Wireless Definitions
Introduction
All definitions are arranged in the alphabetical order in the respective sections.For any comments or you feel there is a copyright violation, please send an email to smdahmed@gmail.com
Communication Systems Definitions
Digital ModulationThe main purpose of Digital Modulation is to transfer a digital bit stream over an analog bandpass channel like a limited radio frequency band. In GSM, we have limited radio frequency bands like the 900MHz band etc.
Modulation
Modulation is a process of changing the key parameters of a signal inorder to use it to convey the required information. The three parameters that can be changed in this modulation process are: frequency (pitch), amplitude (volume) and phase (timing).
GSM
Authentication Procedure1. To check whether the identity provided by the mobile station to the network is acceptable or not.
2. To provide parameters to the mobile station to enable the calculation of a new ciphering key.
Frequency Hopping
Reference: GSM TS: 05.01 Section 6
A mobile radio channel is prone to fading. GSM uses only Slow Frequency Hopping (SFH). Fast Frequency Hopping is used in Spread Spectrum systems(FFH).
In SFH, the operating frequency is changed only once every TDMA frame i.e. 4.615ms. Therefore a MS must transmit at one frequency in one time slot and must hop to a different frequency in the before the next time slot.
In FFH, the changes of operating frequency may happen many times per symbol.
Effects of Frequency Hopping:
The fading appears and disappears every over a distance of one-quarter wavelength which is around 3 feet. Henceforth, when a MS is rapidly moving; it will uncorrelate the fading from one burst to the next. Interleaving spreads each encoded speech block over eight bursts, the affect of fading on any single burst is almost absent. On the contrary when a MS is stationary or moving very slowly; it might be possible that it may remain in the fade for many bursts to come. The effect of fading is UNCORRELATED over frequencies that are of the order of 1MHz., burst-to-burst frequency hopping over frequencies that are far apart will greatly reduce the effect of fading on the slow and stationary MSs.
Hopping sequence is derived by the MS from the parameters broadcast at the channel assignment, namely mobile allocation or MA (set of frequencies on which to hop), the hopping sequence number of the cell (which allows different sequences on homologue cells) and the index offset (to distinguish the different mobiles of the cell using the same mobile allocation).
Pseudo Synchronous Handover
Reference: 3GPP TS 05.10
In GSM phase 2 systems, support for pseudo-synchronous handover is compulsory. In a pseudo-synchronous handover, the MS will keep the timing values for the surrounding BTSs in order to be pre-synchronized to the new BTS upon handover. To obtain this synchonization, the MS must calculate an Observed Time Difference (OTD) between the serving BTS and the other BTSs. Each BTS must maintain a Real Time Difference, RTD between itself and its neighbouring base stations. When handover is performed, the RTD is supplied to the MS, which with the knownledge of the RTD and OTD can calculate the Timing Advance needed to synchronize with the new BTS, and go directly into synchronization. This procedure is defined in GSM 05.10
Security Procedure
Security procedure just points to Ciphering procedure as specified in Section: 4.4.4.4 of 24.008
System Information 1
This is yet to be filled with relevant information.
Timing Advance - TA
Reference: 3GPP TS 05.10
The precise time at which a MS is allowed to transmit a burst of traffic in a timeslot must be adjusted accordingly. Timing Advance is the variable controlling this adjustment.
GPRS
Block Sequence Number - BSN
Reference: Section 9.1.4 of 44.060 3GPP TS.
Each RLC data block contains a block sequence number - BSN which is 7 bits in length. At the time that an in-sequence RLC data block is designated for transmission, the value of BSN is set equal to the value of the send state variable V(S).
Contention Resolution (RLC)
Reference: Section 7.1.2.3 of 04.60 3GPP TS.
Reference: Section 7.1.3.3 of 04.60 3GPP TS.
For One Phase Access -
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In order to uniquely identify the MS when sending on uplink, the RLC header is extended to include the TLLI of the mobile station until contention resolution is completed on MS.
Contention resolution is said to be complete at MS side if the MS receives TLLI sent in the PACKET UPLINK ACK/NACK.
Contention resolution is said to be complete at the Network side if the Network receives a TLLI in the RLC data blocks.
For two phase access -
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The contention resolution is said to be complete at MS side when MS receives a PACKET UPLINK ASSIGNMENT message with the TLLI included by the MS earlier.
The contention resolution is said to be completed in the Network side if the Network receives a TLLI in the RLC data block.
Dual Transfer Mode
Reference: Section 5.4a of 44.060 3GPP TS.
A true Class A mobile might have to use two different radio to be able to transmit on two different frequencies at the same time. This will shoot up the price of the phone due to the co-existence of two different radios. Henceforth, a GPRS mobile equipment may implement the DTM feature. This feature ensures that a GPRS capable mobile need not have to transfer on two different frequencies at the same time, which is coordinated by the network.
Network Control Cell Reselection Modes For GPRS Mobile
Reference:
Mode 0: MS does autonomous cell reselection
Mode 1: MS passes measurement reports to the network but still does autonomous cell reselection
Mode 2: MS passes measurement reports to the network. MS will do cell reselection autonomously if there is a downlink signaling failure or a RACH failure.
Network Mode of Operation
Reference:
| NMO | 1 | 2 | 3 |
| PBCCH | Yes | No | Yes |
| Gs Interface in Network | Yes | No | No |
| CS Paging in GPRS standby state | PPCH/PCCH or CCCH/PCH | CCCH/PCH | CCCH/PPCH |
| PS Paging in GPRS standby state | PPCH | CCCH/PCH | PCCCH/PPCH |
| CS Paging in Packet Call | PDTCH/PACCH | CCCH/PCH | CCCH/PCH |
Quality of Service (QoS) Classes
Reference: Section 6.3 of 3GPP TS 23.107
The GPRS QoS is divided into the following:
GPRS QoS profile
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| Traffic Classes | Latency | Jitter | Throughput | Burstiness |
| Streaming | <500 ms, Bounded | Stringent | Gauranteed | Low |
| Conversational | < 80ms, Bounded | Stringent | N/A | High |
| Interactive | Less than conversational | N/A | Guaranteed | Greater than Conversational |
| Background | N/A | N/A | N/A | N/A |
Reduced BSN
Reference: Section 9.1.4 of 44.060 3GPP TS.
Each downlink RLC/MAC control block contains a Reduced BSN. At the time that an in-sequence RLC/MAC control block is designated for transmission, the value of RBSN is set equal to the value of the control send state variable V(CS).
Relative Reserved Block Period - RRBP
Reference: Section 10.4.5 of the 44.060 3GPP TS
The RRBP value specifies a single uplink block in which the mobile station shall transmit either a PACKET CONTROL ACKNOWLEDGEMENT message or a PACCH block to the network.
Special requirements are specified in the Section: 10.4.5.1.
RLC Variables
Reference: Section 9.1.x of 44.060 3GPP TS
Countdown Value (CV) field - Reference: Section: 10.4.6 of 44.060 3GPP TS
This is set by MS to indicate the Network about the current number of the RLC DATA blocks remaining for the current UPLINK TBF.
V(S) - Send State Variable - The value of V(S) shall be incremented by 1 after transmission of the RLC data block with BSN = V(S).
V(CS) - Control Send State Variable
V(A) - Acknowledge State Variable
V(B) - Acknowledge State Array
V(R) - Receive State Variable - The receive state variable denotes the BSN which has a value one higher than the highest BSN yet received (modulo SNS).
V(Q) - Receive Window State Variable - The receive window state
variable denotes the lowest BSN not yet received (modulo SNS), therefore representing the start of the receive window.
Sequence Number Space - SNS
Sequence Number Space which is 2048 in EGPRS & 128 in GPRS. This is the number of the RLC data blocks that can be sent from an RLC entity at any point of time.
[Need to Confrim]: Each context using the RLC might have its own SNS.
Temporary Logical Link Identifier - TLLI
TLLI is used between the Network and the MS to identify the logical link. TLLI is calculated from the P-TMSI. TLLI is of four types:
1. Local TLLI
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| 1 1 | Bits 29 - 0 from P-TMSI |
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Local TLLI is in the format as above. This is used if the MS is in the same routing area where the P-TMSI was allocated or after a routing area update in the new routing area when the new P-TMSI isnt yet available.
2. Foreign TLLI
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| 1 0 | Bits 29 - 0 from P-TMSI |
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Foreign TLLI is used by MS only for GPRS Attach and Routing Area Updating.
3. Random TLLI
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| 0 1 1 1 1 | Random Bits on MS side |
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Random TLLI is used by MS only for the anonymous PDP context activation (not sure if this is used and allowed practically) & when there is no P-TMSI available.
4. Auxiliary TLLI
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| 0 1 1 1 0 | Random Bits from SGSN |
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This is assigned by the Network when the MS sends anonymous PDP context request. This is done by network mainly to avoid the ambiguities of Random TLLIs for anonymous PDP contexts.
UMTS
A/Gb ModeThis is defined as the mode of operation of MS when UE is connected to the Core Network via GERAN and the A and/or Gb interfaces
Authentication Procedure
Reference: Section 4.3.2 of TS 3GPP 24.008
Authentication procedure is done for the following:
1. To permit Network to check the identity provided by the mobile station is acceptable.
2. To enable the mobile station to calculate the new ciphering key
3. To enable the mobile station to calculate the new integrity key
4. To permit the receiving entity (mobile station??? ) to check the integrity of the network.
Ciphering Key Sequence Number (CKSN)
Reference: 3GPP TS 24.008 Section: 4.3.2.4
In order to allow the start of ciphering over an RR connection without authentication, the ciphering key sequnce numbers are introduced. The sequence number is managed by the network and is indicated to the mobile station in the AUTHENTICATION REQUEST. AUTHENTICATION REQUEST message contains the sequence number allocated to the key(s) which may be computed from the RAND parameter carried in the message.
The mobile station stores this number (i.e. the CKSN) with the key(s) and indicates to the network in the first message (LOCATION UPDATING REQUEST, CM SERVICE REQUEST, PAGING RESPONSE, CM RE-ESTABLISHMENT REQUEST) which sequence number the stored key or set of keys have.
When the CKSN deletion is described, this means that the key(s) associated with the stored key(s) shall be considered invalid.
Compressed Mode Handover
Reference: 3GPP TS 25.212
http://www.commsdesign.com/showArticle.jhtml;jsessionid=ROUTGSHGPQSYQQSNDLPCKH0CJUNN2JVN?articleID=16500056
Analyzing the performance of a mobile radio network during handovers of handsets (UE) between base station (BTS) transceivers is a challenging task. Properly functioning handovers allow calls to continue without interruption and maximize resources through capacity sharing. Poorly executed handovers can disrupt calls, degrade quality-of-service (QoS), and reduce the operating ranges of cell sites—risking severe economic consequences.
The ability for a UE to be handed over reliably between wideband CDMA (W-CDMA) and GSM or TDD systems is particularly important as newer networks are phased into existing infrastructures. W-CDMA systems manage handovers by briefly switching transmissions into compressed mode. Errors that occur during compressed-mode handovers are unpredictable and must be captured in seamless blocks over long periods in order to analyze their characteristics and trace their sources.
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To effectively deal with errors occurring in the compressed mode, designers need to take a three-dimensional approach to measuring the handover process, looking at measurements in the time, frequency, and code domains. Let's analyze why.
Handling Handovers in W-CDMA
The handover is a process by which the radio access network changes the radio transmitters, radio access modes, or radio systems that are used to provide the bearer services to the UE, while maintaining a defined QoS.
Handovers from one cell to another are required in several situations. The most common situation is when the UE moves from one base station coverage area to another. The UE may move between stations within the same radio system or into another system. The W-CDMA standard supports handovers to any GSM or time division duplex (TDD) network frequency bands that meet the specifications.
The multi-standard UE may change its frequency or radio access mode, during a handover to a different cell. The UE may need a handover if its requested service level exceeds the current cell capacity. If a target cell cannot support the combination of bearer services (voice, data, multimedia, etc) that are provided by the current serving cell, some, or all, of the bearer services may be handed over to another cell.
Within the W-CDMA system, handovers are "soft" in order to minimize the interference on neighboring cells and to allow the use of identical carrier frequencies (intra-frequency handovers). In a soft handover, the UE transmits and receives the same signal from both cells simultaneously to make the transition as seamless as possible. Handovers are more complex when a multi-standard UE moves between cells with different carrier frequencies or to a different network, such as GSM ("Inter-frequency Handovers"). Both types of handover are managed with an assist from the UE mobile unit.
The multi-standard UE continuously monitors for the presence of cells with other frequencies and radio access systems that it supports. When the network senses the need for a handover, the BTS measures some system parameters and commands the UE to measure other parameters and report the results. Key parameters include carrier frequency, system type, traffic volume and QoS levels.
Working in the Compressed Mode
When a handover is needed, the BTS directs the UE to operate in a compressed mode. The compressed mode is a method of turning off transmissions for a portion of the 10-ms frame to create gaps that allow time for the UE and BTS to make a prescribed set of measurements. Compressed-mode operation can be achieved by decreasing the spreading factor, removing bits from the data ("puncturing"), or using higher level scheduling to allocate fewer timeslots for user traffic.
In compressed frames, the transmission gap slots are not used for data transmission and the instantaneous transmit power is increased in those slots to maintain quality (BER, FER, etc.) during the periods of reduced processing gain (Figure 1). The value of power increment depends on the transmission time reduction method.
Compressed frames can be set to occur periodically (as in Figure 2 below) or on demand. The rate and type of compressed frames is variable and depends on the environment and the measurement requirements.
Separate compressed mode signals must be defined for uplink and downlink paths as well as for each mode, radio access technology and frequency band supported by the UE. In typical applications, uplink data rates in compressed mode are set by higher protocol layers to twice the normal rate, downlink data rates to twice, or more.
Figure 2 shows examples of a compressed-mode frame structure for uplink operations. Figure 3 illustrates two types of downlink frame structures that differ in the location of the TPC bits. Type A would be used to provide the maximum transmission gap for measurements; Type B optimizes power.
In the compressed mode, a transmission gap pattern sequence is requested by higher-layer BTS protocols and the parameters are passed along to the UE by the BTS. The UE conducts only one set of measurements for each transmission gap pattern sequence. Figure 4 illustrates a compressed-mode sequence of alternating transmission gap patterns while Table 1 lists the parameters that are used to define the sequence.
The First Two Measurement Dimensions
The UE and BTS measure the Layer 1 protocol to determine and report the status of intra-frequency, inter-frequency, inter-system handovers, traffic volume, and QoS levels. First, the BTS transmits a "measurement control message" to the UE including the measurement ID and type of measurement to initiate.
When the reporting is complete, the UE sends a "measurement reporting message" to the BTS with the measurement ID and the results. The measurement control message is broadcast in idle mode within the system Information. When the UE monitors cells at other frequencies, modes, and radio access technologies, the BTS must direct the specific measurement needed to fulfill the requested handover. In W-CDMA, the Layer 1 measurements are reported to higher layers of the protocol. In GSM, the measurements are reported only to the GSM terminal.
Table 2 lists some of the measurements made by the UE and the BTS during compressed mode.
Error conditions that arise during compressed mode measurements and handovers can be brief and unpredictable. To be certain of catching these intermittent problems, power levels, frequency and modulation information must be monitored before, during and after they occur. Data must be captured seamlessly in order to preserve the signal characteristics and reveal the error sources. In depth analysis of error conditions often requires the correlation of signal states in the frequency, time, modulation, and code domains.
The real-time spectrum analyzer employs advanced digital signal processing technology to acquire long seamless records of complex signals and display analysis results without the need for external data processing. For example, the analyzer can be set to acquire a full 10 seconds of signal in a 5 MHz span and analyze the results in multiple display formats. Time and frequency data are recorded simultaneously, revealing even brief, intermittent changes and when they occurred within the long records.
The Third Dimension
To compliment the time and frequency measurements, designers can start analyzing the performance of W-CDMA handoffs in the code domain. To do this, designers should turn to spectogram and codogram measurements.
A spectrogram provides a display of frequency vs. time vs. power density. This measurements shows how well the UE performed at different frequencies during handovers. In the spectogram shown in Figure 5, the vertical axis represents time and the horizontal axis represents frequency while the colors represent frequency and power density.
The codogram is a 3D display of the orthogonal variable spreading factor (OVSF) or channelization code vs. time slot vs. code power. In the codogram shown in Figure 6, the vertical axis is the time slot and the horizontal axis is OVSF while code power is represented by color.
With spectrogram and codogram measurements, more data is captured and no data is missed. Each measurement shows a seamless history of information.
Wrap Up
The compressed mode handover performance of W-CDMA radio system UE and BTS transceivers and networks is complex and should be evaluated simultaneously in the frequency, time, modulation and code domains. The real-time spectrum analyzer can capture intermittent signals in long seamless records and provide measurement results that are correlated in multiple domains. Powerful 3D displays provide clear insight into the system quality and/or potential sources of errors.
References
- 3GPP TS25.212 V3.11.0 Technical Specification Group Radio Access Network Multiplexing and Channel Coding (FDD) (Release 1999).
- 3GPP TS25.215 V3.10.0 Technical Specification Group Radio Access Network; Physical layer—Measurements (FDD) (Release 1999) .
About the Author
Koichi Sega is a product manager in Tektronix's Wireless Product Line. He received a degree in electrical engineering from Tokyo Denki University and can be reached at koichi.sega@tektronix.com.
Follow-On Request & Follow-On Proceed
In UMTS, the PS signalling connection between the mobile station and the network (i.e. SGSN) may either be released right after finishing a GMM specific procedure or prolonged for following mobile station originated activity (e.g. SM or SMS requests).
At present, TS 24.008 describes how the mobile station can request to prolong an established PS signalling connection using GMM protocol signalling (i.e. Follow-on request (FOR) mechanism), but TS 24.008 has no mechanism to inform the mobile station whether the PS signalling connection has actually been prolonged. This lack of information leads to unnecessary signalling and higher service response time when the PS signalling connection is not prolonged, though requested and when the PS signalling connection is prolonged, though not requested.
The introduction of the FOP mechanism for the PS domain achieves signalling and service response time reduction and decreases power consumption in terminals for the GPRS attach and RAU procedures cases.
The Follow-on proceed (FOP) mechanism like in CS domain is introduced. The FOP can be indicated in the ATTACH ACCEPT and ROUTING AREA UPDATE ACCEPT messages by the SGSN. The mobile station acts according to the FOP bit included in the acceptance message of GMM specific procedure. This avoids any unnecessary signalling. If follow-on proceed is indicated and there is any CM sublayer request pending, the mobile station sends appropriate message(s) (for example, ACTIVATE PDP CONTEXT REQUEST) to the SGSN.
Handovers in WCDMA
The term handover is used to describe cell transitions that happen during a voice or video call. These handovers are all network initiated and are categorised as intra-frequency, inter-frequency or inter-RAT (Radio Access Technology). Within a WCDMA network, mobility between cells is managed by handover between different scrambling codes supported by the different cells all working on the same frequency (intra-freq). In this case it is possible for each base station to maintain it’s own link with the mobile, which allows the network to combine two signals and hence achieve a diversity gain at the edge of the cell. This mechanism is referred to as soft handoff diversity gain.
If the multiple links are formed from alternative sectors of the same base station (or node B) then it is referred to as softer handover. Strictly speaking this is a not an actual handover event but more a technique for improving signal quality.
It is sometimes necessary for a 3G handover to take place between frequencies. This may be done in order to maintain coverage or to balance the load between respective carriers. Inter-frequency handover is more complex than intra-frequency due to the fact that during a call, the receiver of a 3G phone is constantly listening to the serving base station. Therefore it either needs to make a handover without measuring the characteristics of the target cell or the phone needs to use what is termed compressed mode in order to make the necessary measurements. When a phone is instructed to make a handover without having gathered a measurement report for the target cell this is termed a ‘blind’ handover. In compressed mode the downlink transmission is briefly halted in order that the phone can make a measurement.
In order to support the period of no transmission on the downlink the signal just prior to and just after the break is sent with either a change to the spreading factor (and appropriate transmit power), or following a change to the higher layer scheduling. These changes are made so that the gap can be accommodated without significant loss of data.
One final handover type supported within 3G networks is the inter-RAT handover from a serving 3G to a target 2G cell. 3G networks use this to provide both 3G service hot spots and full 2G coverage for voice and low data services. For single receiver mobiles the handover will either be blind or will use compressed mode in order to allow the mobile to measure a neighbouring 2G cell.
Identification Procedure
This procedure is defined in Section: 4.3.3 of 3GPP TS 24.008.
This procedure is used by the Network to request a mobile station to provide specific identification parameters to the Network example: IMSI & IMEI.
Iu Mode
This is defined as the mode of operation of the UE when it is connected to the Core Network via the GERAN or UTRAN and the Iu interface.
Layer 2
This is similar to the Data Link Layer of the OSI model. This layer is used for synchronising the bit stream to and from the Physical Layer; and for detection of problems & errors due to transmission over the air. (Is this any different from AS??? )
LSA - Localised Service Area
A localised service area consists of a cell or a number of cells. The cells constituting a
LSA may not necessarily provide contiguous coverage.
QOS - Reliability Class
Reference: 24.008 Section: 10.5.6.5
MS -> Network direction
0 - Subscribed Reliability Class
Network -> MS direction
0 - Reserved
MS <-> Network (both directions)
1 - Unused. If received shall be treated as 2.
2 - Unacknowledged GTP; Acknowledged LLC & RLC, Protected data
3 - Unacknowledged GTP & LLC; Acknowledged RLC, Protected data
4 - Unacknowledged GTP, LLC & RLC, protected data
5 - Unacknowledged GTP, LLC & RLC, Unprotected data
Radio Bearer & Radio Access Bearer - RB & RAB
These definitions are better explained by the picture below:
RB and RAB
Radio Bearer Reconfiguration
Radio Bearer reconfiguration is done if the QoS parameters need to be changed for more or fewer resource OR if the traffic volume measurements indicate more or fewer resources are required.
Security Mode Command Setting by the Network
As compared to GSM (which only means the start of ciphering); security mode command setting by the network in UMTS means the following:
1. Start of Ciphering or to command the restart of ciphering
2. May also be used to indicate to start the integirty protection or modify integrity protection configuration for all the signalling radio bearers.
SoLSA - Support of Localised Service Area
Support of LSA
Traffic volume measurement
Measurement of traffic volume on logical channels and reporting to RRC. Based on the reported traffic volume information, RRC performs transport channel switching decisions.
Transport Channel Reconfiguration
This means reconfiguration of the transport channel parameters (Transport Format Set). The transport channel reconfiguration can be done dynamically for each Transmission Time Interval (TTI).
Transport Format Set - defines certain parameters like block size, TTI & error protection.
Important Message Contents of Transport Channel Reconfiguration:
Reference: Section: 10.2.50 of 3GPP TS 25.331
Activation Time - Used by Network in case of unsynchronized Transport Channel Reconfig.
RRC State Indicator
RB with PDCP Information
Added or Reconfigured Transp. Channel Information (UL & DL) - Contains information of
Transport Format Set; Transport Channel Identity, TTI (only for Release 6).
Maximum allowed UL Transmit Power
Downlink information for each radio link - Primary CPICH & Downlink DPCH information for each radio link.
Transport Channel type switching
Execution of the switching between common and dedicated transport channels based on a switching decision derived by RRC.
Questions
- Integirty Key and Ciphering key clear differences and use.
- TTI, TFS, TFCS
- In which message will a GPRS capable mobile indicate the support of DTM and in which message will a network which implements DTM communicate to the device that it supports DTM? Ans: System Information 13
- Channel Encoding definition
