All about CDMA
All about CDMA About All about CDMA How exactly does CDMA work ? The following was originally written in 1990 as an overview of CDMA principles of operation of the system proposed by QUALCOMM Incorporated to the TIA which has been subsequently standardized as IS-95 CDMA Principles Three basic types of multiple access techniques exist: Frequency Division Multiple Access (FDMA,) Time Division Multiple Access (TDMA,) and Code Division Multiple Access (CDMA) With FDMA, a channel is a relatively narrow band in the frequency domain into which a signal's transmission power is concentrated. Different signals are assigned different frequency channels Interference to/from adjacent channels is limited by the use of band-pass filters which only pass signal energy within the specified narrow frequency band The analog FM cellular system uses FDMA With TDMA, a "channel" consists of a time slot in a periodic train of time intervals making up a frame. A given signal's energy is confined to one of these time slots Adjacent channel interference is limited by use of a time gate that only passes signal energy that is received at the proper time Some systems use a combination of FDMA and TDMA The TIA IS-54 digital cellular standard uses FDMA channels of 30 kHz in bandwidth that are subdivided into six time slots for TDMA, two of which are required for each call. The GSM and DCS-1800 systems use 200 kHz bandwidth channels with 8 TDMA calls per channel With CDMA, each signal consists of a carrier modulated by a different high speed binary sequence, spreading the spectrum of the waveform All signals in the multiple access share the same wideband frequency channel If one looks at CDMA in either the frequency or the time domain, the multiple access signals appear to be on top of each other. The signals are separated in the receivers by using a "correlator" which only accepts signal energy from the selected binary sequence All the other signals contribute only to the noise The signal-to-interference ratio is then determined by the desired signal's power and the total power of other signals The signal-to-interference ratio is improved by the ratio of the spread bandwidth to the information bit rate. This is known as processing gain This article describes the common air interface for a system that uses CDMA to provide a very high capacity digital cellular telephone system The system has been standardized as TIA standard IS-95 In the cellular radio frequency reuse concept, interference is accepted but controlled with the goal of increasing system capacity. CDMA does this effectively because it is inherently an excellent anti-interference waveform CDMA frequency reuse efficiency is determined by the small decrease in signal-to-noise ratio caused by system users in neighboring cells CDMA frequency reuse efficiency is approximately 2/3 compared to 1/7 for narrow band FDMA systems A combination of open loop and closed loop power control (through measurements of the received power at the mobile station and the base station) is used to command the mobile station to make power adjustments so as to maintain a power level for adequate performance. This minimizes interference to other users, helps to overcome fading, and conserves battery power in the mobile station CDMA System Capacity Determinants ; The major parameters that determine the CDMA digital cellular system capacity are the processing gain, the required Eb/N0, the voice duty cycle, the frequency reuse efficiency, and the number of sectors in the cell-site antenna Eb/N0, the ratio of energy per bit to noise power spectral density, is the standard figure of merit by which digital modulation and coding schemes are compared It is directly analogous to carrier-to-noise ratio (C/N) for analog FM modulation. Because of the efficient modulation technique, the powerful error correction coding and the multiple use of diversity, an Eb/N0 of only about 75 dB is required for good performance, even in a dynamic multipath environment This can be compared directly with the required carrier to noise C/N of 18 dB for analog FM operating under the same conditions The power control system (described below) sets transmitter power so that the received Eb/N0 is no greater than required for good link performance. Voice Activity Detection In a typical full duplex two-way voice conversation the duty cycle of each voice is only 35% With the CDMA system, the transmitted data rate is reduced when there is no speech activity, allowing transmitter power to be reduced, substantially reducing interference to other users Since the level of other user interference determines capacity, the capacity is increased by approximately a factor of 2 to 25. Frequency Reuse Efficiency When directional cell-site antennas are used (consider an ideal 120 deg sector antenna, for example), the interference realized is divided by three on the average (it only looks in the direction of 1/3 of the mobile stations) The capacity supportable by the total system is therefore increased by nearly a factor of three Soft Handoff The capacity of the system is limited by the capacity of the reverse link because the forward link is slightly advantaged by the use of orthogonal signaling described below The above system parameters and capabilities combine in the following equation for the approximate capacity of the reverse link: N = W/R * 1/Eb/No * f * g * 1/v where N is the number of calls per sectored cell, W is the system bandwidth (1. 23 MHz), R is the information transmission rate (9600 bps), Eb/No is the ratio of energy per bit to noise power spectral density (75 dB), f is the frequency reuse efficiency (2/3), g is the sectorization gain (3 for 120 deg sectors), and v is the voice duty cycle (1/2) The resulting capacity is 91 calls per cell Note that this is the capacity of a single 1. 23 MHz radio channel devoted to CDMA The values used in the above equation are dependent on the characteristics of the particular environment in which the system is installed and on the quality of the base station antennas used Some environments and installations may achieve higher capacity while some may achieve lower capacity The capacity for the above calculation is about 16 times greater than the capacity of the AMPS system (or the GSM system) operating in the same enviroment with the same antennas. In actual practice, capacity between 10-15 times that of AMPS has been repeatably been demonstrated in very elaborate field -*test*-('")s CDMA Forward Link Waveform Design This IS-95 standard specifies a forward link CDMA waveform design that uses a combination of frequency division, pseudo-random code division and orthogonal signal multiple access techniques Pseudo-random noise (PN) binary codes are used to distinguish signals received at a mobile station from different base stations All CDMA signals in the system share a quadrature pair of PN codes. Signals from different cells and sectors are distinguished by time offsets from the basic code This relies on the property of PN codes that the auto-correlation, when averaged over a few bit times, averages to zero for all time offsets greater than a single code chip time (approximately 1 microsec) The PN codes used are generated by linear shift registers that produce a code with a period of 32768 chips The PN chip rate is 1. 2288 MHz, or exactly 128 times the 9600 bps information transmission rate Two codes are generated, one for each of two quadrature carriers, resulting in quadriphase PN modulation To avoid confusion between the system bandwidth, the PN chip rate and the frequency assignment spacing, note that the PN chip rate is exactly 12288 MHz, the frequency assignment spacing for two adjacent CDMA carriers is exactly 1. 23 MHz (because we use a multiple of 30 kHz,) and that the 3 dB bandwidth is also 123 MHz Frequently, this is rounded off to 125 MHz. The signals are bandlimited by a digital filter that provides a very sharp frequency roll-off, resulting in a nearly square spectral shape that is 123 MHz wide at the 3 dB point The information to be transmitted is convolutionally encoded to provide the capability of error detection and correction at the receiver The code used has a constraint length (encoder memory) of nine and a code rate of one-half (two encoded binary symbols are produced per information bit). The encoded symbols are interleaved to combat fast fading To provide communications privacy, each data channel is scrambled with a user addressed long code PN sequence Thus, a "channel" in the forward link of the specified CDMA system consists of a signal centered on an assigned radio channel frequency, quadriphase modulated by a pair of PN codes with an assigned time offset, biphase modulated by an assigned orthogonal Walsh function, and biphase modulated by the encoded, interleaved, and scrambled digital information signal An important aspect of the forward link waveform design is the use of the pilot signal. The Pilot Channel signal is unmodulated by information and utilizes the zero Walsh function (which consists of 64 zeroes) Thus, the signal simply consists of the basic PN codes The mobile station can obtain synchronization with the nearest base station without prior knowledge of the identity of the base station by searching out the entire length of the PN code The strongest signal's time offset corresponds to time offset of the nearest base station's PN code. After synchronization, the pilot signal is used as a coherent carrier phase reference for demodulation of the other signals from this base station The CDMA reverse link also employs PN modulation using the same 32768 length binary sequences that are used for the forward link Here, however, a fixed code phase offset is used Signals from different mobile stations are distinguished by the use of a very long (2**42 - 1) PN sequence with a user address determined time offset. Because every possible time offset is a valid address, an extremely large address space is provided This also inherently provides a reasonably high level of privacy The transmitted digital information is convolutionally encoded using a rate 1/3 code (three encoded binary symbols per information bit) of constraint length nine The encoded information is then interleaved over a 20 msec interval. The interleaved information is then grouped in six symbol groups (or code words) These code words are used to select one of 64 different orthogonal Walsh functions for transmission The Walsh function "chips" are combined with the long and short PN codes Note that this use of the Walsh function is different than on the forward link. On the forward link, the Walsh function is determined by the mobile station's assigned channel while on the reverse link the Walsh function is determined by the information being transmitted A "channel" on the reverse link of the specified CDMA system consists of a signal centered on an assigned radio channel frequency, quadriphase modulated by a pair of PN codes, biphase modulated by a long PN code with address determined code phase, and biphase modulated by the Walsh encoded, interleaved and convolutionally encoded digital information signal Power Control In a direct sequence CDMA system, maximum capacity is achieved when the Eb/N0 for every user is at the minimum needed to support "acceptable" channel performance Since N0 (noise spectral density) is generated almost entirely by "other" user interference, it is necessary that all signals arrive at, or near the same level of power at the CDMA receiver. This is achieved in the mobile propagation environment by providing dynamic power control for both the mobile station and base station transmitters Reverse Link Power Control Unlike the forward channel, signals arriving at the base station do not fade together, and can vary with respect to one another over a much greater dynamic range (e g 80 dB) thus the power control process for the reverse channel is much different than that used for the forward channel It consists of two elements; open loop estimation of transmit power by the mobile station and closed loop correction of the errors in this estimate by the base station. Open Loop Estimation by the Mobile Station Each mobile station estimates the path loss from the base station to the mobile station by measuring the total power received in the assigned CDMA channel By monitoring total power, rather than using a demodulated signal such as the pilot, this measure can be made rapidly without any knowledge of timing, base station ID, or path conditions Based on this measure, the mobile station's transmitted power is adjusted to match the estimated path loss, so as to arrive at the base station at a predetermined level All mobile stations use the same process, all attempting to arrive with equal power at the base station. Closed Loop Correction by the Base Station Uncontrolled differences in the forward and reverse channels, such as opposite fading which can occur due to their frequency difference, plus mismatches in the mobile station's receive and transmit chains, cannot be estimated by the mobile To reduce these smaller errors, each mobile station corrects its transmit power with information supplied by the base station, via low rate data inserted into each mobiles forward channel The base station derives the correction information by monitoring the reverse channel quality of each mobile, compares this measure to a threshold, and requests either an increase or a decrease, depending on the result In this manner the base station maintains each reverse channel, and thus all reverse channels, at the minimum received power needed to to provide RacceptableS performance. System Timing All digital transmissions are referenced to a common, system-wide time standard which is traceable to and synchronous with Coordinated Universal Time (UTC) As an example, the system could use the GPS time scale, which is distributed world wide with 24 hour per day availability The difference between GPS and UTC is an integer number of seconds, specifically the number of leap second corrections added to UTC since Jan 6, 1980 System Time keeps track of leap second corrections to UTC but does not use these corrections for physical adjustments to the system time clocks. System Time is distributed to and (except for leap second offsets) maintained at every Base Station to within 11 microsecond of UTC Each Base Station contains its own precision frequency standard in the event that time synchronization to other cells is ...
All about CDMA About All about CDMA How exactly does CDMA work ? The following was originally written in 1990 as an overview of CDMA principles of operation of the system proposed by QUALCOMM Incorporated to the TIA which has been subsequently standardized as IS-95 CDMA Principles Three basic types of multiple access techniques exist: Frequency Division Multiple Access (FDMA,) Time Division Multiple Access (TDMA,) and Code Division Multiple Access (CDMA) With FDMA, a channel is a relatively narrow band in the frequency domain into which a signal's transmission power is concentrated. Different signals are assigned different frequency channels Interference to/from adjacent channels is limited by the use of band-pass filters which only pass signal energy within the specified narrow frequency band The analog FM cellular system uses FDMA With TDMA, a "channel" consists of a time slot in a periodic train of time intervals making up a frame. A given signal's energy is confined to one of these time slots Adjacent channel interference is limited by use of a time gate that only passes signal energy that is received at the proper time Some systems use a combination of FDMA and TDMA The TIA IS-54 digital cellular standard uses FDMA channels of 30 kHz in bandwidth that are subdivided into six time slots for TDMA, two of which are required for each call. The GSM and DCS-1800 systems use 200 kHz bandwidth channels with 8 TDMA calls per channel With CDMA, each signal consists of a carrier modulated by a different high speed binary sequence, spreading the spectrum of the waveform All signals in the multiple access share the same wideband frequency channel If one looks at CDMA in either the frequency or the time domain, the multiple access signals appear to be on top of each other. The signals are separated in the receivers by using a "correlator" which only accepts signal energy from the selected binary sequence All the other signals contribute only to the noise The signal-to-interference ratio is then determined by the desired signal's power and the total power of other signals The signal-to-interference ratio is improved by the ratio of the spread bandwidth to the information bit rate. This is known as processing gain This article describes the common air interface for a system that uses CDMA to provide a very high capacity digital cellular telephone system The system has been standardized as TIA standard IS-95 In the cellular radio frequency reuse concept, interference is accepted but controlled with the goal of increasing system capacity. CDMA does this effectively because it is inherently an excellent anti-interference waveform CDMA frequency reuse efficiency is determined by the small decrease in signal-to-noise ratio caused by system users in neighboring cells CDMA frequency reuse efficiency is approximately 2/3 compared to 1/7 for narrow band FDMA systems A combination of open loop and closed loop power control (through measurements of the received power at the mobile station and the base station) is used to command the mobile station to make power adjustments so as to maintain a power level for adequate performance. This minimizes interference to other users, helps to overcome fading, and conserves battery power in the mobile station CDMA System Capacity Determinants ; The major parameters that determine the CDMA digital cellular system capacity are the processing gain, the required Eb/N0, the voice duty cycle, the frequency reuse efficiency, and the number of sectors in the cell-site antenna Eb/N0, the ratio of energy per bit to noise power spectral density, is the standard figure of merit by which digital modulation and coding schemes are compared It is directly analogous to carrier-to-noise ratio (C/N) for analog FM modulation. Because of the efficient modulation technique, the powerful error correction coding and the multiple use of diversity, an Eb/N0 of only about 75 dB is required for good performance, even in a dynamic multipath environment This can be compared directly with the required carrier to noise C/N of 18 dB for analog FM operating under the same conditions The power control system (described below) sets transmitter power so that the received Eb/N0 is no greater than required for good link performance. Voice Activity Detection In a typical full duplex two-way voice conversation the duty cycle of each voice is only 35% With the CDMA system, the transmitted data rate is reduced when there is no speech activity, allowing transmitter power to be reduced, substantially reducing interference to other users Since the level of other user interference determines capacity, the capacity is increased by approximately a factor of 2 to 25. Frequency Reuse Efficiency When directional cell-site antennas are used (consider an ideal 120 deg sector antenna, for example), the interference realized is divided by three on the average (it only looks in the direction of 1/3 of the mobile stations) The capacity supportable by the total system is therefore increased by nearly a factor of three Soft Handoff The capacity of the system is limited by the capacity of the reverse link because the forward link is slightly advantaged by the use of orthogonal signaling described below The above system parameters and capabilities combine in the following equation for the approximate capacity of the reverse link: N = W/R * 1/Eb/No * f * g * 1/v where N is the number of calls per sectored cell, W is the system bandwidth (1. 23 MHz), R is the information transmission rate (9600 bps), Eb/No is the ratio of energy per bit to noise power spectral density (75 dB), f is the frequency reuse efficiency (2/3), g is the sectorization gain (3 for 120 deg sectors), and v is the voice duty cycle (1/2) The resulting capacity is 91 calls per cell Note that this is the capacity of a single 1. 23 MHz radio channel devoted to CDMA The values used in the above equation are dependent on the characteristics of the particular environment in which the system is installed and on the quality of the base station antennas used Some environments and installations may achieve higher capacity while some may achieve lower capacity The capacity for the above calculation is about 16 times greater than the capacity of the AMPS system (or the GSM system) operating in the same enviroment with the same antennas. In actual practice, capacity between 10-15 times that of AMPS has been repeatably been demonstrated in very elaborate field -*test*-('")s CDMA Forward Link Waveform Design This IS-95 standard specifies a forward link CDMA waveform design that uses a combination of frequency division, pseudo-random code division and orthogonal signal multiple access techniques Pseudo-random noise (PN) binary codes are used to distinguish signals received at a mobile station from different base stations All CDMA signals in the system share a quadrature pair of PN codes. Signals from different cells and sectors are distinguished by time offsets from the basic code This relies on the property of PN codes that the auto-correlation, when averaged over a few bit times, averages to zero for all time offsets greater than a single code chip time (approximately 1 microsec) The PN codes used are generated by linear shift registers that produce a code with a period of 32768 chips The PN chip rate is 1. 2288 MHz, or exactly 128 times the 9600 bps information transmission rate Two codes are generated, one for each of two quadrature carriers, resulting in quadriphase PN modulation To avoid confusion between the system bandwidth, the PN chip rate and the frequency assignment spacing, note that the PN chip rate is exactly 12288 MHz, the frequency assignment spacing for two adjacent CDMA carriers is exactly 1. 23 MHz (because we use a multiple of 30 kHz,) and that the 3 dB bandwidth is also 123 MHz Frequently, this is rounded off to 125 MHz. The signals are bandlimited by a digital filter that provides a very sharp frequency roll-off, resulting in a nearly square spectral shape that is 123 MHz wide at the 3 dB point The information to be transmitted is convolutionally encoded to provide the capability of error detection and correction at the receiver The code used has a constraint length (encoder memory) of nine and a code rate of one-half (two encoded binary symbols are produced per information bit). The encoded symbols are interleaved to combat fast fading To provide communications privacy, each data channel is scrambled with a user addressed long code PN sequence Thus, a "channel" in the forward link of the specified CDMA system consists of a signal centered on an assigned radio channel frequency, quadriphase modulated by a pair of PN codes with an assigned time offset, biphase modulated by an assigned orthogonal Walsh function, and biphase modulated by the encoded, interleaved, and scrambled digital information signal An important aspect of the forward link waveform design is the use of the pilot signal. The Pilot Channel signal is unmodulated by information and utilizes the zero Walsh function (which consists of 64 zeroes) Thus, the signal simply consists of the basic PN codes The mobile station can obtain synchronization with the nearest base station without prior knowledge of the identity of the base station by searching out the entire length of the PN code The strongest signal's time offset corresponds to time offset of the nearest base station's PN code. After synchronization, the pilot signal is used as a coherent carrier phase reference for demodulation of the other signals from this base station The CDMA reverse link also employs PN modulation using the same 32768 length binary sequences that are used for the forward link Here, however, a fixed code phase offset is used Signals from different mobile stations are distinguished by the use of a very long (2**42 - 1) PN sequence with a user address determined time offset. Because every possible time offset is a valid address, an extremely large address space is provided This also inherently provides a reasonably high level of privacy The transmitted digital information is convolutionally encoded using a rate 1/3 code (three encoded binary symbols per information bit) of constraint length nine The encoded information is then interleaved over a 20 msec interval. The interleaved information is then grouped in six symbol groups (or code words) These code words are used to select one of 64 different orthogonal Walsh functions for transmission The Walsh function "chips" are combined with the long and short PN codes Note that this use of the Walsh function is different than on the forward link. On the forward link, the Walsh function is determined by the mobile station's assigned channel while on the reverse link the Walsh function is determined by the information being transmitted A "channel" on the reverse link of the specified CDMA system consists of a signal centered on an assigned radio channel frequency, quadriphase modulated by a pair of PN codes, biphase modulated by a long PN code with address determined code phase, and biphase modulated by the Walsh encoded, interleaved and convolutionally encoded digital information signal Power Control In a direct sequence CDMA system, maximum capacity is achieved when the Eb/N0 for every user is at the minimum needed to support "acceptable" channel performance Since N0 (noise spectral density) is generated almost entirely by "other" user interference, it is necessary that all signals arrive at, or near the same level of power at the CDMA receiver. This is achieved in the mobile propagation environment by providing dynamic power control for both the mobile station and base station transmitters Reverse Link Power Control Unlike the forward channel, signals arriving at the base station do not fade together, and can vary with respect to one another over a much greater dynamic range (e g 80 dB) thus the power control process for the reverse channel is much different than that used for the forward channel It consists of two elements; open loop estimation of transmit power by the mobile station and closed loop correction of the errors in this estimate by the base station. Open Loop Estimation by the Mobile Station Each mobile station estimates the path loss from the base station to the mobile station by measuring the total power received in the assigned CDMA channel By monitoring total power, rather than using a demodulated signal such as the pilot, this measure can be made rapidly without any knowledge of timing, base station ID, or path conditions Based on this measure, the mobile station's transmitted power is adjusted to match the estimated path loss, so as to arrive at the base station at a predetermined level All mobile stations use the same process, all attempting to arrive with equal power at the base station. Closed Loop Correction by the Base Station Uncontrolled differences in the forward and reverse channels, such as opposite fading which can occur due to their frequency difference, plus mismatches in the mobile station's receive and transmit chains, cannot be estimated by the mobile To reduce these smaller errors, each mobile station corrects its transmit power with information supplied by the base station, via low rate data inserted into each mobiles forward channel The base station derives the correction information by monitoring the reverse channel quality of each mobile, compares this measure to a threshold, and requests either an increase or a decrease, depending on the result In this manner the base station maintains each reverse channel, and thus all reverse channels, at the minimum received power needed to to provide RacceptableS performance. System Timing All digital transmissions are referenced to a common, system-wide time standard which is traceable to and synchronous with Coordinated Universal Time (UTC) As an example, the system could use the GPS time scale, which is distributed world wide with 24 hour per day availability The difference between GPS and UTC is an integer number of seconds, specifically the number of leap second corrections added to UTC since Jan 6, 1980 System Time keeps track of leap second corrections to UTC but does not use these corrections for physical adjustments to the system time clocks. System Time is distributed to and (except for leap second offsets) maintained at every Base Station to within 11 microsecond of UTC Each Base Station contains its own precision frequency standard in the event that time synchronization to other cells is ...