CDMA 2000 System

One of the major cell/mobile phone or cellular telecommunications technologies today is the CDMA-1/CDMA-2000 system. One of its strengths is that it has focussed on being an evolutionary technology moving from standards such as IS-95 (IS-95A and IS-95B) for CDMA-1 through to standards including IS-2000 and IS-856 for CDMA2000 1X, 1xEv, 1xEV-DO and 1xEV-DV. Currently the standard uses one standard channel under a system known as 1X RTT, although for the future three channels (3X RTT) may be used.

In view of the fact that the CDMA2000 system has been designed to be an evolutionary standard, it is relatively easy to introduce upgrades to the system. This has made it particularly popular with operators because the cost of upgrading to the new standards is much less, and they can have users with a variety of types of phone on the same network. Thus users may operate CDMA-1 phones on the same network as CDMA2000 1X or CDMA2000 1xEV-DV phones.

The first CDMA networks in the form of IS-95/CDMA-1 were the first deployments of CDMA technology, the technology that is being used for all 3G cell phone systems. This formed the basis for a unique evolutionary system as CDMA2000. CDMA2000 is a well established 3G technology, and it is particularly successful in the USA, and Asia Pacific regions as well as having a significant presence in many other parts of the world. It was able to offer 3G services well before W-CDMA became established, and it is now continuing to build on this success.

The idea for using the form of modulation known as direct sequence spread spectrum (DSSS) for a multiple access system for mobile telecommunications came from a California based company called Qualcomm in the 1980s. Previously DSSS had been mainly used for military or covert communications systems as the transmissions were hard to detect, jam and eavesdrop.

The system involved multiplying the required data with another data stream with a much higher data rate. Known as a spreading code, this widened the bandwidth required for the transmission, spreading it over a wide frequency band. Only when the original spreading code was used in the reconstruction of the data, would the original information be reconstituted. It was reasoned that by having different spreading codes, a multiple access system could be created for use in a mobile phone system.

In order to prove that the new system was viable a consortium was set up and Qualcomm was joined by US network operators Nynex and Ameritech to develop the first experimental code division multiple access (CDMA) system. Later the team was expanded as Motorola and AT&T (now Lucent) joined to bring their resources to speed development. As a result the new standard was published as IS-95A in 1995 under the auspices of the Cellular Telecommunications Industry Association (CTIA) and the Telecommunications Industry Association (TIA). As part of the development of CDMA an organisation called the CDMA Development Group (CDG) was formed from the founding network and manufacturers. Its purpose is to promote CDMA and evolve the technology and standards, although today most of the standards work is carried out by 3GPP2.

It then took a further three years before Hutchison Telecom became the first organisation to launch a system. It is now widely deployed in North America, and the Asia Pacific region, but there are also networks in South America, Africa, and the Middle East as well as some in Eastern Europe.

System Basics

The CDMA system was totally unlike any system used before. In the UK the original TACS system had used a channel spacing of 25 kHz and AMPS in the US had used 30 kHz. The new GSM system used 200 kHz channels whilst the US-TDMA standard kept compatibility with AMPS and was based around 30 kHz channels. CDMA, IS-95A, used a 1.25 MHz bandwidth and this was much wider than anything that had been used before. CDMA operates well with a wide bandwidth, but it was limited to 1.25MHz to remain compatible with the spectrum allocations that were available.

CDMA, Code Division Multiple Access, is a multiple access scheme used by many 3G cellular technologies, and other forms of wireless technology. It uses a process called Direct Sequence Spread Spectrum where spreading codes are used to spread a signal out over a given bandwidth and then reconstituting the data in the receiver by using the same spreading code. By supplying different spreading codes to different users, several users are able to utilises the same frequency without mutual interference.

There were other differences as well. CDMA mobiles did not have SIM cards, although recently this has changed. Instead the subscriber data has simply been stored in memory of the mobile with a method of over-the-air programming of this data being available.

CDMA-1 Standard

The first offerings of CDMA under the guise of IS-95 catered for voice as well as data up to a speed of 14.4 kbps. However, with the market moving towards data applications, the IS-95 specification was upgraded to IS-95B to cater for the needs of operators. This new specification allowed packet switched data transmission up to a speed of 64 kbps. IS-95B was first deployed in September 1999 in Korea and has since been adopted by operators in Japan and Peru.

Often IS-95 A and B versions are marketed under the brand name CDMA-1. This is a registered trademark of the CDMA Development Group.

CDMA2000 1X Standard

CDMA-1 had been very successful and was introduced into many countries, but with operators seeing revenue from voice services levelling off, the pressure to migrate to 3G technologies where data speeds were higher and revenue growth could be maintained. As a result of this the IS-2000 standard was developed to enable the higher 3G data rates to be provided.

Within IS-2000 a number of further developments were included. It was envisaged that with many more areas moving towards 3G standards and the old AMPS systems being made obsolete it would be possible to have systems operating on a wider bandwidth. As a result of this the new standards allowed for systems that would use the single channel bandwidth (1X or 1X RTT) and also ones that would use three times the bandwidth (3X). Currently all work is focussed on the 1X systems, with the idea for the 3X (or 3X RTT) systems to be used some time in the future.

CDMA2000 1X can double the voice capacity of CDMA-1 networks and delivers peak packet data speeds of 307 kbps in mobile environments although today's commercial CDMA2000 1X networks (phase 1) support a peak data rate of 153.6 kbps. CDMA2000 1X has been designated a 3G standard and it is now widely deployed.

Evolution

CDMA2000 1X is the basic 3G standard, in fact some people only consider it a 2.75G system, and it is being developed beyond this. In what is termed CDMA2000 1xEv, there are further developments to bring it in line with the UMTS or Wideband CDMA system that is being deployed in Western Europe and many other areas.

The first of these known as CDMA2000 1xEV-DO (EVolution Data Only) is something of a sideline from the main evolutionary development of the standard. It is defined under IS-856 rather than IS-2000, and is as the name indicates is only carries data, but at speeds up to 2.4Mbps in the forward direction and the same as 1X in the reverse direction.

The forward channel forms a dedicated variable-rate, packet data channel with signalling and control time multiplexed into it. The channel is itself time-divided and allocated to each user on a demand and opportunity driven basis. A data only format was adopted so that the system could be optimised for data applications, and if voice is required then a dual mode phone using separate 1X channel for the voice call is required. In fact the “phones” used for data only applications are referred to as Access Terminals or ATs.

The first commercial CDMA2000 1xEV-DO network was deployed by SK Telecom (Korea) in January 2002. Now operators in Brazil Ecuador, Indonesia, Jamaica, Puerto Rico, Taiwan and the USA to mention but a few have all launched networks and more are to follow.

Data and voice

The next logical evolution of the system is to incorporate both data and voice into the standard. This is exactly what CDMA2000 1xEV-DV achieves. This is catered for under Release C of the IS-2000 standard. And is effectively 1X with additional high speed data channels. In this way it is able to provide complete backward compatibility with both CDMA2000 1X and CDMA-1. In addition to this the migration requires comparatively few upgrades to a 1X system and as such it is a very attractive option for network operators. Further developments are available under Release D of the IS-2000 standard. This provides for 3.1 Mbps data in both directions as well as many other upgrades.

CBSE (UGC)-NET DECT System

DECT (Digital Enhanced Cordless Telecommunications) system is widely used for residential, and business cordless phone communications. Designed for short-range use as an access mechanism to the main networks, DECT offers cordless voice, fax, data and multimedia communications, wireless local area networks and wireless PBX. With the flexibility offered by cordless phone communications, DECT has become the major standard for this application and DECT is now in use in over 100 countries worldwide.

DECT development

The standard for DECT or Digital Enhanced Telecommunications system was developed by members of the European Telecommunications Standards Institute (ETSI). The first release of the standard was available in 1992 after which much of the work was focussed on inter-working protocols (DECT/GSM, DECT/ISDN, etc).

As a result of this work, DECT/GSM inter-working has been standardized and the basic GSM services can be provided over the DECT air interface. This enables DECT terminals to inter-work with DECT systems which are connected to the GSM infrastructure. All roaming scenarios based on SIM roaming as described in GSM specifications are applicable.

Along with requirements arising from the growing use of DECT, this work gave rise to a number of extensions to the basic DECT standard. This led to a second release of the standard at the end of 1995. This included facilities including: Emergency call procedures, definition of the Wireless Relay Station (WRS), and an optional direct portable to portable communication feature.

DECT air interface operation

The most common protected spectrum allocation for DECT is 1 880 MHz to 1 900 MHz, but outside Europe spectrum is also available in 1 900 MHz to 1 920 MHz and in 1 910 MHz to 1 930 MHz (several countries in Latin America). In addition to these frequencies there is also a reservation in some countries in the band 2 010 MHz to 2 025 MHz.

DECT carriers have been defined for the whole spectrum range 1880 MHz to 1980 MHz and 2010 MHz to 2025 MHz in the ETSI standard. The basic frequency plan for the 1880 to 1900 MHz DECT band provides for ten channels. The additional frequencies beyond the this allows for expansion of the basic DECT allocation or allows DECT services to be introduced in countries where the basic DECT frequencies are not available. The use of extended or new frequency allocations does not cause regulatory difficulties for roaming DECT handsets as it is mandatory for handsets not to start transmission on carrier frequencies others than those informed by the base station in broadcast messages.

The signal is modulated using a form of modulation called Gaussian Frequency Shift Keying (GFSK) and has a BT of 0.5. This provides the optimum spectral usage for the system.

The system uses dynamic channel allocation and is thereby able to reduce the levels of interference, and ensure that links are set up on the least interfered channels. All DECT equipment scans the frequency allocation at least every 30 seconds as a background activity. This produces a list of free and occupied channels along with the available timeslots to be used for the channel selection, should this be required.

Additionally the DECT portable continuously analyses the signals to ensure that the signals originate from the base station to which it is connected and has access rights. The portable locks onto the h4est base station and checks it can access the base station as detailed in the DECT standard, and the channels with the best signal strength (RSSI-Receive Signal Strength Indication) are used for the radio link as required. This Dynamic Channel Selection and Allocation mechanism guarantees that radio links are always set-up on the least interfered channel available and hence the best performance is obtained.

DECT MC or TDMA or TDD principle

The DECT radio interface employs a number of techniques in its access methodology. The scheme uses Multi-Carrier, Time Division Multiple Access, Time Division Duplex (MC/TDMA/TDD).

The basic DECT system has a total of ten possible carrier frequencies between 1880 and 1900 MHz, i.e.. It is a Multi-Carrier (MC) system.

In addition to this the time dimensions for each carrier is divided to provide timeframes repeating every 10 ms. Each frame consists of 24 timeslots, each of which is individually accessible and may be used for either transmission or reception. For the basic DECT speech service two timeslots-with 5 ms separation-are paired to provide bearer capacity for typically 32 kbps (ADPCM G. 726 coded speech) full duplex connections.

In order to simplify the way DECT can be used when only basic implementations are needed, the allocations of timeslots within the 10 ms timeframe are restricted. The first 12 timeslots are used for downlink transmissions and the remaining 12 are used for the uplink. This reduces the level of complexity, and as this is not needed for basic implementations, it can provide some cost savings.

DECT codecs

The basic telephony speech quality offered by DECT is very high compared to many other wireless systems. This is the result of the use of the ITU-T Recommendation G. 726 codec that is employed. This is a 32 kbit/s ADPCM speech codec and although it uses 32 kbps, the quality it affords is high and there is more than sufficient bandwidth within the system to support it.

TDMA structure

The DECT TDMA structure enables up to 12 simultaneous basic voice connections per transceiver. The system is also able to provide widely varying bandwidths by combining multiple channels into a single bearer. For data transmission purposes error protected net throughput rates of integral multiples of 24 kbps can be achieved. However, the DECT standard defines a maximum data rate of 552 kbps with full security.

What is the DECT GAP profile?

All DECT systems are based on a main standard that is the Common Interface (CI), which is often used in association with the Generic Access Profile (GAP). The GAP profile ensures interoperability of equipment from different providers for voice applications. The GAP defines the minimum interoperability requirements including mobility management and security features. It has different requirements on public and private systems. This means that the GAP is effectively the industry standard for a basic fall-back speech service with mobility management. This basic service is not always used, but instead it forms the fallback that is always be available, especially when requested by a roaming phone, etc

DECT Summary Although DECT has been in use for a number of years now, its flexibility and performance have meant that it is still the major technology used for cordless phones. The standard is maintained by ETSI, and this will enable it to move forward as new requirements appear and technology enables further facilities to be added.

DECT Glossary

·         DECT: Digital Enhanced Cordless Telecommunications

·         DMAP: DECT Multimedia Access Profile

·         DPRS: DECT Packet Radio Service

·         FP: Fixed Part-the base station

·         GAP: Generic Access Profile

·         GSM: Global System for Mobile telecommunications

·         IMT-2000: International Mobile Telecommunications 2000

·         PP: Portable Part-the handset

·         RES: Radio Equipment Systems.

CBSE (UGC)-NET EMC Filter Designs

 In order that an item of electronics equipment can pass its EMC testing and gain its EMC compliance, it is necessary to incorporate various elements into the design. By designing the circuit to meet the electromagnetic compatibility, EMC requirements it is possible to significantly reduce the levels of unwanted signals entering and leaving the unit. One of the major ways in which this can be done is to use an EMC filter or a series of filters.

There are many ways in which EMC filters can be incorporated into a unit from a mechanical viewpoint. They may exist as stand alone EMC filters to be fixed near to the extremities of the unit. They may be mounted on the edge of the electronics board. However, one popular method of incorporating an EMC filter into a unit is to incorporate the filter into the connector itself. This has many advantages in terms of convenience and performance. However, whatever the method used, a filter is often necessary if the electromagnetic compatibility, EMC requirements are to be met.

EMC filter methodology

Although circuits may be well screened to prevent any signal radiated or being picked up by the circuit itself, there are always interconnections to and from the electronics circuit. These wires themselves can conduct unwanted signals into and out of the unit. If the unit is to be able to meet its electromagnetic compatibility, EMC requirements and pass its EMC testing, it is necessary to reduce the levels of unwanted signals that can enter or leave the unit via its interconnections.

In order to enable the unwanted signals to be removed, EMC filters need to be placed in the various lines. The idea is that the interfering signals generally have a frequency above that of the signals normally travelling along the wire or line. By having what is termed a low pass filter as the EMC filter, only the low frequency signals are allowed to pass, and the high frequency interference signals are removed.

These EMC filters can be in one of a variety of formats. Often they may be as simple as a resistor or a ferrite placed around a wire or cable. For more exacting requirements, these EMC filters may need to be made up from a number of components.

The EMC filters may categorised into two main types. One is where the unwanted energy is absorbed by the EMC filter. The other type of filter rejects the unwanted signal and in this case it is reflected back along the line. For EMC filtering applications, the absorptive type is preferred.

EMC filter application

When developing filters for use in electromagnetic compatibility, EMC applications, the EMC filters are nearly always low pass filters, although on occasions bandpass filters may be used. The reason for using low pass filters is that typically interfering signals, i.e.. Ones that are easier to pick up or radiate tend to be at higher frequencies.

These can be filtered by allowing the low frequencies through and rejecting the high frequencies. The cut-off point for the low pass filter used as the EMC filter has to be chosen so that it rejects the unwanted frequencies, but does not have any undue effect on the wanted signal.

Unfortunately this choice is not always easy and it may require some degradation of the wanted signal. The EMC filter placing is of importance. EMC filtering can be placed at any or every level of assembly between segregated areas of circuitry. EMC filters may be placed between segregated areas of a printed circuit board. They may be placed between different boards within a module or sub-assembly, and an EMC filter may be placed between different modules or subassemblies. However, a particularly important place for EMC filters is between the equipment and its external environment. An EMC filter placed here is particularly effective as it will prevent unwanted signals even entering the equipment. Once they enter they are more difficult to contain.

EMC filter design

The EMC filter design is critical to the electromagnetic compatibility, EMC performance. The EMC filter must be capable of providing the required level of attenuation of the unwanted signals while allowing through the wanted signals. In addition to this the EMC filter design must match both the source and load impedances.

Typically for a high impedance circuit, a capacitor connected between the line and ground provides better results, while for low impedance circuits a series inductor placed within the line provides the best results. Often a single component like this designed to have a reactance with little effect at frequencies appropriate to the wanted signals, but a much higher effect at the higher frequencies of the unwanted signal can provide levels of attenuation of up to 30 dB or 40dB in some cases. To improve the performance of one of these basic filters, further components can be added to make multi-component EMC filters. However, to give the required performance they must be configured correctly. One precaution to ensure that inductors face a low impedance sink or source and capacitors face a high impedance.