thanks for the explanation. I really appreciate it.