CDMA spectral efficiency
Encyclopedia
CDMA spectral efficiency refers to the system spectral efficiency in bit/s/Hz/site or Erlang/MHz/site that can be achieved in a certain CDMA based wireless communication system. CDMA techniques (also known as spread spectrum
) are characterized by a very low link spectral efficiency in (bit/s)/Hz as compared to non-spread spectrum systems, but a comparable system spectral efficiency.
The system spectral efficiency can be improved by radio resource management
techniques, resulting in that a higher number of simultaneous calls and higher data rates can be achieved without adding more radio spectrum or more base station sites. This article is about radio resource management specifically for direct-sequence spread spectrum
(DS-CDMA) based cellular systems.
The terminology used in this article is firstly based on 3GPP2 standards.
CDMA is not expected to be used in 4G
systems, and is not used in pre-4G systems such as LTE
and WiMAX
, but is about to be supplemented by more spectral efficient FDE techniques such as OFDMA
.
should be above a certain level. The latter involves covering a certain area and avoiding outage
due to co-channel interference
, noise
, attenuation caused by long distances, fading
caused by shadowing and multipath
, Doppler shift and other forms of distortion
. The grade of service is also affected by blocking due to admission control, scheduling starvation or inability to guarantee quality of service
that is requested by the users.
There are many ways of increasing the system spectral efficiency. These includes techniques to be implemented at the handset level or at the network level. These techniques include the network optimization, vocoder rate encapsulation and other important techniques. The wide issues faced while deploying these techniques are the cost, up-gradation requirements, hardware and software changes (which includes cell phone compatibility corresponding to the changes) to be made and the agreements to be approved from the telecommunication department.
is obvious. It is hence important to initialize interference cancellation
techniques such as pilot interference cancellation (PIC) and forward link interference cancellation (FLIC) together in the network. Quasi-linear interference cancellation (QLIC) is a technique used for both FLIC and PIC.
Along with the forward link, reverse link interference cancellation is also important. Interference will be reduced and the mobiles will have to transmit less power to get the line of sight
with the base station which will in turn increase the battery life of the mobile.
on the reverse fundamental channel (R-FCH) is the method used for gated transmission in a CDMA communication system. A mobile station (mobile phone
) in the CDMA communication system transmits a reverse pilot signal at a reverse gating rate which is different from a forward gating rate in a gated mode, and a base station transmits a forward pilot signal at the forward gating rate different from the forward gating rate in a gated mode.
When the duty cycle
is 1/8, only 1/8 of the whole power control
groups in one frame are transmitted. This behavior is not present in any other CDMA modes.
Another CDMA invention to provide a device and technique for improving a downlink phone capacity and receiving performance by gating an uplink DPCCH signal in a partial period of the power control
group in a mobile communication system. The test set's support for the R-FCH gating mode is disabled (off) by default.
If the test set's R-FCH gating mode is enabled (on) and the mobile station
(MS) supports the gating mode, the MS will gate the R-FCH/R-Pilot Channel when transmitting at 1/8 rate. This will save around 75% of the power on an average on reverse channels.
characteristics, and spreading factors. The traffic channel may consist of one or more code channels such as fundamental channels and supplemental channels.
The conventional channelization code, Walsh code
does not have enough available bits to cope with maximal use. Therefore, the quasi-orthogonal function (QOF), which can process optimal cross-correlation with Walsh code has been used as a method to get around the limitations of the Walsh Codes.
To enhance the overall capacity in such scenarios, alternative sets of orthogonal functions called the quasi-orthogonal functions (QOF), which possess optimal minimax
cross correlation with Walsh code sets of variable length, have been incorporated in IS-2000.
This method uses aggregation of multiple quasi-orthogonal functions with a smaller constellation alphabet size for a single user with a joint multi-channel detector. This method is compared with the alternative method for enhancing the maximum throughput using aggregation of a smaller number of Walsh functions, but with a higher constellation alphabet size (multi-level modulation).
There have been many industrial and academic discussions on the trade-offs with respect to better methods for increasing capacity in IS-2000/3G systems. QOF introduces high amount of interference in the network channels, thus limiting its benefits.
is one of the solutions, as it provides greater coverage granularity than the traditional 3-sector antenna. Instead of 1 BTS, 2 BTS are used and hence the antennas can be separated from each other by 60 degrees instead of 120 degrees.
, also known as space diversity (micro-diversity as well as macro-diversity, i.e. soft handover
, see below), is any one of several wireless diversity schemes that use two or more antennas to improve the quality and reliability of a wireless link.
Often, especially in urban and indoor environments, there is not a clear line-of-sight (LOS) between transmitter and receiver. Instead the signal is reflected along multiple paths before finally being received. Each of these bounces can introduce phase shifts, time delays, attenuations, and even distortions that can destructively interfere with one another at the aperture of the receiving antenna.
Antenna diversity is especially effective at mitigating these multipath propagation situations. This is because multiple antennas afford a receiver several observations of the same signal. Each antenna will experience a different interference environment. Thus, if one antenna is experiencing a deep fade, it is likely that another has a sufficient signal.
Collectively such a system can provide a robust link. While this is primarily seen in receiving systems (diversity reception), the analog has also proven valuable for transmitting systems (transmit diversity) as well.
Inherently an antenna diversity scheme requires additional hardware and integration versus a single antenna system but due to the commonality of the signal paths a fair amount of circuitry can be shared.
With multiple signals there is a greater processing demand placed on the receiver, which can lead to tighter design requirements of the base station. Typically, however, signal reliability is paramount and using multiple antennas is an effective way to decrease the number of drop-outs and lost connections.
and EVRC-WB.
Enhanced Variable Rate Codec B (EVRC-B
) is a speech codec used by CDMA networks. EVRC-B is an enhancement to EVRC and compresses each 20 milliseconds of 8000 Hz, 16-bit sampled speech input into output frames of one of the four different sizes: Rate 1 - 171 bits, Rate 1/2 - 80 bits, Rate 1/4 - 40 bits, Rate 1/8 - 16 bits.
In addition, there are two zero bit codec frame types: null frames and erasure frames, similar to EVRC. One significant enhancement in EVRC-B is the use of 1/4 rate frames that were not used in EVRC. This provides lower average data rates (ADRs) compared to EVRC, for a given voice quality. The new 4GV Codecs used in CDMA2000 are based on EVRC-B. 4GV is designed to allow service providers to dynamically prioritize voice capacity on their network as required.
The Enhanced Variable Rate Codec
(EVRC) is a speech codec used for cellular telephony in cdma2000 systems. EVRC provides excellent speech quality using variable rate coding with 3 possible rates, 8.55, 4.0 and 0.8 kbit/s. However, the Quality of Service
(QoS) in cdma2000 systems can significantly benefit from a codec which allows tradeoffs between voice quality and network capacity, which cannot be achieved efficiently with the EVRC.
Ec/Io
is a notation used to represent a dimensionless ratio of the average power of a channel, typically the pilot channel, to the total signal power. It is expressed in dB.
can be used effectively to provide coverage solutions.
Spread spectrum
Spread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth...
) are characterized by a very low link spectral efficiency in (bit/s)/Hz as compared to non-spread spectrum systems, but a comparable system spectral efficiency.
The system spectral efficiency can be improved by radio resource management
Radio resource management
Radio resource management is the system level control of co-channel interference and other radio transmission characteristics in wireless communication systems, for example cellular networks, wireless networks and broadcasting systems...
techniques, resulting in that a higher number of simultaneous calls and higher data rates can be achieved without adding more radio spectrum or more base station sites. This article is about radio resource management specifically for direct-sequence spread spectrum
Direct-sequence spread spectrum
In telecommunications, direct-sequence spread spectrum is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals...
(DS-CDMA) based cellular systems.
CDMA based standards
Examples of DS-CDMA based cellular systems are:- the 3GPP3GPPThe 3rd Generation Partnership Project is a collaboration between groups of telecommunications associations, known as the Organizational Partners...
/UMTS 3G3G3G or 3rd generation mobile telecommunications is a generation of standards for mobile phones and mobile telecommunication services fulfilling the International Mobile Telecommunications-2000 specifications by the International Telecommunication Union...
radio interfaces WCDMA, HSDPA and HSUPA used globally. - the 3GPP2 2G2G2G is short for second-generation wireless telephone technology. Second generation 2G cellular telecom networks were commercially launched on the GSM standard in Finland by Radiolinja in 1991...
standard cdmaOne (IS-95) and 3G3G3G or 3rd generation mobile telecommunications is a generation of standards for mobile phones and mobile telecommunication services fulfilling the International Mobile Telecommunications-2000 specifications by the International Telecommunication Union...
standards CDMA2000 1x and 1xEV-DO, used especially in the U.S. and South Korea - the Chinese TD-SCDMATD-SCDMATime Division Synchronous Code Division Multiple Access or UTRA/UMTS-TDD 1.28 Mcps Low Chip Rate , is an air interface found in UMTS mobile telecommunications networks in China as an alternative to W-CDMA. Together with TD-CDMA, it is also known as UMTS-TDD or IMT 2000 Time-Division .The term...
system.
The terminology used in this article is firstly based on 3GPP2 standards.
CDMA is not expected to be used in 4G
4G
In telecommunications, 4G is the fourth generation of cellular wireless standards. It is a successor to the 3G and 2G families of standards. In 2009, the ITU-R organization specified the IMT-Advanced requirements for 4G standards, setting peak speed requirements for 4G service at 100 Mbit/s...
systems, and is not used in pre-4G systems such as LTE
3GPP Long Term Evolution
3GPP Long Term Evolution, usually referred to as LTE, is a standard for wireless communication of high-speed data for mobile phones and data terminals. It is based on the GSM/EDGE and UMTS/HSPA network technologies, increasing the capacity and speed using new modulation techniques...
and WiMAX
WiMAX
WiMAX is a communication technology for wirelessly delivering high-speed Internet service to large geographical areas. The 2005 WiMAX revision provided bit rates up to 40 Mbit/s with the 2011 update up to 1 Gbit/s for fixed stations...
, but is about to be supplemented by more spectral efficient FDE techniques such as OFDMA
OFDMA
Orthogonal Frequency-Division Multiple Access is a multi-user version of the popular Orthogonal frequency-division multiplexing digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users as shown in the illustration below...
.
Introduction to radio resource management
The aim of improving the system spectral efficiency is to utilize the limited radio spectrum resources and radio network infrastructure as efficiently as possible. The objective of radio-resource management is typically to maximize the system spectral efficiency under constraint that the grade of serviceGrade of service
In telecommunication engineering, and in particular teletraffic engineering, the quality of voice service is specified by two measures: the grade of service and the quality of service ....
should be above a certain level. The latter involves covering a certain area and avoiding outage
Outage
Outage may refer to:* Network outage* Sun outage* Power outage* Outing...
due to co-channel interference
Co-channel interference
Co-channel interference or CCI is crosstalk from two different radio transmitters using the same frequency. There can be several causes of co-channel radio interference; four examples are listed here....
, noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
, attenuation caused by long distances, fading
Fading
In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
caused by shadowing and multipath
Multipath
In wireless telecommunications, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths...
, Doppler shift and other forms of distortion
Distortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
. The grade of service is also affected by blocking due to admission control, scheduling starvation or inability to guarantee quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
that is requested by the users.
There are many ways of increasing the system spectral efficiency. These includes techniques to be implemented at the handset level or at the network level. These techniques include the network optimization, vocoder rate encapsulation and other important techniques. The wide issues faced while deploying these techniques are the cost, up-gradation requirements, hardware and software changes (which includes cell phone compatibility corresponding to the changes) to be made and the agreements to be approved from the telecommunication department.
Quasi-Linear Interference Cancellation (QLIC)
Due to its large transmission power, the Common pilot channel (CPICH) probably consumes 15 to 20 percentage of the forward as well as the reverse link capacity. Co-channel interferenceCo-channel interference
Co-channel interference or CCI is crosstalk from two different radio transmitters using the same frequency. There can be several causes of co-channel radio interference; four examples are listed here....
is obvious. It is hence important to initialize interference cancellation
Single Antenna Interference Cancellation
Single Antenna Interference Cancellation is a promising technology to boost the capacity of GSM networks without any needed change in the network....
techniques such as pilot interference cancellation (PIC) and forward link interference cancellation (FLIC) together in the network. Quasi-linear interference cancellation (QLIC) is a technique used for both FLIC and PIC.
Along with the forward link, reverse link interference cancellation is also important. Interference will be reduced and the mobiles will have to transmit less power to get the line of sight
Line-of-sight propagation
Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...
with the base station which will in turn increase the battery life of the mobile.
1/8 rate gating on R-FCH (Reverse fundamental channel)
The 1/8 rate gatingGating (telecommunication)
In telecommunication, the term gating has the following meanings:1. The process of selecting only those portions of a wave between specified time intervals or between specified amplitude limits....
on the reverse fundamental channel (R-FCH) is the method used for gated transmission in a CDMA communication system. A mobile station (mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
) in the CDMA communication system transmits a reverse pilot signal at a reverse gating rate which is different from a forward gating rate in a gated mode, and a base station transmits a forward pilot signal at the forward gating rate different from the forward gating rate in a gated mode.
When the duty cycle
Duty cycle
In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration....
is 1/8, only 1/8 of the whole power control
Power control
Power control, broadly speaking, is the intelligent selection of transmit power in a communication system to achieve good performance within the system. The notion of "good performance" can depend on context and may include optimizing metrics such as link data rate, network capacity, geographic...
groups in one frame are transmitted. This behavior is not present in any other CDMA modes.
Another CDMA invention to provide a device and technique for improving a downlink phone capacity and receiving performance by gating an uplink DPCCH signal in a partial period of the power control
Power control
Power control, broadly speaking, is the intelligent selection of transmit power in a communication system to achieve good performance within the system. The notion of "good performance" can depend on context and may include optimizing metrics such as link data rate, network capacity, geographic...
group in a mobile communication system. The test set's support for the R-FCH gating mode is disabled (off) by default.
If the test set's R-FCH gating mode is enabled (on) and the mobile station
Mobile Station
The mobile station comprises all user equipment and software needed for communication with a mobile network.The mobile station refers to global system connected to the mobile network, i.e. mobile phone or mobile computer connected using a mobile broadband adapter. This is the terminology of 2G...
(MS) supports the gating mode, the MS will gate the R-FCH/R-Pilot Channel when transmitting at 1/8 rate. This will save around 75% of the power on an average on reverse channels.
Radio Configuration
The CDMA radio configuration is defined as a combination of forward and reverse traffic channel transmission formats that are characterized by physical layer parameters such as data rates, error-correction codes, modulationModulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
characteristics, and spreading factors. The traffic channel may consist of one or more code channels such as fundamental channels and supplemental channels.
Quasi-Orthogonal functions (QOF)
The forward link of a 3G code-division multiple-access (CDMA) system may become a limiting factor when the number of users increases maximal capsity.The conventional channelization code, Walsh code
Walsh code
In coding theory, the Walsh–Hadamard code, named after the American mathematician Joseph Leonard Walsh and the French mathematician Jacques Hadamard, is an example of a linear code over a binary alphabet that maps messages of length n to codewords of length 2^n...
does not have enough available bits to cope with maximal use. Therefore, the quasi-orthogonal function (QOF), which can process optimal cross-correlation with Walsh code has been used as a method to get around the limitations of the Walsh Codes.
To enhance the overall capacity in such scenarios, alternative sets of orthogonal functions called the quasi-orthogonal functions (QOF), which possess optimal minimax
Minimax
Minimax is a decision rule used in decision theory, game theory, statistics and philosophy for minimizing the possible loss for a worst case scenario. Alternatively, it can be thought of as maximizing the minimum gain...
cross correlation with Walsh code sets of variable length, have been incorporated in IS-2000.
This method uses aggregation of multiple quasi-orthogonal functions with a smaller constellation alphabet size for a single user with a joint multi-channel detector. This method is compared with the alternative method for enhancing the maximum throughput using aggregation of a smaller number of Walsh functions, but with a higher constellation alphabet size (multi-level modulation).
There have been many industrial and academic discussions on the trade-offs with respect to better methods for increasing capacity in IS-2000/3G systems. QOF introduces high amount of interference in the network channels, thus limiting its benefits.
6 Sectorization
There are some places where the utilization of the site is very high and excess softer handoffs occur. For such sites, a 6-sector antennaSector antenna
A sector antenna is a type of directional microwave antenna with a sector-shaped radiation pattern. The word "sector" is used in the geometric sense; some portion of the circumference of a circle measured in degrees of arc...
is one of the solutions, as it provides greater coverage granularity than the traditional 3-sector antenna. Instead of 1 BTS, 2 BTS are used and hence the antennas can be separated from each other by 60 degrees instead of 120 degrees.
Antenna diversity
Antenna diversityAntenna diversity
Antenna diversity, also known as space diversity, is any one of several wireless diversity schemes that uses two or more antennas to improve the quality and reliability of a wireless link. Often, especially in urban and indoor environments, there is no clear line-of-sight between transmitter and...
, also known as space diversity (micro-diversity as well as macro-diversity, i.e. soft handover
Soft handover
Soft handover or soft handoff refers to a feature used by the CDMA and WCDMA standards, where a cell phone is simultaneously connected to two or more cells during a call. If the sectors are from the same physical cell site , it is referred to as softer handoff...
, see below), is any one of several wireless diversity schemes that use two or more antennas to improve the quality and reliability of a wireless link.
Often, especially in urban and indoor environments, there is not a clear line-of-sight (LOS) between transmitter and receiver. Instead the signal is reflected along multiple paths before finally being received. Each of these bounces can introduce phase shifts, time delays, attenuations, and even distortions that can destructively interfere with one another at the aperture of the receiving antenna.
Antenna diversity is especially effective at mitigating these multipath propagation situations. This is because multiple antennas afford a receiver several observations of the same signal. Each antenna will experience a different interference environment. Thus, if one antenna is experiencing a deep fade, it is likely that another has a sufficient signal.
Collectively such a system can provide a robust link. While this is primarily seen in receiving systems (diversity reception), the analog has also proven valuable for transmitting systems (transmit diversity) as well.
Inherently an antenna diversity scheme requires additional hardware and integration versus a single antenna system but due to the commonality of the signal paths a fair amount of circuitry can be shared.
With multiple signals there is a greater processing demand placed on the receiver, which can lead to tighter design requirements of the base station. Typically, however, signal reliability is paramount and using multiple antennas is an effective way to decrease the number of drop-outs and lost connections.
4th Generation Vocoder (4GV)
Qualcomm’s fourth generation vocoder (4GV) is a suite of voice speech codecs expected to be used in future 4G networks as well CDMA networks, that allows the network operators to dynamically prioritize voice quality to increase network capacity while maintaining voice quality. Currently, the 4GV suite offers EVRC-BEVRC-B
Enhanced Variable Rate Codec B is a speech codec used by CDMA networks. EVRC-B is an enhancement to EVRC and compresses each 20 milliseconds of 8000 Hz, 16-bit sampled speechinput into output frames of one of the four different sizes:...
and EVRC-WB.
Enhanced Variable Rate Codec B (EVRC-B
EVRC-B
Enhanced Variable Rate Codec B is a speech codec used by CDMA networks. EVRC-B is an enhancement to EVRC and compresses each 20 milliseconds of 8000 Hz, 16-bit sampled speechinput into output frames of one of the four different sizes:...
) is a speech codec used by CDMA networks. EVRC-B is an enhancement to EVRC and compresses each 20 milliseconds of 8000 Hz, 16-bit sampled speech input into output frames of one of the four different sizes: Rate 1 - 171 bits, Rate 1/2 - 80 bits, Rate 1/4 - 40 bits, Rate 1/8 - 16 bits.
In addition, there are two zero bit codec frame types: null frames and erasure frames, similar to EVRC. One significant enhancement in EVRC-B is the use of 1/4 rate frames that were not used in EVRC. This provides lower average data rates (ADRs) compared to EVRC, for a given voice quality. The new 4GV Codecs used in CDMA2000 are based on EVRC-B. 4GV is designed to allow service providers to dynamically prioritize voice capacity on their network as required.
The Enhanced Variable Rate Codec
Enhanced Variable Rate Codec
Enhanced Variable Rate CODEC is a speech codec used in CDMA networks. It was developed in 1995 to replace the QCELP vocoder which used more bandwidth on the carrier's network, thus EVRC's primary goal was to offer the mobile carriers more capacity on their networks while not increasing the amount...
(EVRC) is a speech codec used for cellular telephony in cdma2000 systems. EVRC provides excellent speech quality using variable rate coding with 3 possible rates, 8.55, 4.0 and 0.8 kbit/s. However, the Quality of Service
Mobile QoS
Quality of service mechanism controls the performance, reliability and usability of a telecommunications service. Mobile cellular service providers may offer mobile QoS to customers just as the fixed line PSTN services providers and Internet Service Provides may offer QoS...
(QoS) in cdma2000 systems can significantly benefit from a codec which allows tradeoffs between voice quality and network capacity, which cannot be achieved efficiently with the EVRC.
Ec/Io optimization
Higher combined Ec/Io, lower traffic channel Ec/Io is required and more BTS power is conserved.Ec/Io
Eb/No
Eb/N0 is an important parameter in digital communication or data transmission. It is a normalized signal-to-noise ratio measure, also known as the "SNR per bit"...
is a notation used to represent a dimensionless ratio of the average power of a channel, typically the pilot channel, to the total signal power. It is expressed in dB.
Forward and reverse link imbalance
There are some remote places where BTS signal penetrates but reverse link of mobile cannot reach back to the base station.Solution is like reducing base station antenna height, down tilt, select lower gains, etc.Excessive soft handoff areas
There are some areas with more soft handoff than necessary. The handoff parameters has to be reduced to save the base station power.Set higher values of T_ADD and T_DROP, and check the sector coverage should not be too high or too low.Improper RF parameters settings
For best quality decrease the FPCH (Forward Pilot Channel) and FER (Frame Error Rate) settings to 1% and for increase the capacity of highly loaded sites, increase the settings of these parameters to more than 3%.Use repeaters for low utilized sectors
Some sites have very low utilization and due to coverage issue, a new site is required in nearby areas. Instead of a new site, a Cellular repeaterCellular repeater
A cellular repeater, cell phone repeater, or wireless cellular signal booster, a type of bi-directional amplifier as commonly named in the wireless telecommunications industry, is a device used for boosting the cell phone reception to the local area by the usage of a reception antenna, a signal...
can be used effectively to provide coverage solutions.
See also
- Frequency reuse
- List of CDMA terminology
- Mobility managementMobility managementMobility management is one of the major functions of a GSM ora UMTS network that allows mobile phones to work. The aim of mobility management is to track where the subscribers are, allowing calls, SMS and other mobile phone services to be delivered to them....
- Near-far problemNear-far problemThe near-far problem or hearability problem is a situation that is common in wireless communication systems, in particular, CDMA. In some signal jamming techniques, the near-far problem is exploited to disrupt communications.-Technical explanation:...
- Pseudorandom noisePseudorandom noiseIn cryptography, pseudorandom noise is a signal similar to noise which satisfies one or more of the standard tests for statistical randomness....