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- 1. Time Division Multiple Access (TDMA)
- 8. TDMA History A A M M P P S S A A n n a a l l o o g g C C e e l l l l u u l l a a r r S S e e r r v v i i c c e e I I S S 5 5 4 4 A A I I S S 5 5 4 4 B B F F i i r r s s t t D D i i g g i i t t a a l l D D u u a a l l - - M M o o d d e e C C e e l l l l u u l l a a r r A A d d d d s s D D i i g g i i t t a a l l T T r r a a f f f f i i c c C C h h a a n n n n e e l l I I S S - - 1 1 3 3 6 6 C C e e l l l l u u l l a a r r A A d d d d s s D D i i g g i i t t a a l l C C o o n n t t r r o o l l C C h h a a n n n n e e l l , , e e n n h h a a n n c c e e m m e e n n t t s s I I S S - - 1 1 3 3 6 6 P P C C S S U U p p c c o o n n v v e e r r t t e e d d I I S S - - 1 1 3 3 6 6 1 1 9 9 8 8 2 2 1 1 9 9 9 9 4 4 1 1 9 9 9 9 6 6 / / 7 7 1 1 9 9 9 9 7 7 / / 8 8
- 9. TDMA Time Slots 1 2 3 4 5 6 Frequency Amplitude Time 6 1 2 3 4 5 6 1 Time Slot Number
- 10. TDMA Network
- 11. TDMA Network NMC Interface to other networks MS MS BTS BTS BTS BS BS MSC MSC MSC MSC VLR VLR HLR EIR AUC OMC OMC BSC BTS BTS BTS BSC 1 2 4 5 7 8 * 0 3 6 9 #
- 12. TDMA Network Interfaces PSTN LAP-Dm RF Test Equipment 1 2 4 5 7 8 * 0 3 6 9 # 1 2 4 5 7 8 * 0 3 6 9 # 1 2 4 5 7 8 * 0 3 6 9 # BTS BTS BTS BSC BSC BSC MSC MSC MSC U m Interface Not Common Interface Not Common Interface Not Common Interface
- 14. TDMA Cell Plan 3 6 6 2 2 1 4 5 7
- 16. TDMA ‘Omni’ Cell Cell Omnidirectional Antenna
- 18. TDMA Sectored Cell Sector 120 Degree Sectored Antenna Old Cell 3 New Sub Cells
- 20. TDMA Channels
- 21. TDMA Traffic Channel RDTC Reverse Digital Traffic Channel FDTC Forward Digital Traffic Channel Digital 1 2 4 5 7 8 * 0 3 6 9 #
- 22. TDMA Control Channels Digital DCCH 1 2 4 5 7 8 * 0 3 6 9 #
- 27. TDMA Handoffs
- 28. TDMA Handoff FDTC ( Forward Digital Traffic Channel) Mobile measures BER & RSSI of FDTC and reports to BS 1 2 4 5 7 8 * 0 3 6 9 #
- 31. TDMA Burst Amplitude Time 3 symbols 3 symbols 156 symbols 3 symbols 1 symbol = 2 bits
- 32. TDMA Burst Data Structure – Reverse Channel 3 3 8 14 61 6 6 61 G R D Sy D S C D Guard Data Synch Data Ramp Up Data SACCH CDVCC 3 R Ramp Up
- 33. TDMA Data Structure – Forward Channel Synch SACCH Reserved Data CDVCC 14 6 65 6 65 6 Sy S D C D Rs Data
- 34. TDMA Burst Equalization Training bits of synchronization data Actual received signal
- 35. TDMA Power Steps 1 2 4 5 7 8 * 0 3 6 9 # Too much power interference = Low talk time Too little power = Dropped calls Adjusted according to received signal strength
- 37. TDMA Signal Flow
- 38. TDMA Voice Path Codec Interleave Vocoder Modulate Burst
- 39. TDMA Vocoder 2 classes of bits are error corrected in different ways Class 1 bits: Error correction, CRC bits and tail bits added Class 2 bits: No error correction Vocoder 20ms speech makes 160 bits Output 8 kbits/s 2 classes of bits output 8:1 compression
- 40. TDMA Interleave Interleave 260 bits 260 bits 8 kbit/s 260 bits 8 kbit/s 260 bits
- 41. TDMA Codec 3 3 8 14 61 6 6 61 G R D Sy D S C D Guard Data Synch Data Ramp Up Data SACCH CDVCC 3 R Ramp Up
- 42. TDMA Modulator Output 48.6 kbits/s /4 DQPSK Input 8 kbits/sec (Error corrected interleaved) Modulate
- 43. TDMA Burst Burst Output /4 DQPSK (bursted) Input 48.6 kbits/s /4 DQPSK
- 44. SUMMARY
Notas del editor
- © Copyright 2001 Global Wireless Education Consortium All rights reserved. This module, comprising presentation slides with notes, exercises, projects and Instructor Guide, may not be duplicated in any way without the express written permission of the Global Wireless Education Consortium. The information contained herein is for the personal use of the reader and may not be incorporated in any commercial training materials or for-profit education programs, books, databases, or any kind of software without the written permission of the Global Wireless Education Consortium. Making copies of this module, or any portion, for any purpose other than your own, is a violation of United States copyright laws. Trademarked names appear throughout this module. All trademarked names have been used with the permission of their owners .
- Partial support for this curriculum material was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE-9972380 and Advanced Technological Education Program under grant DUE‑9950039. GWEC EDUCATION PARTNERS: This material is subject to the legal License Agreement signed by your institution. Please refer to this License Agreement for restrictions of use.
- After of completing this module and all of its activities, you will be able to: Explain how a TDMA system works. Describe TDMA interfaces, channel structure, and cell structure.
- Time Division Multiple Access (TDMA) is a digital transmission technology that is replacing narrowband analog systems in the cellular telecommunications industry. Due to the increased demand for cellular communication, the wireless industry began to explore methods of converting from analog to digital technology in the late 1980’s. In 1987, the TR45.3 Working Group debated the use of a TDMA approach for the digital system, a decision heavily influenced by the TDMA system in Europe called Global System for Mobile Communications (GSM). In 1988, the Cellular Telecommunications Industry Association’s (CTIA) Advanced Radio Technology Committee (ARTS) created User Performer Requirements (UPR), documenting the requirements for the next generation digital system. In 1989, the CTIA chose TDMA as the technology of choice for existing 800 MHz cellular markets and for 1.9 GHz markets. At the same time, the decision was made to let individual carriers make their own technology decisions. In order to reduce development time and considering that the analog system already had a control channel, it was decided to use the same channel sets and control channel for implementing TDMA. This left the need only to develop the time slot structure, modulation technique, message formats, and the method by which a mobile could be assigned to a digital traffic channel as well as an analog traffic channel. Interim standards (IS) were established. IS-54 Rev A added feature sets, caller ID and improved voice quality as well as corrected a variety of other errors. IS-54 Rev B (TIA/EIA 627) added authentication and voice privacy. IS-136 introduced Digital Control Channels and GSM-influenced features. A mobile could now set up a call as well as transmit data over a digital link. Over the last few years, IS-136 has been upgraded to the 1900 MHz PCS band. The major difference in the PCS system is that it is solely digital with no analog channels present.
- TDMA channels are 30 kHz wide. The TDMA frame is 40 msec and consists of six time slots that are 6.667 msec each. There are 162 symbols per time slot and the symbol period is 41.16 msec. Each user is assigned a pair of slots in a channel: Time Slots 1 and 4 = User 1 Time Slots 2 and 5 = User 2 Time Slots 3 and 6 = User 3 When half-rate codecs (coder/decoders) are released, each user will be assigned only one time slot. The TDMA concept permits reuse of the existing cellular plan. The benefit is that three conversations can be carried on each radio frequency. The diagram above illustrates four users assigned to TDMA channels 1, 2, 5 and time slots 1/4, 3/6. Two of the users are on TDMA channel 1, using time slots 1/4 and 3/6. One user is on TDMA channel 2, using time slots 1/4. The fourth user is on TDMA channel 5, using time slot 3/6. The combination of a time slot number and TDMA channel is called a physical channel , while the time slot itself is considered the logical channel . Each digital mobile with 8 kbps “full-rate” speech coders will initially be assigned to two time slots. When new “half-rate” (4 kbps) speech coders are developed, mobiles with that capability will be assigned to single time slots. The system will also be able to accommodate a mixture of mobile types ranging from all (3) full-rate to all (6) half-rate mobiles on a given RF channel, or combination of both.
- The diagram above shows the various elements of a TDMA network . It is based very strongly on the European GSM system. The only real difference is nomenclature. Instead of the base transceiver station (BTS) and base station controller (BSC) being collectively called a base station (BS) system, they are known as a base station.
- TDMA utilizes a flexible air interface that allows for high performance in capacity and coverage, and has unlimited support of mobility. In TDMA, the only common interface is the Air Interface (U m ). Any connection between network elements “behind” the antenna is proprietary. Some manufacturers do allow interworking of equipment but this is based on intercompany relationships and not on a standard. Interoperability between wireless networks is standardized in IS-41.
- TDMA is a cell-based system that relies on channel reuse. There must be a specified distance between the cells using the same channels to avoid co-channel interference . Adjacent channel interference must be considered as well. In the graphic above, the cells with the same numbers are those which can reuse the same channels. The N=7 spacing shown can present a challenge for the service provider in supplying enough analog control channels (as there are only 21 available) to support demand. This problem was eliminated in IS-136, due to the extra capacity and flexibility of digital control channels. ( For more information on the N=7 plan, refer to the GWEC modules , FRP-Cellular Coverage Concepts and RT-RF System Planning.) Theoretically, TDMA should be able to work in an environment where the signal to interference (S/I) requirements are not as stringent as AMPS. Nevertheless, empirical data suggests that TDMA must have at least the same 17 dB carrier to interference ratio (C/I) as recommended for AMPS. Therefore TDMA technology improves system capacity, but does not improve voice quality because the C/I system requirements remain the same as for AMPS. If analog AMPS implementation is poor, TDMA technology will not help. This should suggest that TDMA implementation is not implicit. If analog implementation is inadequate, your digital system may be more trouble than its worth. Note that many customers prefer the voice derived from digital vocoders when the 17dB C/I level is achieved. (For more information on C/I, refer to the GWEC module, FRP-Cellular Coverage Concepts .) TDMA caveats: Sufficient analog control channels Initial loss of trunking efficiency (need to seed the network with digital MS) Single channel set per face No T-coding (ADPCM); must be clear channel Practical cell size is about 12 miles because of delay considerations
- Antennas are required by the base station to both transmit signals to mobile units and receive signals from mobile units. An antenna that sends and receives signals in all directions is an omnidirectional antenna . However, service providers often restrict the directionality of the antenna to provide coverage to specific areas called sectors . These antennas are called directive antennas and are identified by their degree of directionality. If an antenna is focused to serve one-third of its possible 360 range, it is called a 120 directional antenna. If an antenna is focused to serve one-sixth of its possible 360 range, it is called a 60 directional antenna.
- There are two types of cells in a TDMA network: Omnidirectional cells Sectored cells An omnidirectional cell uses an antenna that radiates in all directions, and single or multiple channel sets can be assigned to the cell. In some instances, omnidirectional antennas are used in conjunction with reflectors that pattern-shape the transmitted energy. This allows for contour in border sites where, possibly, roaming agreements could not be obtained or due to the “letter” of the agreement, only certain areas may be covered in the adjacent market. (For more information on omnidirectional antennas, refer to the GWEC module RT - RF Antenna .)
- In the diagram above, solid lines show interference from co-channel base sites to a mobile in the central cell of interest. The dashed lines show interference from co-channel mobiles to a base site in the central cell of interest.
- Capacity requirements will dictate a different approach, namely sectored cells . Antennas in a sectored cell no longer radiate in all directions but have a directional beam that can vary in width, depending on the antenna design. The example above shows three 120-degree sectors in each cell. Each sector can now be assigned a different channel set for control of capacity and increased reuse. Each sector is considered a face . Sectoring further subdivides channel groups and prevents the worst effects of adjacent-and co-channel interference. Sectoring can be visualized to occur either at the cell edge or at the cell center. A cell can be sectorized, thus dividing it into three sub cells by mounting a three-sector, 120 degree antenna on the original antenna tower. The first sector uses the original frequency set and the two new antenna faces reuse other frequency sets from the pattern. Each face is shifted 120 degrees. A cell split can be based on a “D” site by maintaining the central cell as a “D” cell while rotating the rest of the pattern counterclockwise by 120 degrees. The new cells will then have sides one-half the size of the original cells.
- Total interference power is reduced to only 1/3 of what it was in an omnicell for both transmission paths by dividing each cell into three sectors each. Interference from base sites to the mobile of interest comes from different cells than interference from mobiles to the base site of interest. Acronyms - alpha - beta - gamma
- As with AMPS, in the digital wireless environment there are two primary transmission links: the Forward Digital Traffic Channel (FDTC) or downlink and the Reverse Digital Traffic Channel (RDTC) or uplink. The FDTC represents the energy transmitted from the base station down to the mobile station and the RDTC represents the energy transmitted up from the mobile station back to the base station. The traffic channel defines and multiplexes (time-shares) control, voice, and data on a single carrier. To accomplish this, the traffic is divided into several different “logical” channels. Control channels transfer broadcast, paging and access control. Traffic channels transfer voice and data. TDMA (in the 800 MHz band) uses the same channel sets as the pre-existing AMPS system. Consequently, from an interference perspective, the digital system can only perform as well as the analog frequency plan will allow. Additionally, the path-balancing concept is “alive-and-well” in TDMA.
- The first thing that a MS will do after powering on is to try to find a network. In IS-54, it will scan for an Analog Control Channel (ACC). While in IS-136, it will scan for both an ACC and Digital Control Channels (DCCH). Once it locates the strongest channel it will attempt to register with that network using “overhead” messages. The MS can be programmed to scan a particular place for ACCs and DCCHs so that the mobile will register with the network more quickly. When an IS-54 system must co-exist with an AMPS system, access queuing becomes a problem and the analog system cannot coordinate digital radio channel assignment. Dual-mode units can access the system through a secondary dedicated control channel, if provided. The dual-mode unit searches for the digital Protocol Capability Indicator (PCI) on the AMPS control channel. If it cannot find digital capability, it assumes that it has found an older AMPS-only control channel. If the MS finds the PCI bit equal to “0,” it searches for a secondary control channel. If it finds no secondary channel, the unit attempts to access the system as an analog station.
- A control channel can be a different time slot on the same radio frequency (RF) channel as a voice channel. The Forward Digital Control Channel (FDCCH) is not limited to the 21 fixed frequencies and limited paging capacity of analog control channels. Digital Control Channels (DCCH) can be accessed only by IS-136 mobile stations and have the same functionality as their analog counterparts. However, features such as paging classes, temporary mobile station identifier (TMSI), and the fact that the DCCH can be placed in any slot can increase paging capacity. The DCCH has a 16-frame structure called a superframe , that is broadcasted continuously. The logical sub-channels on the FDCCH include the Broadcast Channel (BCCH) . The BCCH is made up of two logical sub-channels which must be acquired by the MS in the order indicated, namely: Fast Broadcast Channel (F-BCCH) that provides general system information that the mobile stations require “quickly,” such as the system identification number (SID). Extended Broadcast Channel (E-BCCH) that provides system information only after the MS has “camped-on” the DCC. The Short message, Paging and Access response Channel (SPACH) provides mobile-specific information related to pages, access, etc. Shared Control Feedback (SCF) flags are part of this access channel and provide information to control channel access by multiple mobiles.
- A fundamental difference between the FDCCH and Reverse Digital Control Channel (RDCCH) is that the reverse channel is bursted by the MS and the forward channel is not bursted. The RDCCH provides an access point for MSs to randomly attempt to access the network. This control channel is called a Random Access Control Channel (RACH ). The RACH is a point-to-point, unacknowledged channel, although the system control field (SCF) sends an acknowledgement message on the downlink channel when the system recognizes that the MS has attempted to contact the system. The MS monitors the SCF channel to determine if it is permitted to attempt access on the RACH. Included in the RDCCH is a “contention-based” Access Response Channel (ARCH) that is a channel shared by all mobile units requesting service.
- The Slow Associated Control Channel (SACCH) is used to transport routine information to and from the MS. The SACCH is a continuous data stream of signaling sent beside speech data. The “slow-channel” uses a few dedicated bits within each time slot and does not affect speech quality. Typically, messages sent via the SACCH have long delays (440 msec) as opposed to the 40 msec required by the Fast Associated Control Channel (FACCH) . The SACCH message is divided into small parts and transmitted over a sequence of twelve time slots. This interleaving helps error correction perform a more effective job. The amount of data that can be sent over the SACCH is limited, which is why it is considered slow. This is fine for routine overhead information, but of no use when large quantities of data need to be sent rapidly. As an example of the mobile station’s use of SACCH, the MS monitors bit error rate, received signal strength, and adjacent cell signal strength. The MS also uses the SACCH to communicate a potential channel problem, change in location, or potential need for a handoff. Dynamic time alignment adjustments are also sent on the downlink using SACCH.
- In the same way that the SACCH is slow, the FACCH is fast but is used for non-routine signaling. As suggested previously, FACCH messages incur 40 msec versus 440 msec of delay. This channel is used when the quantity of data that needs to be transmitted is too large for the SACCH and/or is needed quickly. Whereas SACCH does not affect speech, FACCH control messages replace speech data with signaling. In IS-54, speech quality is degraded non-linearly as more speech frames are replaced. Currently no limit has been placed on the number of speech frames that may be replaced. IS-136 has the capability to “bridge-over” frames of speech data that have been replaced by FACCH. It does this by repeating the last good frame (20 msec) of speech. Most (IS-136) FACCH transmissions go unnoticed by the user. FACCH data is error protected by a ¼ rate convolutional coder. That means that only 1-in-4 of the bits is actually FACCH bits. Neither IS-54 nor IS-136 make allowance for “stealing flags” (as in GSM) to identify the data as speech or signaling, so all data is first decoded as speech. If the decoding process indicates that it is not a valid speech slot, the receiver will attempt to decode it as a FACCH message. If the error correction (EC) decodes correctly, it is a FACCH message. If the EC decodes incorrectly, then it is assumed to be speech data that has errors due to interference.
- A handoff (HO) occurs in any wireless system. When an MS is operating on the boundary of two adjacent cells, it cannot maintain the link with the serving base station and must connect to a new channel from a new serving base station. The link quality is monitored differently for analog channels than it is for digital channels. In an analog system, the base station is constantly monitoring the quality of the supervisory audio tone (SAT) that is always present on the reverse voice channel (REVC) during a call. When the signal level falls below a prescribed level or S/I threshold, the BS will initiate a handoff to a new base station and forward voice channel (FOVC). When a dual-mode subscriber unit is operating on a digital channel, the handoff process is considerably different from AMPS: The mobile station measures the signal strength and estimated bit error rate (BER) of adjacent cells and using SACCH, the MS continuously sends radio channel quality information to the BS. When necessary, the BS directs the MS to tune to a new radio channel. After the MS receives the HO message, it mutes its audio. After retuning, it sends shortened bursts to the candidate cell, if the HO was between cells separated by more than two miles to avoid burst overlap. The new base station determines the necessary time adjustment and commands the MS to realign. The MS un-mutes audio and commences communication.
- The system must be able to cope with handoffs both to and from analog and digital channels. Consequently, there are four different types of handoffs: Analog to analog Analog to digital Digital to analog Digital to digital If the MS is not on a call, it will simply rescan the available control channels every five minutes. When it sees that the present control channel is no longer as strong as ones found in the new scan, it will reregister to the most powerful one available.
- TDMA is a “bursted” system . Consequently, the burst can be examined in the frequency and time domains. The major difference between the modulation formats in TDMA and GSM is that in GSM there is only one bit transmitted per symbol, whereas in TDMA there are two bits per symbol. For example, 156 symbols x 2 = 324 bits in each time slot (frame) as opposed to 148 symbols x 1 =148 bits in the GSM frame. Therefore, TDMA carries 324 bits in each time slot (frame) as opposed to 148 bits in the GSM frame. If observed in time, the axis reveals four distinct areas: 3 guard symbols 3 ramp symbols 156 information symbols 3 ramp symbols The guard and ramp symbols stop succeeding bursts from interfering with one another (overlapping) but carry no useful information. (For more information on GSM, refer to the GWEC module AI- GSM .)
- A TDMA time slot (frame) contains 162 symbols. In the diagram above, the frame seems to contain 165 symbols. The reason for the discrepancy is that the 162 symbols do not include the three leading guard symbols. The burst at that point is “off” and the symbols are really being used as “symbol periods,” to measure time rather than data. The forward channel is not bursted; therefore, this is only a reverse channel issue. The frame is divided into eight areas. Some of these areas are considered sub-channels. These sub-channels should not be confused with RF channels. The sub-channels are: Synchronization channel Slow Associated Control Channel (SACCH) Coded Digital Verification Color Code (CDVCC) The remaining areas contain the guard, ramp symbols, and the data.
- The forward channel structure is similar to that of the reverse channel, with some notable exceptions: The frame contains only 162 symbols but only 156 are available to the user. The six end symbols are reserved for future use. The data is in two blocks of 65 symbols and not in three blocks. The synchronization channels, SACCH and CDVCC, are positioned differently within the frame. There are no guard and ramp symbols as the forward channel is not bursted, but present continuously. Because the forward channel is transmitted continuously, filler bits are added in idle time slots.
- Multipath signals occur when the same source RF signal takes different propagation paths to arrive at the receiver at slightly different times. This fact results in the reception of one or more delayed copies of the same signal, which can cause symbol amplitude and phase distortion. TDMA has relatively short bit intervals, requiring “adaptive equalization.” Adaptive equalization uses the characteristics of a known signal to separate multipath signals from an unknown received signal. The TDMA equalizer must be able to extract multipath signals with delays of up to 20 microseconds. Equalization must also compensate for the Doppler shift of the burst frequency as a result of the movement of the MS. The BS instructs the MS to place a known “training sequence” into the synchronization channel. It will also put the same sequence in its own synchronization channel in the forward link. This training sequence field provides a standard pattern to accommodate equalizer adaptive calibration. The base station and mobile, as part of their design, know what the received signal should prefer. If it does not see what is expected, the equalizer will set an inverse filter to counter the effects of the environment on the transmitted signal.
- The BS monitors the reverse voice channel power level in the analog system, or the MS reports on a digital channel and uses the results to decide whether the power should be adjusted (increased or decreased) based upon the Received Signal Strength Indicator (RSSI) reading. The challenge for an MS is to use just enough power to overcome any signal degradation caused by path losses and not too much to create interference that will impact frequency reuse or cause batteries in the MS to be depleted too quickly. TDMA power classes include: Class 1 4 W max, 6 mw min, 1.333W average Class 2 1.6 W max, 6 mw min, .533W average Class 3 0.6 W max, 6 mw min, .2W average (reduces minimum cell radius for microcellular systems) Class 4 Dual-mode MSs transmit in bursts, so their average output power is properly measured during the burst period. Differential Quadrature Phase Shift Keying (DQPSK) modulation, coupled with a 33% transmit duty cycle for full-rate TDMA, causes a typical measurement of about 30% of peak burst power.
- Digital Verification Color Code (DVCC) is the digital equivalent to the analog SAT tone. Each base station has its own unique “color-code” used to detect interference from neighboring cells and to indicate that the mobile station is operating (off hook) on a channel. A unique DVCC for each cell ensures that the correct MS is communicating with the proper base station. The DVCC is transponded by the MS, much like the SAT tone. There are 255 unique codes available. In some systems, the 255 codes are divided between contiguous markets, with no reuse assumed. In this manner, if any interference is observed, the DVCC will indicate which of the markets is causing the problem.
- Above is a simplified block diagram of a digital cellular phone and the signal flow. The voice to be transmitted enters the microphone and is input into the vocoder . The vocoder digitizes the audio and then uses complex algorithms to minimize the amount of data needed to represent the information to be sent. This data stream is then interleaved to add more protection against lost bursts, and then input into a channel codec that adds additional bits needed for channel coding. The data is then sent to the Pi/4 DQPSK modulator and up converted to the cellular frequency band. This RF carrier with modulation is then filtered and bursted at the appropriate time. The following slides further explain each of these steps in the voice path.
- The primary purpose of the vocoder is to digitize voice and minimize the amount of data needed to represent and reconstruct the original voice signal. TDMA uses a vocoder to model the tone and noise generation of the human throat, and the acoustic filtering of the mouth and tongue, by breaking the speech up into long-term short-term vector sum blocks. The TDMA system uses Vector Sum Excited Linear Predictive (VSELP) vocoders . The VSELP vocoder digitizes the voice and outputs an 8 kbit/s data stream that represents the voice. The two vector blocks are coded according to standardized "codebooks." Fewer bits are applied to the long-term block since these elements of speech change slowly and contain little information. More bits are used to represent the short-term sounds as they change much more rapidly. Speech is sampled in 20 msec time increments (blocks) that create 160-bit/speech blocks. Assuming the 64 kbps input to the vocoder and a 8:1 compression ratio, an 8 kbps rate at the output of the vocoder is obtained (i.e., 260 bits/.02 seconds=8 kbps). This vocoder achieves good voice quality with a data rate of only 8 kbit/s.
- As a countermeasure against an unreliable transmission path (to combat Rayleigh fading), TDMA bits spread themselves over several TCH bursts. By the process of interleaving, the data is spread over several time slots on the radio path, thus reducing the probability of total corruption of speech and reduction in voice quality or loss of speech. The data blocks are then interleaved so that any bursts that may be lost do not cause loss of sequential data blocks, and hence large amounts of speech. The 8 kbps rate equates to the “payload or throughput,” less error correction and cyclic redundancy check (CRC) bits. The actual “line-rate” is 13 kbps (i.e., 260 bits/.02 seconds=13 kbps) .
- The channel codec performs several functions: Adds control channel information and supervisory messages needed to maintain and establish the channel. Adds the CDVCC . Adds the SACCH. Adds a synchronization sequence that is used for applying equalization filtering to correct for the effects of fading and multipath. Adds guard and ramp bits to ensure that no data is lost due to burst overlap and/or ramp-up/ramp-down time.
- The data is now ready to be modulated by a Pi/4 DQPSK modulator . Pi/4 is used because the design of a QPSK modulator is more complex. Pi/4 DQPSK has no instantaneous phase changes that would cause the output amplifier to go into saturation. By making all phase transitions relative to the previous state, there are only smooth phase changes so that the amplifiers and modulator are easier to produce.
- The final part of the transmission is to burst the modulated data in the correct time slots and carry the data on the top of the burst.
- Some of the key concepts about TDMA that were discussed in this module include the following: Three calls per 30 kHz (Six time slots per 30 kHz) Approximately three times voice channel capacity Digitalize voice (8 kbps) Other TDMA networks (GSM) The cellular industry decided to adopt a hybrid frequency division multiple access/time division multiple access (FDMA/TDMA) approach in 1990. The decision was made to continue to use 30 kHz wide channels. Each channel is divided into six time slots. The voice coder (vocoder) technology requires that two time slots be used for each phone. Therefore, three conversations can share the same frequency as a result of the concept of “time division”. Future improvements in vocoder technology will likely lead to only one time slot being needed for each phone. At that point, up to six conversations will be able to share the same frequency. When TDMA capability is added to an existing analog FM system, a portion of the 800 MHz cellular spectrum must be dedicated for use as TDMA channels. In other words, some of the existing 30 kHz wide channels that were used for FM must be retired from FM service and used for new TDMA service.
- TDMA provides many advantages, as listed above and on the next two slides.
- Additionally and more specifically, dual-band 800/900 MHz systems offer the following competitive advantages: Identical applications and services are provided to subscribers operating in both bands. Carriers can use the same switch for 800 MHz and 900 MHz services. Seamless interworking between 800 MHz and 900 MHz networks through dual-band/dual mode phones. Using dual-mode, dual-band phones, subscribers on a TDMA 900 MHz channel can handoff both to and from a TDMA channel on 800 MHz as well as to and from an analog AMPS channel.
- There are some disadvantages of TDMA. Users roaming from one cell to another are not allotted a time slot. Therefore, if all the time slots in the next cell are already occupied, the call will be disconnected. Likewise, if all the time slots in the cell in which a user is in are already occupied, the user will not receive a dial tone. A signal coming from a tower to a handset might come from any one of several directions. It may have bounced off of several different buildings before arriving, which can cause interference. One way of getting around this interference is to put a time limit on the system. The system will be designed to receive, treat, and process a signal within a certain time limit. After the time limit has expired, the system ignores signals. The sensitivity of the system depends on how far it processes the multipath frequencies. Even though the multipath signal only travels for a thousandth of a second, these multipath signals cause problems. All cellular architectures, whether microcell- or macrocell-based, have a unique set of propagation problems. Macrocells are particularly affected by multipath signal loss, a phenomenon usually occurring at the cell fringes where reflection and refraction may weaken or cancel a signal. TDMA capacity is limited by the six time slot structure. With half-rate mobiles, each digital mobile with 8 kbps full-rate speech codes will initially be assigned to two time slots. This means each radio frequency pair can support up to three voice calls. New half-rate (4 kbps) speech coders could increase that capacity to six voice calls per radio frequency by coding speech into a single time slot pair. TDMA is ultimately limited by the six time-slot structure.