CDMA Overview ACCESS SCHEMES For radio systems there are two resources, frequency and time. Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA). Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA). In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections. 
CODING CDMA uses unique spreading codes to spread the baseband data before transmission. The signal is transmitted in a channel, which is below noise level. The receiver then uses a correlator to despread the wanted signal, which is passed through a narrow bandpass filter. Unwanted signals will not be despread and will not pass through the filter. Codes take the form of a carefully designed one/zero sequence produced at a much higher rate than that of the baseband data. The rate of a spreading code is referred to as chip rate rather than bit rate. See coding process page for more details.
CODES CDMA codes are not required to provide call security, but create a uniqueness to enable call identification. Codes should not correlate to other codes or time shifted version of itself. Spreading codes are noise like pseudo-random codes, channel codes are designed for maximum separation from each other and cell identification codes are balanced not to correlate to other codes of itself. See codes page for more details. 
THE SPREADING PROCESS WCDMA uses Direct Sequence spreading, where spreading process is done by directly combining the baseband information to high chip rate binary code. The Spreading Factor is the ratio of the chips (UMTS = 3.84Mchips/s) to baseband information rate. Spreading factors vary from 4 to 512 in FDD UMTS. Spreading process gain can in expressed in dBs (Spreading factor 128 = 21dB gain). See spreading page for more details.
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POWER CONTROL
CDMA is interference limited multiple access system. Because all users transmit on the same frequency, internal interference generated by the system is the most significant factor in determining system capacity and call quality. The transmit power for each user must be reduced to limit interference, however, the power should be enough to maintain the required Eb/No (signal to noise ratio) for a satisfactory call quality. Maximum capacity is achieved when Eb/No of every user is at the minimum level needed for the acceptable channel performance. As the MS moves around, the RF environment continuously changes due to fast and slow fading, external interference, shadowing , and other factors. The aim of the dynamic power control is to limit transmitted power on both the links while maintaining link quality under all conditions. Additional advantages are longer mobile battery life and longer life span of BTS power amplifiers
See UMTS power control page for more details.
HANDOVER
Handover occurs when a call has to be passed from one cell to another as the user moves between cells. In a traditional "hard" handover, the connection to the current cell is broken, and then the connection to the new cell is made. This is known as a "break-before-make" handover. Since all cells in CDMA use the same frequency, it is possible to make the connection to the new cell before leaving the current cell. This is known as a "make-before-break" or "soft" handover. Soft handovers require less power, which reduces interference and increases capacity. Mobile can be connected to more that two BTS the handover. "Softer" handover is a special case of soft handover where the radio links that are added and removed belong to the same Node B.
See Handover page for more details.
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CDMA soft handover |
MULTIPATH AND RAKE RECEIVERS
One of the main advantages of CDMA systems is the capability of using signals that arrive in the receivers with different time delays. This phenomenon is called multipath. FDMA and TDMA, which are narrow band systems, cannot discriminate between the multipath arrivals, and resort to equalization to mitigate the negative effects of multipath. Due to its wide bandwidth and rake receivers, CDMA uses the multipath signals and combines them to make an even stronger signal at the receivers. CDMA subscriber units use rake receivers. This is essentially a set of several receivers. One of the receivers (fingers) constantly searches for different multipaths and feeds the information to the other three fingers. Each finger then demodulates the signal corresponding to a strong multipath. The results are then combined together to make the signal stronger.
Main UMTS Codes
Here us a summary of the main UMTS FDD codes:
Here us a summary of the main UMTS FDD codes:
Synchronisation Codes | Channelisation Codes | Scrambling Codes, UL | Scrambling Codes, DL | |
Type | Gold Codes Primary Synchronization Codes (PSC) and Secondary Synchronization Codes (SSC) | Orthogonal Variable Spreading Factor (OVSF) codes sometimes called Walsh Codes | Complex-Valued Gold Code Segments (long) or Complex-Valued S(2) Codes (short) Pseudo Noise (PN) codes | Complex-Valued Gold Code Segments Pseudo Noise (PN) codes |
Length | 256 chips | 4-512 chips | 38400 chips / 256 chips | 38400 chips |
Duration | 66.67 µs | 1.04 µs - 133.34 µs | 10 ms / 66.67 µs | 10 ms |
Number of codes | 1 primary code / 16 secondary codes | = spreading factor 4 ... 256 UL, 4 ... 512 DL | 16,777,216 | 512 primary / 15 secondary for each primary code |
Spreading | No, does not change bandwidth | Yes, increases bandwidth | No, does not change bandwidth | No, does not change bandwidth |
Usage | To enable terminals to locate and synchronise to the cells' main control channels | UL: to separate physical data and control data from same terminal DL: to separate connection to different terminals in a same cell | Separation of terminal | Separation of sectors |
WCDMA Spreading
TDD WCDMA uses spreading factors 4 - 512 to spread the base band data over ~5MHz band. Spreading factor in dBs indicates the process gain. Spreading factor 128 = 21 dB process gain). Interference margin is calculated from that:
Interference Margin = Process Gain - (Required SNR + System Losses)
· Required Signal to Noise Ration is typically about 5 dB
· System losses are defined as losses in receiver path. System losses are typically 4 - 6 dBs
Overview of Spreading Process
UMTS Power Control
Open loop power control is the ability of the UE transmitter to sets its output power to a specific value. It is used for setting initial uplink and downlink transmission powers when a UE is accessing the network. The open loop power control tolerance is ± 9 dB (normal conditions) or ± 12 dB (extreme conditions)
Inner loop power control (also called fast closed loop power control) in the uplink is the ability of the UE transmitter to adjust its output power in accordance with one or more Transmit Power Control (TPC) commands received in the downlink, in order to keep the received uplink Signal-to-Interference Ratio (SIR) at a given SIR target. The UE transmitter is capable of changing the output power with a step size of 1, 2 and 3 dB, in the slot immediately after the TPC_cmd can be derived. Inner loop power control frequency is 1500Hz.
The serving cells estimate SIR of the received uplink DPCH, generate TPC commands (TPC_cmd) and transmit the commands once per slot according to the following rule: if SIRest > SIRtarget then the TPC command to transmit is "0", while if SIRest < SIRtarget then the TPC command to transmit is "1". Upon reception of one or more TPC commands in a slot, the UE derives a single TPC command for each slot, combining multiple TPC commands if more than one is received in a slot. Two algorithms are supported by the UE for deriving a TPC_cmd. Which of these two algorithms is used, is determined by a UE-specific higher-layer parameter, "PowerControlAlgorithm".
Algorithm 1:
Open loop power control is the ability of the UE transmitter to sets its output power to a specific value. It is used for setting initial uplink and downlink transmission powers when a UE is accessing the network. The open loop power control tolerance is ± 9 dB (normal conditions) or ± 12 dB (extreme conditions)
Inner loop power control (also called fast closed loop power control) in the uplink is the ability of the UE transmitter to adjust its output power in accordance with one or more Transmit Power Control (TPC) commands received in the downlink, in order to keep the received uplink Signal-to-Interference Ratio (SIR) at a given SIR target. The UE transmitter is capable of changing the output power with a step size of 1, 2 and 3 dB, in the slot immediately after the TPC_cmd can be derived. Inner loop power control frequency is 1500Hz.
The serving cells estimate SIR of the received uplink DPCH, generate TPC commands (TPC_cmd) and transmit the commands once per slot according to the following rule: if SIRest > SIRtarget then the TPC command to transmit is "0", while if SIRest < SIRtarget then the TPC command to transmit is "1". Upon reception of one or more TPC commands in a slot, the UE derives a single TPC command for each slot, combining multiple TPC commands if more than one is received in a slot. Two algorithms are supported by the UE for deriving a TPC_cmd. Which of these two algorithms is used, is determined by a UE-specific higher-layer parameter, "PowerControlAlgorithm".
Algorithm 1:
· The power control step is the change in the UE transmitter output power in response to a single TPC command
Algorithm 2:
Algorithm 2:
· If all five estimated TPC command are "down" the transmit power is reduced by 1 dB
· If all five estimated TPC command are "up" the transmit power is increased by 1 dB
· Otherwise the transmit power is not changed
Transmitter power control range
The transmit power of the downlink channels is determined by the network. The power control step size can take four values: 0.5, 1, 1.5 or 2 dB. It is mandatory for UTRAN to support step size of 1 dB, while support of other step sizes is optional. The UE generates TPC commands to control the network transmit power and send them in the TPC field of the uplink DPCCH. Upon receiving the TPC commands UTRAN adjusts its downlink DPCCH/DPDCH power accordingly.
Outer loop power control is used to maintain the quality of communication at the level of bearer service quality requirement, while using as low power as possible. The uplink outer loop power control is responsible for setting a target SIR in the Node B for each individual uplink inner loop power control. This target SIR is updated for each UE according to the estimated uplink quality (BLock Error Ration, Bit Error Ratio) for each Radio Resource Control connection. The downlink outer loop power control is the ability of the UE receiver to converge to required link quality (BLER) set by the network (RNC) in downlink.
Power control of the downlink common channels are determined by the network. In general the ratio of the transmit power between different downlink channels is not specified in 3GPP specifications and may change with time, even dynamically.
Additional special situations of power control are Power control in compressed mode andDownlink power during handover.
UMTS Handover
There are following categories of handover (also referred to as handoff):
· Hard Handover
Hard handover means that all the old radio links in the UE are removed before the new radio links are established. Hard handover can be seamless or non-seamless. Seamless hard handover means that the handover is not perceptible to the user. In practice a handover that requires a change of the carrier frequency (inter-frequency handover) is always performed as hard handover.
Hard handover means that all the old radio links in the UE are removed before the new radio links are established. Hard handover can be seamless or non-seamless. Seamless hard handover means that the handover is not perceptible to the user. In practice a handover that requires a change of the carrier frequency (inter-frequency handover) is always performed as hard handover.
· Soft Handover
Soft handover means that the radio links are added and removed in a way that the UE always keeps at least one radio link to the UTRAN. Soft handover is performed by means of macro diversity, which refers to the condition that several radio links are active at the same time. Normally soft handover can be used when cells operated on the same frequency are changed.
Soft handover means that the radio links are added and removed in a way that the UE always keeps at least one radio link to the UTRAN. Soft handover is performed by means of macro diversity, which refers to the condition that several radio links are active at the same time. Normally soft handover can be used when cells operated on the same frequency are changed.
· Softer handover
Softer handover is a special case of soft handover where the radio links that are added and removed belong to the same Node B (i.e. the site of co-located base stations from which several sector-cells are served. In softer handover, macro diversity with maximum ratio combining can be performed in the Node B, whereas generally in soft handover on the downlink, macro diversity with selection combining is applied.
Softer handover is a special case of soft handover where the radio links that are added and removed belong to the same Node B (i.e. the site of co-located base stations from which several sector-cells are served. In softer handover, macro diversity with maximum ratio combining can be performed in the Node B, whereas generally in soft handover on the downlink, macro diversity with selection combining is applied.
Generally we can distinguish between intra-cell handover and inter-cell handover. For UMTS the following types of handover are specified:
· Handover 3G -3G (i.e. between UMTS and other 3G systems)
· FDD soft/softer handover
· FDD inter-frequency hard handover
· FDD/TDD handover (change of cell)
· TDD/FDD handover (change of cell)
· TDD/TDD handover
· Handover 3G - 2G (e.g. handover to GSM)
· Handover 2G - 3G (e.g. handover from GSM)
The most obvious cause for performing a handover is that due to its movement a user can be served in another cell more efficiently (like less power emission, less interference). It may however also be performed for other reasons such as system load control.
The most obvious cause for performing a handover is that due to its movement a user can be served in another cell more efficiently (like less power emission, less interference). It may however also be performed for other reasons such as system load control.
· Active Set is defined as the set of Node-Bs the UE is simultaneously connected to (i.e., the UTRA cells currently assigning a downlink DPCH to the UE constitute the active set).
· Cells, which are not included in the active set, but are included in the CELL_INFO_LIST belong to the Monitored Set.
· Cells detected by the UE, which are neither in the CELL_INFO_LIST nor in the active set belong to the Detected Set. Reporting of measurements of the detected set is only applicable to intra-frequency measurements made by UEs in CELL_DCH state.
The different types of air interface measurements are:
The different types of air interface measurements are:
· Intra-frequency measurements: measurements on downlink physical channels at the same frequency as the active set. A measurement object corresponds to one cell.
· Inter-frequency measurements: measurements on downlink physical channels at frequencies that differ from the frequency of the active set. A measurement object corresponds to one cell.
· Inter-RAT measurements: measurements on downlink physical channels belonging to another radio access technology than UTRAN, e.g. GSM. A measurement object corresponds to one cell.
· Traffic volume measurements: measurements on uplink traffic volume. A measurement object corresponds to one cell.
· Quality measurements: Measurements of downlink quality parameters, e.g. downlink transport block error rate. A measurement object corresponds to one transport channel in case of BLER. A measurement object corresponds to one timeslot in case of SIR (TDD only).
· UE-internal measurements: Measurements of UE transmission power and UE received signal level.
· UE positioning measurements: Measurements of UE position.
The UE supports a number of measurements running in parallel. The UE also supports that each measurement is controlled and reported independently of every other measurement.
The UE supports a number of measurements running in parallel. The UE also supports that each measurement is controlled and reported independently of every other measurement.
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