Basics of Mobile Networks

What are Mobile Networks?

Mobile Networks are the ones designed to allow the user telephone or equipment to move anyplace in the area covered by this network even during a conversation or data connection. Mobile Networks should permit moving at car speed without losing connection. Current mobile network provide voice and data connection even traveling in a high-speed train with more than 300 Km/h speed.

 GSM  UMTS  LTE  NB-LTE  Antenna  SON
 GPRS  HSPA  LTE FDD and TDD Carrier Aggregation  Cell  Digital Dividend
 EDGE  CSFB MIMO  Coverage  MBMS
 GSM-R  VoLTE IMT  Band  eMBMS
 LTE Advanced IMT-2020 (5G)  Small Cells / HetNet  Wifi Offload
 LTE-U  Backhaul network
LTE Advanced Pro


GSM

GSM initially stands for “Group Special Mobile” and for “Global System for Mobile Communication” later on. It wasn’t the first mobile network system but it was the first totally digital and the one providing a total mobile telephony revolution. Before GSM mobile telephony was something exclusive and only available for few people. Mobile telephony started to be available for everyone with GSM. GSM is also named 2G or second generation. First generation refers to previous analog systems.

GPRS

GPRS stands for “General Packet Radio System”. Initially GSM focus was only voice connections. GSM provided data connections but at really reduced speed: 9600 bits per second. GPRS was a new revolution providing speeds up to 144000 bits per second but only from the network to the mobile equipment. From mobile equipments to the network GPRS maintain the same 9600 Bit/S speed but the reason is not the system but battery consumption. GPRS use all time slots not used by voice connections aggregated in one single data connections. This method can only be used in the network because, if mobile equipment use two or three time slots, it needs double or triple power. Because of technology and battery consumption reasons time slot aggregation is not used by the mobile device. GPRS is valid at all as most of data connections are internet connections where most of the data traffic is from the network to the mobile equipment. This technology is named 2.5 generation.

EDGE

EDGE stands for “Enhanced Data Rate for GSM Evolution”. It is the GPRS evolution improving the speed but only next to mobile network antenna. Data modulation (the way how the bits are converted in radio waves traveling through the air) is different. In one way this modulation provide a higher speed but (there is no miracles) it is only valid in shorter distances. GPRS use the same modulation than normal GSM and the coverage is the same. EDGE modulation doesn’t work far from the antenna and it is automatically changed to GPRS. EDGE technology is named 2.75 generation.

GSMR or GSM-R

GSM-R stands for  “GSM Railways”. GSM-R system convert railways companies in telecommunication operators. Anyway this networks are designed specifically for train communication with railway system, both voice and data. It is not designed at all for providing internet service in the train. GSM technology is the only one, neither GPRS nor EDGE. Main advantage is to connect the train to railway systems at high-speed train speed. Classical railway communication technologies don’t allow this connection with more than 250 Km/h speed. Additionally it allows connection when the train is moving in another country using roaming functionality between railway operators.

UMTS

UMTS stands for “Universal Mobile Telecommunication System”. UMTS is named 3G or third generation. This technology is totally different to second generation (even EDGE). Compared with 2G technology UMTS provide video calls and data connections up to 384 thousands of bits per second (KBit/S). Not too much compared with EDGE but EDGE is the limit of 2G and this is the starting point in 3G. 2G didn’t disappear with 3G deployment due to several reasons. More antennas are needed for the same coverage and the real fact is 3G coverage never reached 2G one. Additionally there is no miracles in data communication: more speed more power. Some people disconnect 3G to save battery consumption. No operator has disconnect 2G technology and operators with only 3G license have roaming agreements with operators providing 2G coverage for the cases there is no 3G one. One of the main disadvantage of 3G it is not possible to provide coverage to far and near devices at the same time. When there are mobile equipments connected near the antenna it reduce power retiring coverage to far mobiles. This behaviour has no solution and force to deploy more antennas than 2G. It is estimated it is needed three 3G antennas per 2G antenna.

HSPA

HSPA stands for “High Speed Packet Access”. HSPA improve 3G technology providing data connections up to 14.4 MBit/S even higher than usual fix line ADSL connections. This technology is named 3.5G. There are two options: HSDPA (High Speed Downlink Packet Access) improving connections from mobile network to user device, and HSUPA (High Speed Uplink Packet Access) improving user device to mobile network connection as well. HSUPA provide user device to mobile network speed up to 2 MBit/S. In some way HSPA is the main advantage in front of 2G because original 3G technology has no real advantage compared with EDGE considering UMTS technology is more complex and difficult to deploy. HSPA technology evolved even more reaching 88 Mbps from mobile network to user device and 22 Mbps from user device to mobile network. This evolution was named HSPA+ and 3.75G too.

LTE

LTE stands for “Long Term Evolution”. LTE is the fourth mobile network generation or 4G and, as UMTS, is completely different from 2G or 3G. Currently LTE coverage is limited but  it is increasing really fast. Every time there are more mobile telephones supporting this technology and it is supported, of course, by the most famous: Apple iPhone. Technology used is well-known OFDM (Orthogonal Frequency Division Multiplexing). It appear complex but it has been used previously and it is more simple than 3G. LTE solve the 3G problem with far and near devices. As 2G technology LTE provide a constant coverage. Will LTE replace 2G and 3G technology? Currently all operators are deploying LTE but maintaining 2G and 3G technology and this situation will be maintained during some years. Based on the experience operators will decide which technology to switch off for reducing maintenance costs and network complexity.

LTE FDD and TDD

FDD and TDD are the two options considered in LTE standard. There are reserved bands for FDD (most of them) and other band reserved for TDD. Depending on purchased license operator should use the right technology for the assigned band. For the user there is no different as mobile phones support both. FDD LTE use two different frequencies in the same band for every direction. Communication from mobile network to user device is named “downlink” or “downstream” and it use one frequency. Communication from user device to mobile network is named “uplink” or “upstream” using other frequency in the same band. LTE TDD use the same frequency for both directions. Communication is alternating “downlink” and “uplink” direction. These changes are very fast and they are not perceived and user perception is there is a both direction communication. This technology is more efficient as, most of the times, there are more data traffic from the network to the device and “uplink” direction use to be “Idle”. LTE FDD is wasting “uplink” capacity (same than downlink) and LTE TDD has near the same capacity with half of bandwidth.

This the theory but, actually, LTE TDD has some limitations. Transition between “uplink” and “downlink” direction cannot be decided by one antenna but all antennas simultaneously. If two contiguous antennas are using different directions there is a big interference between them and system doesn’t work. Synchronization between all the antennas for transmitting and receiving at the same time makes this technology less efficient than expected. Additionally, if two operators are using this technology in contiguous bands force them to synchronized. Current situation is most of the operators use LTE FDD. Nowadays there are 448 LTE licenses in the world and only 26 are LTE TDD.

CSFB

CSFB stands for “Circuit Switching Fall Back”. LTE network is a pure data communication one and it can be used only for data connections. For voice connections it is foreseen VoLTE (Voice over LTE) technology but operators are introducing this technology slowly. In the meantime VoLTE is available our mobile phone, when calling, switch to UMTS or GSM network for doing this voice call. Once voice call is finished our device come back to LTE connection. This procedure is automatic and it is done by our mobile phone without any action from our side. In this moment this is the usual mode in LTE mobiles.

VoLTE

VoLTE stands for “Voice over LTE”. This technology provides voice calls on LTE network. VoLTE is not a new technology but the classical VoIP (Voice over IP) adapted to LTE environment. VoLTE use IMS (IP Multimedia Subsystem) and SIP (Session Initiation Protocol) protocol already used widely in fix networks. VoLTE is not widely used as it is necessary a good coverage in 90% of the country in order to obtain good service. Anyway, in case of lack of 4G coverage, there is an automatic switch to 2G or 3G to avoid call drop.

Nowadays there are mobiles phones supporting VoLTE like iPhone 6. For SmartPhones without this functionality operators provide a VoLTE application which could be downloaded by subscriber to start using it. Usual strategy is VoLTE calls don’t spend our data package but these calls are not for free. Operator charge them with the same price than using 2G or 3G. Some operators allow VoLTE calls over Wifi when our mobile is connected to this kind of networks. Operator strategy for VoLTE will influence in the success of this technology. We have to consider applications like WhatsApp have substituted legacy  SMS and MMS and currently WhatsApp is offering VoIP calls similar to VoLTE ones.

LTE Advanced

Before finishing LTE deployment LTE Advanced technology is already available. LTE Advanced is really the only technology fulfilling 4G requirements. Additionally it is the only one presented by 3GPP organization to ITU as fourth generation. Then we have to consider LTE like pre-4G and LTE Advanced as the actual 4G. For improving data connection speed several technologies are used: More efficient modulation, MIMO (Multiple Input Multiple Output) with 2, 3 or 4 different antennas working simultaneously, Carrier aggregation using several bands simultaneously adding the capacity of all of them. Target is to reach 3 GBps speed from mobile network to user device and 1.5 GBps from user device to mobile network.

LTE Advance Pro

It is the newest LTE standard officially named as 4.5 generation to be available during 2016. LTE Advanced Pro will provide downloading speeds over 1 Gbit/S faster than more advanced FTTH fixed networks. In order to reach this speed it includes the last advances in Carrier Aggregation, Beamforming, MIMO and QAM technologies. Anyway these are not the main LTE Advanced Pro characteristics as these technologies were already present in LTE Advanced.

LTE Advance Pro allow the use of unlicensed 5 GHz band for Carrier Aggregation and currently used for Wifi. This feature is called LTE-U, LTE Unlicensed or LAA. For a completely different purpose LTE Advanced Pro include a new device category thoughts for “Internet of the Things” with a reduce speed but low consumption and cost as well.

IMT

IMT stands for “International Mobile Telecommunication”. This name is assigned by ITU (United Nations specialized agency for Information and Communication Technologies). ITU is the highest standardization organization belonging to United Nations. During long time ITU allowed first to GSM project and 3GPP organization afterwards to be in charge of mobile networks standards. Both organizations named technologies like GSM, GPRS, UMTS, HSDPA, LTE, LTE Advanced, etc. However ITU have decided a different names for main technologies: IMT-2000 for third generation or UMTS, IMT-Advanced for fourth generation or LTE (Actually LTE Advanced). The fifth generation is not existing yet but it has already the name: IMT-2020.

LTE-U

LTE-U stands for LTE “Unlicensed”. All operators use “Licensed” bands. They should pay an amount of money to country regulators or governments for the right to use assigned band or part of a band. No other company can use this assigned frequencies in order to assure there is no interference and the operator can provide the service assuring service quality. In parallel with “licensed” bands there are public bands which could be use for anyone like Wifi bands. Classical Wifi band is 2.4 GHz and recently 5 GHz as well. This last band is the one LTE-U would use. As it is a high frequency band it has high-capacity and they think to use complementary to increase capacity in some specific high traffic areas. The problem is this band is a public one and it is not regulated and there is no organization deciding in case of conflicts with Wifi users or any other operator. As service quality cannot be assured currently is being tested but there is no one operator using it.

NB-LTE

NB-LTE or Narrowband LTE is a new technology though for IoT (Internet of the Things) or M2M (Machine To Machine).  We refer to all elements in our life connected to Internet allowing us to control them from anywhere. The most repeated example is the refrigerator but we could be calm as it is not expected our refrigerator purchasing in the supermarket. Some examples of IoT already available are vending, smart-grid, connected car, home automation, etc. In all these cases the needed bandwidth is really small but the most important point is consumption and the target is to be powered by alkaline batteries during months. NB-LTE will be available soon and fulfil both requirements: minimum bandwidth and minimum consumption.

Carrier Aggregation

Carrier Aggregation is a technology increasing dramatically connection bandwidth included both in LTE Advanced and in UMTS with the name DC-HSPA (Dual Channel HSPA) . Network operators try to cover as much bands as possible even with the same technology. High frequency bands have high-capacity but low coverage. In the opposite low-frequency bands have a better coverage but lower capacity. The best would be to connect in several bands at the same time and this is carrier aggregation. Our device connect simultaneously in all the bands available in our position even with different antennas adding the capacity of each connection. This technology is already available in most devices but sometime it is necessary to activate it.

MIMO

MIMO stands for “Multiple Input Multiple Output”. It is called “Spatial Multiplexing” as well. This technology is used not only in LTE but in latest UMTS releases and Wifi as well. Two antennas in our Wifi router (sometimes 3 or 4) indicates MIMO technology is used.

Both transmitting antennas transmit different data in the same frequency or band. At the reception there are two antennas as well receiving the sum of both transmissions but every one has followed a different way. Processing both transmissions it is possible to distinguish which transmission is coming from which transmitting antenna. Adding both transmissions speed is double with the same bandwidth. MIMO is not limited to two antennas and it is possible to use 4, 8 or more.

IMT-2020 (5G)

Currently all operators are focused in LTE or 4G deployment, but most important telecommunications companies are working in next mobile generation which would be the fifth one and it will be applied to mobile networks beyond 2020.  Target is to reach 100 Mbps capacity widely available and even 10 Gbps in special environments. Additionally connection should have around 1 mS latency needed for voice connections using VoLTE technology. Other important point is the possibility of low performance/cost devices or low consumption ones though for “Internet of the Things”. This “white paper” is so extensive and includes future fifth generation characteristics. Every time there are more information about how 5G will be but we could start with “white paper” document in the following link:

https://www.ngmn.org/uploads/media/NGMN_5G_White_Paper_V1_0.pdf

Biggest operators have already plans to start testing 5G technologies but, what these operators are testing?

  • Evolution of OFDM modulation (Orthogonal Frequency Division Multiplexing) used in 4G. These evolutions are F-OFDM (Filtered OFDM), SC FDM (Single Carrier Frequency Division Multiplexing) or OFDMA (Orthogonal Frequency Division Multiple Access).
  • New connection profiles. In 4G there are only normal connections and NB-LTE for IoT devices. In 5G it is considered new connection profiles like messaging, VoIP, etc.
  • New bands from 6 GHz to 60 GHz. If band frequency is 10 times higher its capacity is 10 times bigger as well. It is clear these bands of tens of GHz will have a huge capacity but it is needed to test propagation and coverage.

Operators’ objective is to test these technologies in an environment close similar to the real one.

Antenna

In fact there is not only one antenna but several ones and It used to be named “site”. Every site have a group of antennas in order to give a better coverage to more users. Normally there are three groups of antennas and each one covering 120 degrees around the site. These groups of antennas are really impressive but the reason is to support several bands and being directive. Power use to be less than 100 W for all the antennas, more than enough for mobile application. Sites is the most expensive element in the network as they should pay to the owner of the building or space, provide electricity and maintain sometimes in places difficult to reach. An operator use to have around 10.000 or 20.000 sites depending on country size.

Cell

Cell is the element our mobile phone is connected to. Normally a site use to have three cells covering 120 degrees each one. Additionally, if we have several technologies, each technology has a different cell even covering the same area. The same technology in two different bands (2G in 900 MHz and 1800 MHz) has the same effect: two different cells even sharing the same antenna. Every mobile phone is connected to only one cell but it has information about the nearest one for doing a fast change if necessary.

Coverage

Coverage is more limited by mobile phone power than cell power. Cell power could be tens of watts and it has directive antennas concentrating power. Additionally it is in a high place without any element in front. But mobile phone has 1 Watts or 2 Watts (depending on the band) transmitting in all directions and it use to be in our pocket or inside a bag. Then coverage improvement has to consider the point of view of the mobile phone. Operators have to deploy more sites to be more close to the device in all situation. Our mobile phone in an open space has a better coverage, we can use a headset for talking with our phone this open space. Some mobile phones allow an external antenna we can put in the top of our car when driving. Other point to consider is poser limitation: bands from 700 MHz to 900 Hz have a 2 W limitation in front of higher frequency bands limited to 1 W. Then low-frequency bands have a better coverage (but lower capacity).

Band

Band is a frequency range assigned to a specific use. There are public bands like Wifi one and “licensed” bands. Government or government departments assign the use of licensed bans to one or several companies for a specific use and during a specific time. Normally these companies pay a license for this use. In case of several companies using the same band every company has a frequency range inside the band different to the other companies. The band is identified by the central frequency but it is a frequency range, in case of 900 MHz band the frequency range from 890 MHz to 915 MHz. Inside the band there is one range for the communication from the antenna to the mobile phone (the highest frequency one) and other range for the communication from mobile phone to mobile network (the lowest frequency range). If some operators every range should be splitted in one subrange for each operator. Because of mobile network huge development every time more bands are assigned for this use.

As a general rule any technology works in any band. Country governments select the band and the use for this band specified in the license assigned to the operator. Lower frequency bands provide a better coverage but they have less capacity and, in the opposite, higher frequency bands have worst coverage forcing to deploy more sites but higher capacity in each site. Operators try to receive license for as much bands as possible transmitting in all of them.

Small cells

It is a small system covering a small area but easy to deploy and improving the coverage of mobile network in specific places like enterprises, business areas or airports. Small cells provide 3G and 4G additional coverage and they are deployed as an added value to special customers. As the owner of the building or space receives a benefit there is no charge for the space used and they use the electricity for free.

HetNet

HetNet or “Heterogeneous Networks” is a network which combine normal cells (named “macrocells”) and small cells (named micro or pico-cells depending on cell size). Normal or macro cells know all cells around named “neighbor” cells. When a mobile device changes from one cell to another this change is guided by the network which recommend the best change. This is not the case of small cells where there is no knowledge about the rest of the network. In this case mobile device should decide the best cell to connect. Heterogeneous Networks are more complex to design and this complexity should be managed properly to obtain a real benefit of small cells.

Backhaul network

At the beginning of GSM mobile telephony the basic link to connect network elements was E1 link. E1 link is a 2 MBit/S connection organized in 32 channels of 64 KBit/S each one. With GPRS arrival it was needed TCP/IP connections similar to Internet. Legacy E1 links continued for voice connections and this new TCP/IP connections was used for data connections. This was the beginning of backhaul networks. Backhaul networks are similar to Internet but used internally to connect all network elements. Nowadays VoIP technology is used for voice connections and all connections (voice and data) are TCP/IP through operator backhaul network. All network elements have an internal IP address and communicate to others network elements with TCP/IP protocol.

SON

SON stands for “Self Organizing Networks”. Currently network design and optimization is a very complex task with a lot of people working on it. In one way the different antennas interact between them and one site should be configured with the information of the nearest sites in order to allow mobile device to change from one antenna to the other without dropping calls. In other way people are moving and sometimes they are concentrated in unexpected places. In the office in the morning but in their houses in the night, futbol match, music festivals, etc. Target for SON networks is a self-adapting one with a minimum management effort. SON will be shortly a mandatory feature in the new systems deployed in all mobile networks.

Digital Dividend

The UHF (Ultra High Frequency) band use for TV broadcast cover from 470 MHz to 862 MHz organized in channels every 8 MHz numbered from channel number 21 to channel number 69. Analog TV broadcast for TV companies have reserved channels in the same way operators have reserved frequency ranges. But analog TV have more requirements in the channels assigned to each broadcast. Two different broadcasts cannot use contiguous channels and it is necessary to maintain empty channels in order to avoid interferences. But TV broadcast is moving to digital DVB-T standard. This technology distributes four TV broadcasts in each channel. Additionally there is no problem with interferences and contiguous channel can be used without problems. As UHF band is used in a more efficient way it is not needed such amount of channels and most of the countries are changing regulation for using higher channels for mobile networks. The new bands are 700 MHz, 800 MHz and 850 MHz. This change in the use of higher UHF channels is called digital dividend.

MBMS

MBMS stands for “Mobile Broadcast Multicast Service”. UMTS antennas transmit a TV signal with less resolution than normal TV suitable for a mobile device screen and not spending too much bandwidth. This TV signal is transmitted in broadcast mode and all devices supporting this feature can receive it. There is no answer to the network and all devices receive the same signal. As there is no answer from the device battery consumption is reduced. This technology was delayed several times because of the impact in cell capacity, and it has been moved to LTE.

eMBMS

eMBMS is the evolution of MBMS technology. As LTE has a higher capacity there is less impact in cell capacity when introducing some TV channels. This technology continues in testing phase but it is next to be available. Expected configuration is the broadcast of five TV channels with a total capacity needed of 1 MBit/S. That means every cell have 1 MBit/S capacity less due to this broadcast but no all the cells in a site will transmit eMBMS. Compared with normal digital TV normal definition channel needs 5 MBit/S and high-definition 20 MBit/S.

Wifi Offload

It is another technology tested by operators but not deployed due to different non technical reasons. As most of mobile devices support Wifi connections operator could deploy Wifi access points in high traffic environments adding capacity to the mobile network. Mobile devices would connect to Wifi access point using authentication based on SIM information doing a soft transition from mobile network to Wifi connection increasing connection speed and offloading mobile network. Reasons for not deploying this technology are completely different from technical. In this high traffic areas use to be Wifi operators (sometimes the same than mobile) charging Wifi connections separately. As it is possible to charge this service separately it has no sense to provide it for free.