20-06-2012, 12:35 PM
Forward broadcast channels
[attachment=24887]
Every BTS dedicates a significant amount of output power to a pilot channel, which is an unmodulated PN sequence (in other words, spread with Walsh code 0). Each BTS sector in the network is assigned a PN offset in steps of 64 chips. There is no data carried on the forward pilot. With its strong autocorrelation function, the forward pilot allows mobiles to determine system timing and distinguish different BTS's for handoff.
When a mobile is "searching", it is attempting to find pilot signals on the network by tuning to particular radio frequencies, and performing a cross-correlation across all possible PN phases. A strong correlation peak result indicates the proximity of a BTS.
Other forward channels, selected by their Walsh code, carry data from the network to the mobiles. Data consists of network signaling and user traffic. Generally, data to be transmitted is divided into frames of bits. A frame of bits is passed through a convolutional encoder, adding forward error correction redundancy, generating a frame of symbols. These symbols are then spread with the Walsh and PN sequences and transmitted.
BTSs transmit a sync channel spread with Walsh code 32. The sync channel frame is \frac{80}{3} ms long, and its frame boundary is aligned to the pilot. The sync channel continually transmits a single message, the Sync Channel Message, which has a length and content dependent on the P_REV. The message is transmitted 32 bits per frame, encoded to 128 symbols, yielding a rate of 1200 bit/s. The Sync Channel Message contains information about the network, including the PN offset used by the BTS sector.
Once a mobile has found a strong pilot channel, it listens to the sync channel and decodes a Sync Channel Message to develop a highly-accurate synchronization to system time. At this point the mobile knows whether it is roaming, and that it is "in service".
BTSs transmit at least one, and as many as seven, paging channels starting with Walsh code 1. The paging channel frame time is 20 ms, and is time aligned to the IS-95 system (i.e. GPS) 2-second roll-over. There are two possible rates used on the paging channel: 4800 bit/s or 9600 bit/s. Both rates are encoded to 19200 symbols per second.
The paging channel contains signaling messages transmitted from the network to all idle mobiles. A set of messages communicate detailed network overhead to the mobiles, circulating this information while the paging channel is free. The paging channel also carries higher-priority messages dedicated to setting up calls to and from the mobiles.
When a mobile is idle, it is mostly listening to a paging channel. Once a mobile has parsed all the network overhead information, it registers with the network, then optionally enters slotted-mode. Both of these processes are described in more detail below.
Forward traffic channels
The Walsh space not dedicated to broadcast channels on the BTS sector is available for traffic channels. These channels carry the individual voice and data calls supported by IS-95. Like the paging channel, traffic channels have a frame time of 20ms.
Since voice and user data are intermittent, the traffic channels support variable-rate operation. Every 20 ms frame may be transmitted at a different rate, as determined by the service in use (voice or data). P_REV=1 and P_REV=2 supported rate set 1, providing a rate of 1200, 2400, 4800, or 9600 bit/s. P_REV=3 and beyond also provided rate set 2, yielding rates of 1800, 3600, 7200, or 14400 bit/s.
For voice calls, the traffic channel carries frames of vocoder data. A number of different vocoders are defined under IS-95, the earlier of which were limited to rate set 1, and were responsible for some user complaints of poor voice quality. More sophisticated vocoders, taking advantage of modern DSPs and rate set 2, remedied the voice quality situation and are still in wide use in 2005.
The mobile receiving a variable-rate traffic frame does not know the rate at which the frame was transmitted. Typically, the frame is decoded at each possible rate, and using the quality metrics of the Viterbi decoder, the correct result is chosen.
Traffic channels may also carry circuit-switch data calls in IS-95. The variable-rate traffic frames are generated using the IS-95 Radio Link Protocol (RLP). RLP provides a mechanism to improve the performance of the wireless link for data. Where voice calls might tolerate the dropping of occasional 20 ms frames, a data call would have unacceptable performance without RLP.
Under IS-95B P_REV=5, it was possible for a user to use up to seven supplemental "code" (traffic) channels simultaneously to increase the throughput of a data call. Very few mobiles or networks ever provided this feature, which could in theory offer 115200 bit/s to a user.
Block Interleaver
After convolution coding and repetition, symbols are sent to a 20 ms block interleaver, which is a 24 by 16 array.
Capacity
IS-95 and its use of CDMA techniques, like any other communications system, have their throughput limited according to Shannon's theorem. Accordingly, capacity improves with SNR and bandwidth. IS-95 has a fixed bandwidth, but fares well in the digital world because it takes active steps to improve SNR.
With CDMA, signals that are not correlated with the channel of interest (such as other PN offsets from adjacent cellular base stations) appear as noise, and signals carried on other Walsh codes (that are properly time aligned) are essentially removed in the de-spreading process. The variable-rate nature of traffic channels provide lower-rate frames to be transmitted at lower power causing less noise for other signals still to be correctly received. These factors provide an inherently lower noise level than other cellular technologies allowing the IS-95 network to squeeze more users into the same radio spectrum.
Active (slow) power control is also used on the forward traffic channels, where during a call, the mobile sends signaling messages to the network indicating the quality of the signal. The network will control the transmitted power of the traffic channel to keep the signal quality just good enough, thereby keeping the noise level seen by all other users to a minimum.
The receiver also uses the techniques of the rake receiver to improve SNR as well as perform soft handoff.
Layer 2
Once a call is established, a mobile is restricted to using the traffic channel. A frame format is defined in the MAC for the traffic channel that allows the regular voice (vocoder) or data (RLP) bits to be multiplexed with signaling message fragments. The signaling message fragments are pieced together in the LAC, where complete signaling messages are passed on to Layer 3.
List of CDMA2000 networks
India
• Reliance Communications, Tata Teleservices and MTS India are major wireless services providers on CDMA2000 EV-DO. Govt-owned BSNL is also offering EVDO services, though voice services comes with CDMA2000-1X only. MTS India was the world's first mobile operator to deploy EVDO Rev B phase II network in Jaipur. EVDO Rev B Phase II network of MTS India is capable to offer upto 9.8 Mbps (2 x 4.9Mbit/s) downlink using two carriers of 1.25MHz spectrum. Soon Tata Tele also rolled out EVDO Rev B phase I network to deliver 6.2 Mbps (2 x 3.1Mbps) of peak downlink using two carriers of 1.25MHz spectrum. Both operators' EVDO Rev B services are limited to dongle based services and branded as MTS MBlaze Ultra and Tata Docomo Photon Max.
United States
• Alaska Communications Systems
• Alltel
• Cellcom
• Claro Wireless (formerly Verizon Wireless in Puerto Rico)
• Cricket Communications (Leap Wireless)
• C Spire Wireless
• Exit Mobile (Verizon Wireless)
• MetroPCS
• nTelos
• Sprint Nextel
• U.S. Cellular
• Verizon Wireless
• Virgin Mobile (Sprint Nextel)
• Voitel Mobile (Verizon Wireless)
CDMA2000
From Wikipedia, the free encyclopedia
CDMA2000 (also known as IMT Multi Carrier (IMT MC)) is a family of 3G[1] mobile technology standards, which use CDMAchannel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B.[2] All are approved radio interfaces for the ITU's IMT-2000. CDMA2000 has a relatively long technical history and is backward-compatible with its previous 2G iteration IS-95 (cdmaOne). In the United States, CDMA2000 is a registered trademark of the Telecommunications Industry Association (TIA-USA).[3]
•
1X
CDMA2000 1X (IS-2000), also known as 1x and 1xRTT, is the core CDMA2000 wireless air interface standard. The designation "1x", meaning 1 times Radio Transmission Technology, indicates the same radio frequency (RF) bandwidth as IS-95: a duplex pair of 1.25 MHz radio channels. 1xRTT almost doubles the capacity of IS-95 by adding 64 more traffic channels to the forward link, orthogonal to (in quadrature with) the original set of 64. The 1X standard supports packet data speeds of up to 153 kbit/s with real world data transmission averaging 60–100 kbit/s in most commercial applications.[4] IMT-2000 also made changes to the data link layer for greater use of data services, including medium and link access control protocols and QoS. The IS-95 data link layer only provided "best efforts delivery" for data and circuit switched channel for voice (i.e., a voice frame once every 20 ms).
[edit]1xEV-DO
Main article: Evolution-Data Optimized
CDMA2000 1xEV-DO (Evolution-Data Optimized), often abbreviated as EV-DO or EV, is a telecommunications standard for the wireless transmission of data through radio signals, typically for broadband Internet access. It uses multiplexing techniques including code division multiple access (CDMA) as well as time division multiple access (TDMA) to maximize both individual user's throughput and the overall system throughput. It is standardized by 3rd Generation Partnership Project 2 (3GPP2) as part of the CDMA2000 family of standards and has been adopted by many mobile phone service providers around the world – particularly those previously employing CDMA networks. It is also used on the Globalstarsatellite phone network.[5]
U.S. Cellular
U.S. Cellular is a large multi-regional carrier offering service in a number of states. The company uses CDMA3G technology and is deploying LTE4G technology in 2012-2013. Most CDMA coverage is cellular-band (850 MHz), with some PCS (1900 MHz) areas. LTE 4G service is provided in the 700 MHz and 1700 MHz (AWS) bands, expanding into the 850 and 1900 MHz bands in the future.
United States Cellular Corporation, d.b.a.U.S. Cellular (NYSE: USM), owns and operates the sixth largest[1] wireless telecommunications network in the United States, behind Verizon Wireless, AT&T Mobility, Sprint Nextel, T-Mobile USA, and Metro PCS. As of 2011, they serve about 6.1 million customers in 126 markets in 26 U.S. states.[2] The company has its headquarters in Chicago, Illinois.
Customer satisfaction
U.S.Cellular touts its service, having one of the highest customer retention rates among its competitors, as reported by some market research firms.[6]
J. D. Power and Associates continually rates U.S. Cellular as having "Highest in Wireless Call Quality Performance" for several of its coverage regions.[7] The North Central Region (IL,IN, MI, OH, WI) has received the award from 2006-2011.[8]
Consumer Reports does an annual survey of wireless service providers. The company has the highest call quality and network satisfaction of any national carrier as stated by the survey.[9]
Network
Originally, U.S. Cellular used Digital AMPS "TDMA" cell phones in most markets, but the company has completed shifting over to 1xRTT CDMA technology. After the switch, U.S. Cellular has discontinued all analog and TDMA services. In 2009, U.S. Cellular started converting its network to EVDO which offers 3G speeds. U.S. Cellular plans to eventually use GSM based LTE for its future 4G network.[10]
The company offers national 3G coverage through roaming agreements. Native coverage is mainly in the Pacific Northwest, Midwest, parts of the East and New England. Although headquartered in Chicago, U.S. Cellular did not offer service in the Chicago metropolitan area until it acquired territories from PrimeCo Communications between 2002 and 2003, after the formation of Verizon Wireless.[11]
4G Rollout
U.S. Cellular announced that it will start offering 4G coverage to customers beginning in the first quarter of 2012. The company decided to go with LTE for its 4G coverage. The rollout is planned for selected cities in Iowa, Wisconsin, Maine, North Carolina, Texas and Oklahoma. These include some of U.S. Cellular's leading markets such as Milwaukee, Madison and Racine, Wis.; Des Moines, Cedar Rapids and Davenport, Iowa; Portland and Bangor, Maine; and Greenville, N.C.
Comparison of mobile phone standards
Global System for Mobile Communications (GSM, around 80–85 % market share) and IS-95 (around 10–15 % market share) were the two most prevalent 2G mobile communication technologies in 2007.[1] In 3G, the most prevalent technology was UMTS with CDMA-2000 in close contention.
All radio access technologies have to solve the same problems: to divide the finite RF spectrum among multiple users as efficiently as possible. GSM uses TDMA and FDMA for user and cell separation. UMTS, IS-95 and CDMA-2000 use CDMA. WIMAX and LTE use OFDM.
• Time-division multiple access (TDMA) provides multiuser access by chopping up the channel into sequential time slices. Each user of the channel takes turns to transmit and receive signals. In reality, only one person is actually using the channel at a specific moment. This is analogous to time-sharing on a large computer server.
• Frequency-division multiple access (FDMA) provides multiuser access by separating the used frequencies. This is used in GSM to separate cells, which then use TDMA to separate users within the cell.
• Code-division multiple access (CDMA) This uses a digital modulation called spread spectrum which spreads the voice data over a very wide channel in pseudorandom fashion using a user or cell specific pseudorandom code. The receiver undoes the randomization to collect the bits together and produce the original data. As the codes are pseudorandom and selected in such a way as to cause minimal interference to one another, multiple users can talk at the same time and multiple cells can share the same frequency. This causes an added signal noise forcing all users to use more power, which in exchange decreases cell range and battery life.
• Orthogonal Frequency Division Multiple Access (OFDMA) uses bundling of multiple small frequency bands that are orthogonal to one another to provide for separation of users. The users are multiplexed in the frequency domain by allocating specific sub-bands to individual users. This is often enhanced by also performing TDMA and changing the allocation periodically so that different users get different sub-bands at different times.
In theory, CDMA, TDMA and FDMA have exactly the same spectral efficiency but practically, each has its own challenges – power control in the case of CDMA, timing in the case of TDMA, and frequency generation/filtering in the case of FDMA.
For a classic example for understanding the fundamental difference of TDMA and CDMA imagine a cocktail party, where couples are talking to each other in a single room. The room represents the available bandwidth:
TDMA: A speaker takes turns talking to a listener. The speaker talks for a short time and then stops to let another couple talk. There is never more than one speaker talking in the room, no one has to worry about two conversations mixing. The drawback is that it limits the practical number of discussions in the room (bandwidth wise).
CDMA: any speaker can talk at any time; however each uses a different language. Each listener can only understand the language of their partner. As more and more couples talk, the background noise (representing the noise floor) gets louder, but because of the difference in languages, conversations do not mix. The drawback is that at some point, one cannot talk any louder. After this if the noise still rises (more people join the party/cell) the listener cannot make out what the talker is talking about without coming closer to the talker. In effect, CDMA cell coverage decreases as the number of active users increases. This is called cell breathing.
[attachment=24887]
Every BTS dedicates a significant amount of output power to a pilot channel, which is an unmodulated PN sequence (in other words, spread with Walsh code 0). Each BTS sector in the network is assigned a PN offset in steps of 64 chips. There is no data carried on the forward pilot. With its strong autocorrelation function, the forward pilot allows mobiles to determine system timing and distinguish different BTS's for handoff.
When a mobile is "searching", it is attempting to find pilot signals on the network by tuning to particular radio frequencies, and performing a cross-correlation across all possible PN phases. A strong correlation peak result indicates the proximity of a BTS.
Other forward channels, selected by their Walsh code, carry data from the network to the mobiles. Data consists of network signaling and user traffic. Generally, data to be transmitted is divided into frames of bits. A frame of bits is passed through a convolutional encoder, adding forward error correction redundancy, generating a frame of symbols. These symbols are then spread with the Walsh and PN sequences and transmitted.
BTSs transmit a sync channel spread with Walsh code 32. The sync channel frame is \frac{80}{3} ms long, and its frame boundary is aligned to the pilot. The sync channel continually transmits a single message, the Sync Channel Message, which has a length and content dependent on the P_REV. The message is transmitted 32 bits per frame, encoded to 128 symbols, yielding a rate of 1200 bit/s. The Sync Channel Message contains information about the network, including the PN offset used by the BTS sector.
Once a mobile has found a strong pilot channel, it listens to the sync channel and decodes a Sync Channel Message to develop a highly-accurate synchronization to system time. At this point the mobile knows whether it is roaming, and that it is "in service".
BTSs transmit at least one, and as many as seven, paging channels starting with Walsh code 1. The paging channel frame time is 20 ms, and is time aligned to the IS-95 system (i.e. GPS) 2-second roll-over. There are two possible rates used on the paging channel: 4800 bit/s or 9600 bit/s. Both rates are encoded to 19200 symbols per second.
The paging channel contains signaling messages transmitted from the network to all idle mobiles. A set of messages communicate detailed network overhead to the mobiles, circulating this information while the paging channel is free. The paging channel also carries higher-priority messages dedicated to setting up calls to and from the mobiles.
When a mobile is idle, it is mostly listening to a paging channel. Once a mobile has parsed all the network overhead information, it registers with the network, then optionally enters slotted-mode. Both of these processes are described in more detail below.
Forward traffic channels
The Walsh space not dedicated to broadcast channels on the BTS sector is available for traffic channels. These channels carry the individual voice and data calls supported by IS-95. Like the paging channel, traffic channels have a frame time of 20ms.
Since voice and user data are intermittent, the traffic channels support variable-rate operation. Every 20 ms frame may be transmitted at a different rate, as determined by the service in use (voice or data). P_REV=1 and P_REV=2 supported rate set 1, providing a rate of 1200, 2400, 4800, or 9600 bit/s. P_REV=3 and beyond also provided rate set 2, yielding rates of 1800, 3600, 7200, or 14400 bit/s.
For voice calls, the traffic channel carries frames of vocoder data. A number of different vocoders are defined under IS-95, the earlier of which were limited to rate set 1, and were responsible for some user complaints of poor voice quality. More sophisticated vocoders, taking advantage of modern DSPs and rate set 2, remedied the voice quality situation and are still in wide use in 2005.
The mobile receiving a variable-rate traffic frame does not know the rate at which the frame was transmitted. Typically, the frame is decoded at each possible rate, and using the quality metrics of the Viterbi decoder, the correct result is chosen.
Traffic channels may also carry circuit-switch data calls in IS-95. The variable-rate traffic frames are generated using the IS-95 Radio Link Protocol (RLP). RLP provides a mechanism to improve the performance of the wireless link for data. Where voice calls might tolerate the dropping of occasional 20 ms frames, a data call would have unacceptable performance without RLP.
Under IS-95B P_REV=5, it was possible for a user to use up to seven supplemental "code" (traffic) channels simultaneously to increase the throughput of a data call. Very few mobiles or networks ever provided this feature, which could in theory offer 115200 bit/s to a user.
Block Interleaver
After convolution coding and repetition, symbols are sent to a 20 ms block interleaver, which is a 24 by 16 array.
Capacity
IS-95 and its use of CDMA techniques, like any other communications system, have their throughput limited according to Shannon's theorem. Accordingly, capacity improves with SNR and bandwidth. IS-95 has a fixed bandwidth, but fares well in the digital world because it takes active steps to improve SNR.
With CDMA, signals that are not correlated with the channel of interest (such as other PN offsets from adjacent cellular base stations) appear as noise, and signals carried on other Walsh codes (that are properly time aligned) are essentially removed in the de-spreading process. The variable-rate nature of traffic channels provide lower-rate frames to be transmitted at lower power causing less noise for other signals still to be correctly received. These factors provide an inherently lower noise level than other cellular technologies allowing the IS-95 network to squeeze more users into the same radio spectrum.
Active (slow) power control is also used on the forward traffic channels, where during a call, the mobile sends signaling messages to the network indicating the quality of the signal. The network will control the transmitted power of the traffic channel to keep the signal quality just good enough, thereby keeping the noise level seen by all other users to a minimum.
The receiver also uses the techniques of the rake receiver to improve SNR as well as perform soft handoff.
Layer 2
Once a call is established, a mobile is restricted to using the traffic channel. A frame format is defined in the MAC for the traffic channel that allows the regular voice (vocoder) or data (RLP) bits to be multiplexed with signaling message fragments. The signaling message fragments are pieced together in the LAC, where complete signaling messages are passed on to Layer 3.
List of CDMA2000 networks
India
• Reliance Communications, Tata Teleservices and MTS India are major wireless services providers on CDMA2000 EV-DO. Govt-owned BSNL is also offering EVDO services, though voice services comes with CDMA2000-1X only. MTS India was the world's first mobile operator to deploy EVDO Rev B phase II network in Jaipur. EVDO Rev B Phase II network of MTS India is capable to offer upto 9.8 Mbps (2 x 4.9Mbit/s) downlink using two carriers of 1.25MHz spectrum. Soon Tata Tele also rolled out EVDO Rev B phase I network to deliver 6.2 Mbps (2 x 3.1Mbps) of peak downlink using two carriers of 1.25MHz spectrum. Both operators' EVDO Rev B services are limited to dongle based services and branded as MTS MBlaze Ultra and Tata Docomo Photon Max.
United States
• Alaska Communications Systems
• Alltel
• Cellcom
• Claro Wireless (formerly Verizon Wireless in Puerto Rico)
• Cricket Communications (Leap Wireless)
• C Spire Wireless
• Exit Mobile (Verizon Wireless)
• MetroPCS
• nTelos
• Sprint Nextel
• U.S. Cellular
• Verizon Wireless
• Virgin Mobile (Sprint Nextel)
• Voitel Mobile (Verizon Wireless)
CDMA2000
From Wikipedia, the free encyclopedia
CDMA2000 (also known as IMT Multi Carrier (IMT MC)) is a family of 3G[1] mobile technology standards, which use CDMAchannel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B.[2] All are approved radio interfaces for the ITU's IMT-2000. CDMA2000 has a relatively long technical history and is backward-compatible with its previous 2G iteration IS-95 (cdmaOne). In the United States, CDMA2000 is a registered trademark of the Telecommunications Industry Association (TIA-USA).[3]
•
1X
CDMA2000 1X (IS-2000), also known as 1x and 1xRTT, is the core CDMA2000 wireless air interface standard. The designation "1x", meaning 1 times Radio Transmission Technology, indicates the same radio frequency (RF) bandwidth as IS-95: a duplex pair of 1.25 MHz radio channels. 1xRTT almost doubles the capacity of IS-95 by adding 64 more traffic channels to the forward link, orthogonal to (in quadrature with) the original set of 64. The 1X standard supports packet data speeds of up to 153 kbit/s with real world data transmission averaging 60–100 kbit/s in most commercial applications.[4] IMT-2000 also made changes to the data link layer for greater use of data services, including medium and link access control protocols and QoS. The IS-95 data link layer only provided "best efforts delivery" for data and circuit switched channel for voice (i.e., a voice frame once every 20 ms).
[edit]1xEV-DO
Main article: Evolution-Data Optimized
CDMA2000 1xEV-DO (Evolution-Data Optimized), often abbreviated as EV-DO or EV, is a telecommunications standard for the wireless transmission of data through radio signals, typically for broadband Internet access. It uses multiplexing techniques including code division multiple access (CDMA) as well as time division multiple access (TDMA) to maximize both individual user's throughput and the overall system throughput. It is standardized by 3rd Generation Partnership Project 2 (3GPP2) as part of the CDMA2000 family of standards and has been adopted by many mobile phone service providers around the world – particularly those previously employing CDMA networks. It is also used on the Globalstarsatellite phone network.[5]
U.S. Cellular
U.S. Cellular is a large multi-regional carrier offering service in a number of states. The company uses CDMA3G technology and is deploying LTE4G technology in 2012-2013. Most CDMA coverage is cellular-band (850 MHz), with some PCS (1900 MHz) areas. LTE 4G service is provided in the 700 MHz and 1700 MHz (AWS) bands, expanding into the 850 and 1900 MHz bands in the future.
United States Cellular Corporation, d.b.a.U.S. Cellular (NYSE: USM), owns and operates the sixth largest[1] wireless telecommunications network in the United States, behind Verizon Wireless, AT&T Mobility, Sprint Nextel, T-Mobile USA, and Metro PCS. As of 2011, they serve about 6.1 million customers in 126 markets in 26 U.S. states.[2] The company has its headquarters in Chicago, Illinois.
Customer satisfaction
U.S.Cellular touts its service, having one of the highest customer retention rates among its competitors, as reported by some market research firms.[6]
J. D. Power and Associates continually rates U.S. Cellular as having "Highest in Wireless Call Quality Performance" for several of its coverage regions.[7] The North Central Region (IL,IN, MI, OH, WI) has received the award from 2006-2011.[8]
Consumer Reports does an annual survey of wireless service providers. The company has the highest call quality and network satisfaction of any national carrier as stated by the survey.[9]
Network
Originally, U.S. Cellular used Digital AMPS "TDMA" cell phones in most markets, but the company has completed shifting over to 1xRTT CDMA technology. After the switch, U.S. Cellular has discontinued all analog and TDMA services. In 2009, U.S. Cellular started converting its network to EVDO which offers 3G speeds. U.S. Cellular plans to eventually use GSM based LTE for its future 4G network.[10]
The company offers national 3G coverage through roaming agreements. Native coverage is mainly in the Pacific Northwest, Midwest, parts of the East and New England. Although headquartered in Chicago, U.S. Cellular did not offer service in the Chicago metropolitan area until it acquired territories from PrimeCo Communications between 2002 and 2003, after the formation of Verizon Wireless.[11]
4G Rollout
U.S. Cellular announced that it will start offering 4G coverage to customers beginning in the first quarter of 2012. The company decided to go with LTE for its 4G coverage. The rollout is planned for selected cities in Iowa, Wisconsin, Maine, North Carolina, Texas and Oklahoma. These include some of U.S. Cellular's leading markets such as Milwaukee, Madison and Racine, Wis.; Des Moines, Cedar Rapids and Davenport, Iowa; Portland and Bangor, Maine; and Greenville, N.C.
Comparison of mobile phone standards
Global System for Mobile Communications (GSM, around 80–85 % market share) and IS-95 (around 10–15 % market share) were the two most prevalent 2G mobile communication technologies in 2007.[1] In 3G, the most prevalent technology was UMTS with CDMA-2000 in close contention.
All radio access technologies have to solve the same problems: to divide the finite RF spectrum among multiple users as efficiently as possible. GSM uses TDMA and FDMA for user and cell separation. UMTS, IS-95 and CDMA-2000 use CDMA. WIMAX and LTE use OFDM.
• Time-division multiple access (TDMA) provides multiuser access by chopping up the channel into sequential time slices. Each user of the channel takes turns to transmit and receive signals. In reality, only one person is actually using the channel at a specific moment. This is analogous to time-sharing on a large computer server.
• Frequency-division multiple access (FDMA) provides multiuser access by separating the used frequencies. This is used in GSM to separate cells, which then use TDMA to separate users within the cell.
• Code-division multiple access (CDMA) This uses a digital modulation called spread spectrum which spreads the voice data over a very wide channel in pseudorandom fashion using a user or cell specific pseudorandom code. The receiver undoes the randomization to collect the bits together and produce the original data. As the codes are pseudorandom and selected in such a way as to cause minimal interference to one another, multiple users can talk at the same time and multiple cells can share the same frequency. This causes an added signal noise forcing all users to use more power, which in exchange decreases cell range and battery life.
• Orthogonal Frequency Division Multiple Access (OFDMA) uses bundling of multiple small frequency bands that are orthogonal to one another to provide for separation of users. The users are multiplexed in the frequency domain by allocating specific sub-bands to individual users. This is often enhanced by also performing TDMA and changing the allocation periodically so that different users get different sub-bands at different times.
In theory, CDMA, TDMA and FDMA have exactly the same spectral efficiency but practically, each has its own challenges – power control in the case of CDMA, timing in the case of TDMA, and frequency generation/filtering in the case of FDMA.
For a classic example for understanding the fundamental difference of TDMA and CDMA imagine a cocktail party, where couples are talking to each other in a single room. The room represents the available bandwidth:
TDMA: A speaker takes turns talking to a listener. The speaker talks for a short time and then stops to let another couple talk. There is never more than one speaker talking in the room, no one has to worry about two conversations mixing. The drawback is that it limits the practical number of discussions in the room (bandwidth wise).
CDMA: any speaker can talk at any time; however each uses a different language. Each listener can only understand the language of their partner. As more and more couples talk, the background noise (representing the noise floor) gets louder, but because of the difference in languages, conversations do not mix. The drawback is that at some point, one cannot talk any louder. After this if the noise still rises (more people join the party/cell) the listener cannot make out what the talker is talking about without coming closer to the talker. In effect, CDMA cell coverage decreases as the number of active users increases. This is called cell breathing.