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- 1. Code Division Multiple Access (CDMA)
- 8. CDMA History Early 1989 NADC IS-54 established CDMA development started Proposed by Qualcomm Late 1989 First demonstration May 1995 IS-95A standard published by TIA committee TR45.5 Dec. 1995 TSB-74 introduces 14400 bps TCH June 1996 J-STD-008 CDMA PCS standard July 1998 IS-95B (a consolidation of J-STD-008 and TSB-74) 3rd Generation (3G)
- 9. CDMA Around the World Israel USA Singapore Philippines Korea Japan
- 10. CDMA Network
- 12. CDMA Network Interface to other networks MS MS BTS BTS BTS BS BS MSC MSC VLR VLR HLR EIR AuC OS BSC MSC MSC BSC BTS BTS BTS 1 2 4 5 7 8 * 0 3 6 9 #
- 13. CDMA Network Interfaces PSTN Analog Interface using DTMF or MF signaling Internal Interface Defined SS7 ISDN BRI/PRI Frame Relay Ai Interface RF Test Equipment Um Interface Abis A Interface Air Interface 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
- 14. CDMA Concept
- 15. Direct Spread Frequency Hopping
- 16. CDMA Communication FDMA and TDMA CDMA
- 21. CDMA Capacity Capacity limit is fixed at 8 x number of ARFCNs per cell Capacity limit is ‘soft’, Increases with decrease in quality
- 22. CDMA Capacity What? It’s getting loud in here!
- 23. CDMA Modulation and Demodulation f c f c 0 0 Walsh Code Spreading Walsh Code Correlator Baseband Data Baseband Data Decoding & Deinterleaving Encoding & Interleaving
- 24. CDMA Power Control Acquisition Only 1 2 4 5 7 8 * 0 3 6 9 # Too much power Interference Low talk time Too little power Dropped calls Receive + Transmit Power = -73 dB Assume forward and reverse losses are equal
- 27. Omni Cell Cell Omnidirectional Antenna
- 28. Sectored Cell Sector Sectored Antenna Cell
- 29. CDMA Cell Plan 1 1 1 1 1 1 1 1 1
- 30. CDMA Channels
- 31. CDMA Pilot Channel Pilot Channel 1 Pilot Channel 3 Pilot Channel 2 Sector 2 Sector 3 Sector 1 1 2 4 5 7 8 * 0 3 6 9 #
- 33. CDMA Traffic Channel Forward Link Traffic Channel Reverse Link Traffic Channel 1 2 4 5 7 8 * 0 3 6 9 #
- 36. CDMA Handoffs: Soft Handoff and Hard Handoff
- 37. CDMA Soft Handoff Channel 1 Channel 2 Sector 2 Sector 1 1 2 4 5 7 8 * 0 3 6 9 #
- 38. CDMA Softer Handoff Channel 1 Channel 2 Sector 2 Sector 1 1 2 4 5 7 8 * 0 3 6 9 #
- 39. CDMA Hard Handoff and Hand-Down Channel 1 Channel 2 Sector 2 Sector 1 1 2 4 5 7 8 * 0 3 6 9 # 1 2 4 5 7 8 * 0 3 6 9 #
- 40. CDMA Forward Voice Path
- 41. CDMA Forward Voice Path Vocoder Interleave Modulator Power Correct Error Privacy Voice Control Channelize Coverage
- 49. CDMA Reverse Voice Path
- 50. CDMA Reverse Voice Path Spreading Data Channelize Scrambling Vocoder Interleave Correct Error Modulator
- 57. Summary
Editor's Notes
- © 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 CDMA system works. Describe CDMA interfaces, channel structure, and cell structure.
- Qualcomm, Inc. of San Diego, California, was the first company to propose a second generation digital system based on spread spectrum and code division multiple access (CDMA). CDMA uses code division to allow multiple users on the system at the same time. Time division multiple access (TDMA) uses time division to split a channel into time slots, and frequency division multiple access (FDMA) uses frequency division to split it into frequency ranges, CDMA allows all users to transmit at the same time in the same frequency “passband”, thus dismissing the idea of transmitting on a user-specific “channel”. Put another way, in TDMA, users are assigned to different, non-overlapping time slots to avoid interference from each other. In CDMA, users transmit and receive at all times. The way in which user signals are distinguished from each other, with virtually no or little interference, is by using codes with special properties, thus accounting for “code” in CDMA. Additionally, all users in CDMA are using frequencies in a passband and each user’s frames are encoded with a special pseudorandom noise (PN) code . Code division was such a radical idea that many engineers did not think it could be made practical. However, in 1989, Qualcomm proposed a spread spectrum system and within a year, had demonstrated it successfully. Within three years, the Telecommunications Industry Association (TIA) Committee TR-45.5 published the IS-95A CDMA standard. This standard proposed a 9600 bps vocoder that was increased to a 14400 bps vocoder with the introduction of Technical Services Bulletin (TSB) 74. In the middle 1990’s, Joint Standard-008 was published for CDMA operation in the newly released 1900 MHz band.
- There are two third-generation (3G) CDMA standards. One of the main differences between the standards is the channel bandwidth, for example, 3.6864 MHz for CDMA2000 and 3.84 MHz for W-CDMA. CDMA2000, which is an outgrowth of IS-95 (to be discussed later in this module) is endorsed primarily by the United States and Korea. W-CDMA, a new standard, is endorsed primarily by Europe and Japan.
- In CDMA, multiple users share the same frequencies in a band at the same time. Unique channels are created by having each user’s information modulated by a unique, high bit-rate code sequence that is essentially uncorrelated with that assigned to any other user. The number of users on the system at any given time is limited only by the number of unique code sequences assigned at that time. Theoretically, as many users are allowed as there are unique code sequences. However, CDMA is an interference limited system and there is a natural trade-off between capacity and call quality. This means that the more users you allow to simultaneously access the system, the more overall interference in the system. This in turn can lead to higher frame erasure rates and lower call quality for the users even before you run out of unique codes (i.e., the call will sound poorly if too many users are allowed on the system). You could still assign more codes and physically allow more users onto the system but the system call quality would degrade and cause poor service to customers. Most operators make a judgment call and limit access below the available codes simply to optimize system performance and keep customers happy with call quality. In the real world, CDMA typically provides 6 to 10 times the capacity of analog. The system capacity, or the number of users that can be supported simultaneously in a given CDMA frequency channel, depends on the mutual interference between the two user codes. Although the interference between any two codes is very low, the accumulative interference from many codes can exceed the tolerable level. The interference rejection rate is a function of the ratio between the code rate and the information rate. This ratio is known as the processing gain . CDMA uses processing gain to make the radio frequency (RF) link more reliable.
- The design of CDMA networks is modeled on existing standards for advanced mobile phone service (AMPS) and digital AMPS (D-AMPS) created by Committees TR45 and TR46. The advantage of this is that it has inter-operability capability with the analog and D-AMPS systems. Unfortunately, there needs to be further development before CDMA systems can be overlaid onto the European global system for mobile communications (GSM), if ever. Although the network component names differ slightly, the function of a CDMA network component can be shown to be the same as those in GSM and D-AMPS networks. Mobile stations (MS) talk to the base station (BS) system over the air interface. BS systems consist of a base transceiver station (BTS) and a base station controller (BSC ). The BTS (or cell site) provides the RF link to subscribers. Users communicate directly with the BTS. The BSC provides data routing, voice coding, and some handoff functions. Only one BSC is required for the system. The BSC function can and often is located at the MSC. The mobile switching center (MSC) controls traffic among a number of different BSSs. Multiple BSC’s can be used to help the MSC control call processing for the system if there are numerous cell sites on a MSC. Each MSC has a visitors location register (VLR) in which roaming mobiles are tracked so the network will know where to find them. Each MSC is connected to a home location register (HLR), an authentication center (AuC), and an equipment identity register (EIR ), so the system can facilitate fraud control and verify that users and equipment are legitimate. Facilities also exist within the system for operations and maintenance center (OMC) and network management center (NMC). These are usually centralized and provide remote support to the MSC. The MSC has an interface to other networks such as private land mobile networks (PLMN), public switched telephone networks (PSTN) and integrated services digital networks (ISDN).
- Unlike AMPS, which has many proprietary interfaces, all interfaces in a CDMA system are established in various standards, except for the backhaul. This approach allows different manufacturers to design and manufacture individual parts of the network. This is similar to GSM but unlike D-AMPS. The air interface (U m ) is the protocol between the MS and BTS. The radio interface uses RF signaling as layer one and a modification of the ISDN protocol as layers two and three. All mobile station and BTS vendors adhere to it strictly . The A bis interface is the interface between the BTS and the BSC. The physical is an E1. Layer two uses ISDN signaling and layer three is an ISDN modification. While it is well documented, most vendors have a proprietary version that differs in support of optional elements as well as how they support operations, administration and maintenance (OA&M). A-interface between the MSC and the BS is supported by all MSC vendors. The A-interface uses CCITT CCS7 for the lower three layers to transport modified ISDN call-control signaling. The information carried on this interface pertains to BS system management, call handling, and mobility management. The air interface groups functionality under call-associated signaling. The interfaces are not specific to CDMA and are commonly used in PSTNs for call set-up.
- CDMA can be implemented through frequency hopping or direct spread spectrum coding. Frequency hopping transmits part of the information on a frequency and then “hops” to another frequency to send another piece of the information. The information can be decoded only by receivers that have the hopping sequence key, and can retune accordingly. Many discrete frequencies are used across the entire allocated spectrum, one at a time. Direct spread spectrum CDMA spreads the information over the entire allocated spectrum centered around a carrier frequency at the same time. The IS-95 standard uses this method of encoding.
- Conceptually, FDMA and TDMA access schemes are relatively simple to understand. Both use the channelization approach to frequency management. Alternatively, CDMA is a spread spectrum access technology and it does not follow the channelization principle of traditional radio communications systems. In CDMA systems, channels and communication are combined into the same channel, which shows amplitude that varies over the frequency but describes no distinct pattern over time. In place of narrow channels, the information is transmitted (spread) over a very wide channel with several mobile stations simultaneously using that very wide channel. As previously discussed , IS-95 describes is a direct sequence CDMA access scheme. In a direct sequence system, a low bit-rate information signal is combined with a known, high bit-rate psuedorandom, noise-like signal known as a pseudorandom binary sequence (PRBS). The base station and the mobile station both know the PRBS (secret code). At the mobile station, the receiver accepts signal energy from the selected PRBS and removes (or discriminates against) other codes that do not match. This leaves the information-bearing signal. CDMA allows several parallel communications to take place but to be differentiated from each other by these “secret codes.” Like other systems, CDMA has a forward link and a down link. All communication in a CDMA system takes place over 1.23 MHz frequency blocks. CDMA channels can co-exist within the AMPS frequency bands. 1.23 MHz-wide CDMA channels have been defined for the 80 MHz cellular and the 1900 MHz PCS band. Link separation (between the forward and reverse links) is 45 MHz in a cellular system and 80 MHz in a PCS system. The forward link is used to transmit from the BTS to the mobile station. The reverse link is used to transmit from the mobile station to the BTS.
- An alternative view is that the original message is modulated (in simplest methods, this is multiplication) by the PRBS signal which produces a spectrum that is spread over a wide bandwidth due to the high PRBS bit rate. In the detection process, receivers compare the received signal with their PRBS code. Only those receivers that have the same PRBS code that encoded the message can detect the presence of a message. All other receivers “see” the PRBS encoded message as noise. This means that for those receivers that have the proper PRBS code the message is detected and decoded. For those receivers that do not have the PRBS code of the message, they note the presence of a slightly increased noise level.
- CDMA requires the engineer to adopt a completely new view or paradigm. Neither frequencies nor time slots describe channels. Base stations do not transmit at different frequencies, even to different users. The key to understanding this is to think about what is going on inside the frequency block, i.e., invisible to standard measurement techniques. In the forward channel , channels are assigned by unique digital codes called Walsh codes . There are 64 unique combinations of Walsh codes, hence 64 forward channels in each cell or in each sector. (A cell is defined by the base station’s antenna propagation pattern. Sectorization splits the cells 360 degrees into subcells or sectors.) Each Walsh code sequence is 64 chips in length, and a chip is a binary digit (1 or 0). Walsh codes are orthogonal codes, which is a mathematical property guaranteeing that “exclusive-or-ing” any two of them results in an output consisting of an equal number of 1s and 0s. Exclusive-or-ing the same Walsh code with itself produces a large value equal to the number of bits in the code. This process enables receivers to detect the signal for which they have the proper code and to “not hear” those signals with the other codes. Walsh codes define the type of transmission channel. This means that the correlation between the codes is zero, as defined mathematically to mean the integration of the product between the two codes over the code duration. Walsh code 0 always describes the pilot channel . Pilot channels use short code PN offsets to identify which cell site or sector the mobile is receiving. Walsh codes 1 through 7 always describe the paging channels . Paging channels send information to the mobile station, including call initiation information, channel assignment, system parameters, authentication messages, general page messages, or system orders. Walsh code 32 always describes the synchronization (sync) channel . Sync channels are used for timing synchronization and carrying other key information. The sync channel is demodulated once the mobile tracks the pilot channel. All remaining codes describe individual communication channels .
- All base stations use the same pseudorandom noise (PN) sequence (Walsh 0). However, each base station selects from 512 different PN off-sets that are differentiated by phase relationship. Once a mobile station has synchronized to a pilot channel, it is simple to find the adjacent pilots, which are multiples of 64 clock cycles away from the known one. Each base station has a global positioning system (GPS) receiver to synchronize itself to all other base stations because timing accuracy between base stations is vital for the CDMA system to function.
- The reverse channel product has the same functionality as the forward channel but it goes about it in a different manner. Walsh codes cannot be used to identify mobile stations for two reasons: Each mobile would need to have accurate timing source (for example, its own GPS receiver) and all need to be exactly the same distance from a base station. There would only be 64 mobiles per system. The reverse channel uses another code called a long code to spread, scramble/randomize, and differentiate mobile stations in the reverse direction. The long code has 4.3 billion combinations. There is a clock in the system that repeats once every 41 days. Masking the clock from each mobile station creates individual identification codes which in turn creates many very secure channels. A reverse channel is identified by its CDMA RF carrier frequency and the unique long code PN offset on the individual handset. There are two types of CDMA reverse channels: Traffic channels are used by individual users during their actual calls to transmit traffic to the base transceiver station. There are as many traffic channels as there are CDMA phones in the world. Access channels are used by mobile stations not yet in a call to transmit registration requests, call setup requests, pager responses, order responses, and other signaling information. Access channels are paired with paging channels. There can be up to 32 access channels per paging channel. To initiate communication with the base station and to respond to a paging channel message, a mobile station uses an access channel.
- The number of channels in an FDMA system can be calculated by determining the number of available “frequency” channels. TDMA system capacity can be calculated by multiplying the number of available time slots by the number of available channels. In a CDMA cell, the maximum number of channels can increase and decrease (soft limit) depending upon the demand.
- A common way to describe the differences between FDMA, TDMA and CDMA is an office building analogy. Imagine the floor of an office building divided into work spaces (cubes). Consider the floor space analogous to the frequency band and the cubes to the individual channels. In an FDMA environment such as AMPS, each cube can accommodate a pair of people conversing only with each other. As additional pairs enter the office, they are assigned to an available cube. As occupancy increases, the noise level on the floor increases because the pairs speak louder to be heard. The walls separating the cubes are not effective in blocking the noise. Applying the same analogy to a TDMA system, such as D-AMPS, three pairs of people now reside in each cube. Each pair is required to take turns talking in sequence, and the process is repeated without any deviations. In this way, each pair gets to talk but not at the same time. Any new pairs entering the floor space receive a cube assignment and information about the order in which they may speak. When all the cubes are occupied with allocated pairs, newcomers will have to wait until space becomes available. Again, the noise level on the floor increases as occupancy increases because the pairs speak louder to be heard. In a CDMA system, the floor space would remain but the cubes would be removed, making the entire space available to all users simultaneously. To ensure intelligible conversation, each pair must now talk in a different language. Even though there is a considerable noise level, the pairs can converse because they are using unique languages and the ear can distinguish relevant information from the background noise. There are no “ hard limits ” such as the number of cubes that limit pairs on the floor. As more people enter the floor space, the noise level increases until all conversation has to cease. In this way, CDMA system puts a soft (but monitored) limit on capacity.
- The diagram above depicts baseband data being modulated then demodulated back to baseband. Once the baseband data has been encoded and interleaved*, it appears as if the baseband information has disappeared into the channel noise. The fact that the data could be hidden in a block of interference is why the military spent considerable time and effort to develop spread spectrum applications. The baseband signal can be 7 dB below the channel noise. During demodulation, the signal is decoded and de-interleaved to recover the intelligence. The reason that data can be demodulated is because of the special characteristics of Walsh codes that allow the “conversations” to be distinguished from each other due to the orthogonality of the Walsh functions. *Interleaving is a data communication technique to reduce the number of undetected error bursts.
- Power control is of paramount importance in a CDMA system. The power at the base station, as received from each user, must be made nearly equal to that of the others in order to minimize interference and maximize the total system capacity. There are two types of uplink power control in the CDMA system, open loop power control and closed loop power control. Open loop power control is a course adjustment that assumes the physical channel is symmetrical (equal losses in both directions), and stipulates that the sum of the forward and reverse paths must equal –73 dB (-76 dB in PCS networks). For example, assume that the received power from the base station is -85 dBm. Then for the total power to be -73 dBm, the open loop power setting must be +12 dBm: [-73] – [-85] = +12 dBm Closed loop power control is a fine adjustment that is constantly changing in +/- 1 dB steps. The signal must either be increasing or decreasing; it can never stay the same in closed loop. Open loop power control has no base station feedback loop; the mobile station is in control. There is a base station feedback loop in closed loop power control and the base station has control. To accomplish downlink power control, the base station periodically reduces transmitted power (unsolicited). This process continues until the mobile station senses an increase in the frame error rate (FER) and requests additional power. The base station responds by increasing the transmitted power slightly. This is a closed-loop process . Rogue mobiles are those that transmit at power levels higher than directed. These mobiles do not obey power control commands and can degrade system performance. In this instance, a malfunction timer can be used, a lock order can be issued, or standard transmitter “disable” orders can be sent to the mobile station.
- 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. (For more information on omnidirectional and directive antennas, refer to the GWEC module RT - RF Antenna or Antennas and Propagation for Wireless Communication Systems by Simon R. Saunders.)
- There are two types of cells in a CDMA 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 to allow for contour in border sites where roaming agreements could not be obtained or only certain areas may be covered in the adjacent market due to the contractual agreement. Omni cells are one way of implementing a cell site. A single antenna will put the same channel into each cell, but the PN offset (explained later) will be different.
- Capacity requirements dictate a different approach, namely sectored cells . Antennas in a sectored cell do not radiate equally in all directions like omnidirectional cells 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 is considered a face .
- CDMA uses codes assigned to individual voice channels. Because CDMA is designed to decode signals correctly in the presence of high levels of interference, adjacent cells can use the same (co-channel) frequencies. This is an N=1 cell plan . If AMPS and CDMA are intended to co-exist in a serving area, the operator needs to clear a number of AMPS channels per sector for the first (and succeeding) CDMA channels. Each CDMA channel occupies 1.23 MHz of spectrum which corresponds to about 40 AMPS channels. Typically, three AMPS channels are replaced in each sector. A 900 kHz guard-zone is required from the CDMA channel center frequency on each end as well. This needs to be coupled with creating a buffer zone between the CDMA and AMPS cell using the same frequency. This buffer zone has to be 1.77 MHz. The first CDMA carrier in the AMPS A-band at channel number 283 and the first CDMA carrier in the B-band is at channel number 384. Guard bands need to be introduced between the different bands to ensure that CDMA carriers do not interfere. There can be potentially nine CDMA carriers across the AMPS frequency allocation. (For more information on AMPS, refer to the GWEC module AI-AMPS .)
- The CDMA pilot signal is a timing source used in system acquisition and is used as a measurement device during handoffs. It is transmitted in every cell. Unlike TDMA systems, the pilot signal is only used to synchronize to the strongest sector. The diagram above illustrates how the MS can (and does) evaluate several pilot signals simultaneously. This is important to the MS’s ability to participate in mobile-assisted handoffs (MAHO) .
- The pilot channel is a structural beacon that does not contain a character stream. It is transmitted at all times by each base station on each active CDMA frequency. This signal is tracked continuously by each mobile station. The pilot channel is present only in the downlink and is used by the mobile to: Obtain initial system acquisition. Provide time, frequency, and phase tracking of the signals from the cell site. This information allows the mobile station to: Determine if the system is digital capable. Provide a reference clock for channel demodulation. Use the information as a reference signal level for handoff decisions. The MS must initialize to the pilot channel before attempting to access any other CDMA control channel. System information is decoded from the sync channel. The sync channel is found easily once the mobile has locked onto the pilot channel. The pilot channel data is an unmodulated series of 0s, “spread” using a Walsh code of 0. CDMA soft handoff requires base stations to operate in sync with each other. CDMA base stations have global positioning system (GPS) receivers and use the GPS not for geographic positioning information, but as a common system clock to establish synchronization. This is critical in high mobility situations to rapidly process multiple signals from multiple sources, compute the best path, and then perform the soft handoff. (Source: Wireless Personal Communication Systems by David J. Goodman , page 208)
- By definition, the forward link and reverse link traffic channels allow communication to take place between the base station and mobile station.
- Once a strong pilot channel is located, the mobile station listens to the corresponding sync channel for system information. The sync channel is spread using Walsh code 32 and will be demodulated whenever the mobile station tracks the pilot channel. The sync channel carries a data stream of system identification and parameter information used by mobiles during system acquisition. The mobile station resynchronizes at the end of every call. The sync channel operates at a fixed rate of 1200 bps and carries: System identification number/Network identification number (SID/NID) Cell site identification BS protocol revision level Paging channel data rate CDMA channel number Pilot transmit power Pilot PN off-set Comparing this to the sync channel in GSM, it appears that the pilot and sync channels of the GSM and CDMA system have each other’s roles. (For more information on GSM, refer to the GWEC module AI-GSM .)
- The paging channel is used by the base station to transmit system overhead information and mobile station-specific messages such as pages, system parameters information, and call setup orders. For each paging channel that the base station transmits, the base station continually transmits valid paging channel messages. These messages may include the null message which consists of two 0’s. While Walsh code 1 is the primary paging channel, the paging channel is spread using Walsh codes between 1 and 7. The paging channel can operate at a data rate of either 4800 or 9600 bps. System operators determine the appropriate numbers of paging channels to implement. The paging channel is used to send messages informing the mobile station of certain information. Access parameter messages control how the mobile initiates calls. The channel assignment message and the CDMA channel list control how the mobile is assigned a traffic channel and which channels are available. Other messages include the following: Authentication messages General page messages System orders
- Handoff is the process by which a mobile station maintains communication with the mobile services switching center when traveling from the coverage area of one base station to that of another. A mobile station can execute a handoff while it is either in the idle state or in call. An idle handoff occurs while a mobile station has moved from the coverage area of one base station into the coverage area of another base station while it is in the idle state. If the mobile station detects a pilot channel signal from another base station that is sufficiently stronger than that of the current station, the mobile station determines that an idle handoff should occur. Soft handoff is a handoff in which the mobile station starts communications with a new base station without interrupting communications with the old one. Soft handoffs occur between base stations using identical CDMA frequency assignments. The use of rake receivers (four receivers in each mobile and at base stations) allows for soft handoffs and overcomes multi-path fading conditions. Soft handoff allows the serving cell and a candidate cell to temporarily serve the call during the transition, while the mobile station combines the two signals to produce a higher quality link. The mobile station monitors the two channels and when it decides that only one of the channels is strong enough, the mobile station requests the base station to terminate the soft handoff. Soft handoffs are designed to prevent dropped calls the way hard handoffs do, but real-world conditions still result in some dropped calls. Hard handoffs occur when base stations have different CDMA frequency assignments. The transition process reduces the probability of a dropped call, reduces “ping pong,” (boundary handoffs), and eliminates the “break-before-make” in the handoff process. Requirements for a soft handoff include the following: All links must be on the same CDMA frequency. All links must use the same traffic frame offset. Participating base transceiver stations must be connected to the same base station controller.
- A similar process takes place when the mobile station moves from one sector to another in the same cell. In the softer handoff , the parallel path is supported and cell diversity combines the signals from both sectors. The MSC is notified of the activity but does not participate directly. No additional mobile switching center/cell path is set up for the softer handoff. There is a caveat, however. In a poorly designed system, it is possible for a mobile station to access the system, immediately go into soft handoff, and maintain that status throughout the call. This situation would have an impact on system resources and reduce the overall capacity of the system.
- A hard handoff in CDMA is the traditional “make-before-break” used by AMPS, D-AMPS and GSM. The mobile station assists in the process by taking measurements from adjacent channels and reporting them to the MSC. Hard handoffs occur between base stations having different CDMA frequency assignments. The handoff typically takes from one-half to one second to complete. A CDMA-to-CDMA hard handoff can take place in any of the following situations: The mobile station is transitioning between two cells operating on different CDMA frequencies. The mobile station is transitioning between two cells operating in the same CDMA frequency but the traffic channels assigned in both cells have their frames aligned differently. The mobile station is transitioning between cells which are connected to different MSCs, whether in the same or in different CDMA systems (referred to as inter- and intra-system handoffs, respectively.) The term hand-down is used to describe the handoff that occurs between a CDMA and traditional AMPS system at the system boundary. This process is also “hard” in nature and results in the characteristic break in the conversation.
- The CDMA forward voice path from the base station to the mobile contains eight modules.
- Vocoding reduces the bit rate needed to represent speech. CDMA specifies a variable-rate vocoder in place of the fixed-rate vocoder used in D-AMPS and GSM. The vocoder rate varies based on thresholds that change according to background noise levels, and activates the higher vocoder rates used for user speech. The result is noise suppression and good voice quality, even in a noisy environment. The overall required user power is also decreased. The vocoder also provides variable rate coding, adjusting the bit rate to the voice activity. If you talk faster, a higher bit rate (up to the full data rate, typically 9.6 kbps in current generation vocoders) is used to adequately code your voice and if you slow down, a slower bit rate is used. This helps keep overall system noise at an overall lower level which allows for greater system capacity. In a typical two-way conversation, the duty-cycle of each voice is approximately 50%. This means that each person is talking only 50% of the time. CDMA takes advantage of this by compressing voice traffic and reducing that transmission rate during the silent (or idle) periods. This process reduces interference to users in the same cell as opposed to reducing interference between cells as with D-AMPS and GSM. The use of voice activity detection (VAD) also reduces the average mobile station transmit power requirements. The primary IS-95 vocoder operates at an 8 kbps rate within a standard 9.6 kbps digital data stream. This is known as Rate Set 1 and the rates supported are 9.6, 4.8, 2.4 and 1.2 kbps of digital data. To provide better voice quality (at the expense of capacity reduction and coverage loss), a Rate Set 2 has been standardized. Rate Set 2 operates at 14.4 kbps and supports 14.4, 7.2, 3.6 and 1.8 kbps of digital data. Both base stations and mobile stations support either Rate Sets 1 or 2. Extended variable rate coding (EVRC) uses new algorithms to produce “toll-quality” speech at a lower rate (8 kbps).
- The forward error correction (FEC) circuit provides protection for the information bits by providing repetition and recovery capability through error detection/correction. Repetition is required to maintain a constant input to the forward interleave circuit and additional countermeasures against the damaging effects of the radio channel (fading). The FEC output will always be 19.2 kbps; therefore the different input bit rates are treated slightly differently. Rate Set 1 does not repeat any symbols if the FEC input is 19.2 kbps. It repeats each bit twice if the input is 9.6 kbps, repeats each bit four times if the input is 4.8 kbps, and so on. Repeated symbols have lower power so the aggregate across all the symbols remains the same.
- CDMA uses a matrix system for interleaving. Interleaving “shuffles” the information so it does occur contiguously. Any errors that occur will be distributed in the block and can be corrected more easily. Data is input to a 4 X 4 matrix by rows and read out by columns. Block interleaving is used to combat the effects of Rayleigh fading. Rayleigh fading is frequency selective fading that causes errors in large blocks of contiguous data. If contiguous information is sent in consecutive frames, fading makes it very difficult to reconstruct the information. (For more information on Rayleigh fading, refer to the GWEC modules RT-RF Antenna and RT-RF Propagation .) An error of short duration will not destroy adjacent bits of the message. Instead, bits that are rather far apart, e.g., AE, will have the errors after interleaving rather than adjacent bits AB before interleaving.
- Long code was discussed previously in conjunction with reverse link channelization. In the forward link, the same code sequence is used to scramble the voice data and provide an additional level of privacy. A process of exclusive-or-ing the voice data with masked long code produces a scrambled voice signal that can be unscrambled later by using the long code. The rate at which the data is scrambled is 64 times lower than the long code generator rate. A decimator is used to pass only every 64 th bit from the masked long code generator into the exclusive-or circuit. Exclusive-or-ing (XOR) is a process by which 1s and 0s are added using a modification in the normal rules of binary addition. Example A shows the Truth Table of normal binary addition. Example B shows the Truth Table of XOR’d addition. Example A Example B (1+1=1, with a carry) (1+1=0, nothing to carry) 1 1 0 0 0 0 1 0 1 1 1 1 Input X Input X Input X 1 1 0 0 0 0 1 0 1 0 1 1 Input X Input X Input X
- CDMA is an interference-limited system based on the number of users. As already discussed, CDMA has a soft capacity limit. Each user is a noise source on the shared channel and the noise contributed by users is cumulative, which creates a practical limit to how many users a system will sustain. Precise power control of mobile stations is critical to help maximize system capacity and increase the battery life of the mobile stations. The goal is to keep each mobile station at the absolute minimum power level necessary to ensure acceptable service quality. Ideally, the power received at the base station from each mobile station should be the same. Mobile stations that transmit excessive power increase interference to other mobile stations. Closed loop power control substantially diminishes the disparity between uplink and downlink power levels. Power is more important in the uplink for two reasons. First, downlink power is measured continuously on the unmodulated pilot signal. The uplink power measurement is more complex. Second and more importantly, uplink power maximizes uplink capacity. The base station makes fine adjustments to the estimated power level (from the open loop power control) in 1dB steps. Puncturing the voice data with a power control sub-channel facilitates setting mobile station power. The rate at which the voice data is punctured is 1536 times lower than the long code rate. Any data lost from the voice channel can be recovered by using error correction in the mobile station. Adjustments can be made every 1.25 msec (800 times/sec), so power levels will increase or decrease by in 1dB increments, almost constantly. The dynamic range is +/-24 dB of the open-value and the total dynamic range (open and closed) is 80 dB.
- Channelization in the forward link is achieved by spreading (increasing the data rate) using a Walsh code generator running at 1.2288 mcps (64 times faster than the data rate). Each bit of the 19.2 kbps input becomes the XOR of a 64 bit Walsh code, which is unique to that channel. At this point, the output has been “spread” (19.2 kbps x 64 bits = 1.2288 mcps) over a larger bandwidth with each original bit being represented by 64 Walsh bits. The mobile station considers each 64-bit/symbol-block input and XORs them with the same Walsh code that created them. The result will be a majority of either 1s or 0s, which is the original data bit. Even if errors occur, typically either the number of 1’s or 0’s will dominate the data block. Due to the orthogonal nature of the Walsh code, exclusive-or-ing one Walsh code with a different one will result in an output consisting of an equal number of 1’s and 0’s. The receiver will not be able to determine the input bit. NOTE: kcps = kilo chips per second. Chip is the nomenclature for a binary signal element in a digital spread spectrum carrier. The word chip distinguishes these short signal elements from the source bits of an information signal and the channel bits of an information signal protected by a channel code. The modulated digital spread spectrum signal in turn modulates a radio carrier to produce the transmitted signal. The ratio of the digital carrier chip rate (Wch/s) to the source information rate (Rb/s) in a spread spectrum signal is referred to as the processing gain (G). (Source: Wireless Personal Communications Systems by David J. Goodman, page 209) G= Wch/s X ch/b Rb/s
- Once the Walsh spreading is done, the data is then quadrature spread (covered) with a base station-specific PN sequence, know as a short code . This process gives it a base station-specific identity and results in a quadrature phase shift keying (QPSK) output with a choice of 512 possible offsets. NOTE: I = In-phase control Q= Quadrature (out-of-phase) channel
- The reverse voice path is slightly different from the forward path but provides same essential functions.
- The same vocoders are used in both the reverse and forward paths. A “gated” variable rate scheme is used to reduce the average power in the uplink channel. This reduces the interference levels throughout the CDMA system, while improving capacity and performance.
- Error correction (EC) is applied to all bits in the same way. The only difference here is that the output of the convolutional coder is now 28.8 kbps. The lack of a clock in the mobile station dictates improved error correction over the uplink, hence the additional EC bits. One-half rate (2:1) at 14.4 k bps and one-third rate (1:3) at 9.6 kbps, convolutional coders are used.
- Reverse interleaving is the same as forward channel interleaving. It is read in by rows and out by columns.
- Reverse data spreading randomizes the data in an easy-to-recover scheme and spreads data to 307.2 kcps. The method for increasing the data rate and scrambling the data in the reverse link is called 64ary modulation . The name is more complex than the modulation scheme. This function substitutes groups of six code bits for all 64 bits of a Walsh code. These can only have one of 64 values. Coincidentally, there are 64 Walsh codes output from the modulator. Hence, the 64ary modulator could be thought of as a look-up table with a rate increase on the output (28.8 kbps/6 x 64 = 307.2 kcps).
- Channelization (spreading) is achieved using the same long codes as at the base station. This is so because of capacity and of all mobiles needing to be the same distance from the base station and synchronized to one another.
- In the reverse path, there is no need to hide the Walsh codes but the short code is used to scramble the sequence. A half-chip delay is added to the output of the Q modulator so that the I and Q modulators do not both make zero crossings at the same time. This reduces the complexity of the output amplifier design.
- CDMA differs from other systems in that it has a frequency reuse of one , meaning that every customer utilizes every cell. Interference is managed by keeping the power from every user in the base station nearly equivalent, i.e., within two decibels. Customers who are close to the edge of the cell will be powered way down. Customers at the edge of the cell can be brought up toward their full power of 200 mW. There is less interference with other users in the same cell, as well as with users in other cells by maintaining such tight and accurate power control, which is the bane of all other multiple-access techniques. Keeping the power low maximizes battery life. In tests, it is consistently found that the average transmitted power is below 10 mW. In side-by-side comparisons, the transmitted power is approximately 25 dB below AMPS. One of the biggest problems in cellular telephony today is that calls often get dropped. This typically happens as the call is being handed off from one station to another. To address this, CDMA supports soft handoff. One base station is not dropped until contact is established with another. In peripheral areas, CDMA provides more reliable communications and uses less power because just enough power is transmitted to reach the base station that has the best reception from where the mobile phone located.
- CDMA offers the following benefits: High capacity, with no hard limits. Up to ten times the capacity of analog networks. Tradeoff users for quality. Easy frequency planning. Every frequency can be used in every cell. Simplified planning, lower operating costs. Greater coverage with fewer cells. Both startup (broader coverage)and higher capacity cells. Tradeoff capacity for coverage area. Multipath interference actually improves signal. Well suited to in-building transmissions. Powerful technology platform for PCS Advanced digital features and services (SMS, data service, ISDN type). Packetized structure easily supports data. Facilitates interleaving of voice and data signals. Excellent call quality. Soft handoffs improve call quality. No call degradation on handoffs. Important improvement for data transmission. Improved privacy. Built-in message encryption on every call. Private (personal) code available. Hand portable friendly. Low power terminals, long battery life.