02-02-2010, 10:09 AM
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1. INTRODUCTION
Code Division Spread Spectrum is one form of CDMA communication technique optimized for the cost conscious, high volume and short cycle consumer electronics market. Its primary application is in digital cordless telephony today. Meanwhile, it is being applied to wireless devices for the last 100-meter connection to the Internet, and in the wireless local loop to provide sub-$20 handset cost, tariff free mobile voice services in some third world countries.
Why is CD/SS technology suitable for consumer electronics And how does it compare to other long-range wireless transmission techniques such as DSS and Wide band CDMA This report will introduce the history, theory and practice, and highlight some technical areas for comparison.
Consumer electronics are, by heritage, an Open Architecture market. All forms of technology finds its niche, which eventually help every other technology. This philosophy drives the Lanwave view of this market, and forms the cornerstone to our business approach from sales to research and engineering.
1.1 SPREAD SPECTRUM MULTIPLE ACCESS
Spread spectrum multiple access (SSMA) uses signals which have a transmission bandwidth that is several orders of magnitude greater than the minimum required RF bandwidth. A pseudo noise (PN) sequence converts a narrow band signal to a wideband noise like signal before transmission. SSMA also provides immunity to multipath interference and robust multiple access capability. Spread spectrum multiple access is not very bandwidth efficient when used by a single user. However, since many users can share the same spread spectrum bandwidth without interfering with one another, Spread spectrum systems become bandwidth efficient in multiple user environment. It is exactly this situation which is of interest to wireless system designers. There are two main types of spread spectrum multiple access techniques;
Frequency hopped multiple Access.
Direct sequence multiple Access (DS).
The direct sequence multiple access is also called Code Division multiple access (CDMA).
1.2 CODE DIVISION MULTIPLE ACCESS (CDMA)
In CDMA systems, the narrow band message signal is multiplied with a very large bandwidth signal called the spreading signal. The spreading signal is a pseudo-noise code sequence that has a chip rate which is orders of magnitudes greater than the data rate of the message. All users in a CDMA system, as seen from fig (1) use the same carrier frequency and may transmit simultaneously. Each user has his own pseudo random code word which is approximately orthogonal to all other codewords. The receiver performs a time correlation operation to detect only the specified desired codeword. All the other codewords will appear as noise due to decorrelation. For detection of the message signal, the receiver needs to know the code word used by the transmitter. Each use operates independently with no knowledge of the other users.
In CDMA, the power of multiple users at the receiver determines the noise floor after decorrelation. If the power of each user within a cell is not controlled such that they do not appear equal to the base station receiver, then the near-far problem occurs.
The near-far problem occurs when many mobile users share the same channel. In general, the strongest received mobile signal will capture the demodulator at the base station. In CDMA, stronger received signal levels raise the noise floor at the base station demodulators for weaker signals, thereby decreasing the probability that weaker signals will be received. To combat the near-far problem, power control is used in most CDMA implementations. Power control is provided by each base station in a cellular system and assures that each mobile within the base station coverage area provides the same signal to the base station receiver. This solves the problem of a nearby subscriber overpowering the base station receiver and drowning out the signals of far away subscribers. Power control is implemented at the base station by rapidly sampling the radio signal strength indicator (RSSI) levels of each mobile and then sending a power change command over the forward radio link. Despite the use of power control within each cell, out-of-cell mobiles provide interference which is not under the control of the receiving base station.
Depending on the system, the actual implantation of CDMA may be different. There are three CDMA systems that need consideration the IS-95 2G digital cellular standard that specifies the Um interface in the 800-900 MHz bands for cellular radio in the United States, the JTC Std-008 for the 1,900MHz PCS bands, and the emerging WCDMA and CDMA2000 standards for IMT-2000.
Figure 1.
CDMA in which each channel is assigned to a unique PN code which is orthogonal to the PN codes used by the other users.
1.3 Wireless communications
The primary difference between analog and digital transmission is in the addition of a coding engine known as the wireless DSP base band chip. In the cordless phone and similar voice devices, a second voice DSP chip for source digitization and information compression is used.
As the consumer information sources increasingly turn digital, and device applications increasingly tied to the Internet, digital wireless transmission is becoming necessary, natural and more popular every day, which is not for its performance benefit over analog transmission alone.
The component count, manufacturing cost and engineering, are vastly different in between them. The dichotomy is illustrated by the fact that technology suppliers in this market are largely different and besides solutions from Europe, tends to offer unique and open architecture not yet standardized by committees.
However, the consumers and the product channels are the same. So how these digital wireless products shall take advantage of new technology in new solutions to help the consumer; and how they are engineered, manufactured and their benefits told through the channel; become pivotal to the delight and satisfaction of all parties in the supply chain.
So what does digital transmission offer that analog donâ„¢t And how do these new abilities bring better solutions to the consumer Summarized below are 7 unique digital advantages classified under the dictionary definition of Personal, Communication and Networking:
Personal:
1. Encrypted for exclusive, secured communication.
2. Authenticated logon preventing interception faults.
Communication:
3. Natural and interactive, full duplex in between parties.
4. Multi lingual access with voice and data, combined to allow for future
service upgrades.
5. Supporting circuit and packet connection and connectionless links to the
backbone environment.
Network
6. STAR and BUS topology, flexibly supporting point-to-point and point-to-
multipoint networking.
7. Open architecture protocol, or system command and control, easily
configurable and customizable by the consumer solution designer.
It has been shown from research, that when the base band DSP chip spreads its signal over a wider bandwidth using the correct ways and means, the resultant transmission performance would be vastly improved over analog and narrow band methods. (C.E. Shannon et. al., since 1950.) Performance in these cases is measured by the range, security, clarity and quantity of information transmitted and received.
The practice of spread spectrum communication in military and aerospace applications has over 50 years of history amongst first world countries. But until 10-years ago this technology was confined by the US Government away from commercial use. Planetary satellites in the transmission of images employed the technology over astronomical distances, while military radar and radios use anti-jamming and antieavesdropping communications in the battlefield. Henceforth the ability and robustness of this technology is well proven in non-cost sensitive applications. The challenges today are to commercialize the technology for the consumer electronics market worldwide.
2. Personal communications services.
The term personal communications services (PCS) refers to a wide variety of wireless access and personal mobility services provided through a small terminal, with a goal of enabling communications at any time, at any place, and in any form. Business opportunities for such services are tremendous, since every person (not just every home) could be equipped, as long as the service is fairly inexpensive.
Several PCS systems have been developed to meet rapid growth prompted by heavy market demand. Most of them are connected to the public switched telephone network (PSTN) to provide access to wireline telephones. Examples include high-tier digital cellular systems (mobile phone systems) for widespread vehicular and pedestrian services:
Global System for Mobile Communication (GSM)
IS-136 TDMA based Digital Advanced Mobile Phone Service (DAMPS)
Personal Digital Cellular (PDC)
IS-95 CDMA-based cdmaOne System
and low-tier telecommunication system standards for residential, business and public cordless access applications:
Cordless Telephone 2(CT)
Digital Enhanced Cordless Telephone (DECT)
Personal Access Communications Systems (PACS)
Furthermore, wideband wireless systems have been developed to accommodate Internet and multimedia services. Examples include:
cdma-2000 evolved from cdmaOne
W-CDMA proposed by Europe
SCDMA proposed by China/Europe
3. Wireless System Architecture
Wireless technologies have grown rapidly in the telecommunications industry.
Two of the most popular are:
Cellular telephony.
Cordless and low-tier PCS telephony
These technologies have similar architectures, as shown in the figure (2).
The wireless system architecture consists of three major interconnected
subsystems that interact with the users through certain network interfaces. The subsystems are:
Base Station Subsystem (BSS) or Radio Subsystem
Network and Switching Subsystem (NSS)
Operation Support subsystem (OSS)
The BSS, also known as the radio subsystem provides and manages radio transmission paths between the Mobile Stations and the Mobile Switching Centres (MSCs). The BSS also manages the radio interface between the mobile stations and all the subsystems of wireless systems. Each BSS consists of many Base Station Controllers (BSCs) which connect the MS to the NSS via the MSCs. The NSS manages the switching functions of the system and allows the MSCs to communicate with other networks such as PSTN and ISDN. The OSS supports the operation and maintenance of wireless systems and allows system engineers to monitor, diagnose, and troubleshoot all aspects of the wireless systems. This subsystem interacts with the other subsystems, and is provided solely for the staff of the wireless systems operating company which provides service facilities for the network.
figure (2)
Wireless System Architecture.
The mobile stations (MS) communicate with the Base Station Subsystem (BSS) over the radio air interface. The BSS consists mainly BSCs which connect to a single MSC, and each BSC typically controls upto several hundred Base Trans-receiver stations (BTSs). Some of the BTSs may be co-located at the BSC, and others may be remotely distributed and physically connected to the BSC by a microwave page link or dedicated leased lines. Mobile handoffs between two BTSs under the control of same BSC are handled by the BSC, and not the MSC. This greatly reduces the switching burden of the MSC.
The interface which connects the Mobile station to the BTS is called the air interface. The interface which connects the BTS to a BSC is called the Abis interface. The Abis interface carries traffic and maintenance data. The BSCs are physically connected via dedicated/leased lines or microwave page link to the MSC. The interface between the BSC and a MSC is called the A interface.
4. Development of wireless networks
4.1 First generation wireless networks: Analog voice
First generation cellular and cordless telephone networks are based on analog technology all first generation cellular systems use FM modulation, and cordless telephones use a single base station to communicate with a single portable terminal.
A typical example of a first generation cellular telephone system is the Advanced Mobile Phone Services (AMPS) system used in the United States.
Basically, all first generation systems use the transport architecture shown in the figure (3) below.
figure (3)
Communication signalling between mobile, base station and MSC is first generation wireless networks.
In first generation cellular networks, the system control for each market resides in the MSC, which maintains all mobile related information and controls each mobile hand-off. The MSC also performs all the network management functions, such as call handling and processing, billing, and fraud detection within the market. The MSC is connected with the PSTN via landline trunked lines (trunks) and a tandem switch. MSCs are connectd with other MSCs via dedicated signalling channels for exchange of location, validation and call signalling information.
4.2 Second generation wireless network: Digital voice
The first generation of mobile phones was analog; the second generation was digital. Second generation wireless systems employ digital modulation and advanced call processing capabilities. Examples of second generation wireless systems include the Global System for Mobile (GSM), the TDMA and CDMA U.S. digital standards (the Telecommunications Industry Association IS-54 and IS-95 standards), Second generation Cordless Telephone (CT2), the British standard for cordless telephony, the Personal Access Communications Systems (PACS) local loop standard, and Digital European Cordless Telephone (DECT), which is the European standard for cordless and office telephone.
Second generation wireless networks have introduced new network architectures that have reduced the computational burden of the MSC.GSM has introduced the concept of Base Station Controller(BSC) which is inserted between several base stations and the MSC. In PACS the BSC is called the radio port controller unit. This architectural change has allowed the data interface between the Base Station Controller and the MSC to be standardized, thereby allowing carriers to use different manufacturers for MSC and BSC components. This trend in standardization and interoperability is new to second generation wireless networks. Eventually, wireless network components such as MSC and BSC, will be available as off-the-shelf components, much like their wireline telephone counterparts.
In contrast to first generation systems, which were designed primarily for voice, second generation wireless networks have been specifically designed to provide paging, and other data are services such as facsimile and high-data rate network access. The network controlling structure is more distributed in second generation wireless systems, since mobile stations assume greater control functions. In second generation wireless networks, the hand-off process is mobile-controlled and is known as mobile assisted hand-off(MAHO).The mobile units in these networks perform several other functions not performed by first generation subscriber units, such as received power reporting adjacent base station scanning, data encoding and encryption.
4.2.1 Second-Generation CDMA
Code division multiple access (CDMA) is a spread spectrum based technique for multiplexing, as introduced in section 1.2, that provides an alternative to TDMA for 2G cellular networks. This section begins with an overview of features of CDMA and its advantages and disadvantages and also a look at the most widely used standard IS-95.
The features of CDMA include the following:
Many users of a CDMA system share the same frequency. Either TDD or FDD may be used.
Unlike TDMA or FDMA, CDMA has a soft capacity limit. Increasing the number of users in a CDMA system raises the noise floor in a linear manner. Thus, there is no absolute limit on the number of users in CDMA. Rather, the system performance gradually degrades for all users as the number of users is increased, and improves as the number of users is decreased.
Multipath fading may be substantially reduced because the signal is spread over a large spectrum. If the spread spectrum bandwidth is greater than the coherence bandwidth of the channel, the inherent frequency diversity will mitigate the effects of small-scale fading.
Channel data rates are very high in CDMA systems. Consequently, the symbol (chip) duration is very short and usually much less than the channel delay spread. Since PN sequences have low autocorrelation, multipath which is delayed by more than a chip will appear as a noise. A RAKE receiver can be used to improve reception by collecting time delayed versions of the required signal.
Since CDMA uses co-channel cells, it can use macroscopic spatial diversity to provide soft handoff. Soft handoff is performed by the MSC, which can simultaneously monitor a particular user from two or more base stations. The MSC may choose the best version of the signal at any time without switching frequencies.
Self-jamming is a problem in CDMA system. Self-jamming arises from the fact that the spreading sequences of different users are not exactly orthogonal, hence in the dispreading of a particular PN code, non-zero contributions to the receiver decision statistic for a desired user arise from the transmission of other users in the system.
The near-far problem occurs at a CDMA receiver if an undesired user has a high detected power as compared to the desired user.
With CDMA, all user data, and in most implementations the control channel and signaling information are transmitted at the same frequency at the same time. All the CDMA systems employ direct sequence spread spectrum and powerful error control codes. The primary significance of CDMA is that by employing a variety of physical layer schemes, it is possible to reuse frequencies in all cells unlike the traditional cellular telephony.
4.2.1.1 Advantages and drawbacks of CDMA systems
CDMA has a number of advantages for a cellular network:
Frequency diversity: Because the transmission is spread out over a larger bandwidth, frequency-dependent transmission impairments, such as noise bursts and selective fading, have less effect on the signal.
Multipath resistance: In addition to the ability of DS-SS to overcome multipath fading by frequency diversity, the chipping codes used for CDMA not only exhibit low cross correlation but also low auto correlation. Therefore, a version of the signal that is delayed by one chip interval does not interfere with the dominant signal as much as in other multipath environments.
Privacy: Because spread-spectrum is obtained by the use of noise like signals, where each user has a unique code, privacy is inherent.
Graceful degradation: In CDMA as more number of users can access the system simultaneously, the noise level and hence error rate increases gradually.
Soft handoffs: There is no loss of continuity with the base station in case of soft hand-offs.
Drawbacks of CDMA cellular systems:
Self jamming
Near-far problem
Soft-handoff: It is more complex than hard handoff used in TDMA and FDMA schemes.
4.2.1.2 A 2G-CDMA system: IS-95 Digital Cellular System
Qualcomm proposed the IS-95 CDMA radio system for digital cellular phone applications. It was optimized under existing U.S. mobile cellular system constraints. The CDMA system uses the same frequency in all cells and all sectors. This system design has been standardized by the TIA and many equipment vendors sell CDMA equipment that meet the standards.
The IS-95 CDMA system operates in the same frequency band as the AMPS system using FDD with 25MHz in each direction. The uplink (mobile to base station) and downlink (base station to mobile) bands use frequencies from 869 to 894 MHz, respectively.
Modulation and coding features of the IS-95 system are listed in table
Table 1.
Modulation and coding features of IS-95 CDMA system.
Modulation QPSK
Chip rate 1.2288 Mcps
Nominal data rate 9,600 bps
Filtered bandwidth 1.25 MHz
Coding Convolutional and Viterbi decoding
Interleaving With 20m-sec span
4.3 Third generation wireless networks: Digital voice and
Data.
Third generation wireless systems evolve from mature second generation system. The aim of the third generation systems is to provide a single set of standards that can meet a wide range of wireless applications and provide universal access throughout the world.
3G systems are expected to offer better system capacity and higher data transmission speed to support wireless internet access and wireless multimedia services (including audio, video and images). To bridge 2G technologies to 3G, EDGE and GPRS were introduced; they are typically referred to as 2.5G technologies.
Based on the preceding discussions, the concepts of 3G systems introduce two paradigm shifts:
The shift from voice-centric traffic to data centric traffic demands a packet based infrastructure instead of traditional circuit based infrastructure.
Data applications continue to evolve: As a result, advanced application protocols and human interfaces become very crucial in practical applications. End users will demand the same capabilities whether from wireless or wireline services.
3G wireless communication requires a very broad band spectrum and fast data rate to support high-quality Internet access and multi-media services. Bandwidth, however is always limited. Table 2 lists the existing spectrums used by 2G systems and the new spectrum allocated for 3G.
Table 2.
Terrestrial spectrum allocation for 2G and 3G systems.
SPECTRUM BANDWIDTH SYSTEMS
800MHz 50MHz AMPS,IS-95,
IS-136
900MHz 50MHz GSM-900
1500MHz 48MHz JAPAN PDC
1700MHz 60MHz KOREAN PCS
1800MHz 150MHz GSM-1800
1900Mz 120MHz PCS
2100MHz 155MHz 3G
According to the table, only 25 percent (155MHz out of 628MHz) of the spectrum is newly created for terrestrial 3G usage. Since the 2G systems will eventually be upgraded into 3G, the 2G spectrums will be reused for 3G. Furthermore , to use the resources more efficiently, better channel and source-coding techniques, such as space-time coding and grammar-based lossless data compression, are being developed for MPEG-4,with toll quality voice at a data rate much lower than 8Kbps.
To accommodate flexible 3G applications that are compatible with the existing internet applications, significant efforts have been devoted to enhance portable handset capabilities, including the WAP with micro browser, and new man-machine interfaces, such as voice recognition.
4.3.1 3G-CDMA systems
The CDMA-based 3G standards selected from numerous proposals to ITU have become the major stream for IMT 2000. In particular, W-CDMA and cdma2000 are two major proposals for third-generation systems. Even though both systems are CDMA-based, many distinguishing features can be identified, as listed in table 3.
For one, W-CDMA uses dedicated time division multiplexing (TDM) pilot signal, whereby channel estimation information is collected from another signal stream. This approach reduces the overall pilot power. In contrast, cdma2000 uses common code division multiplexing (CDM) pilot, whereby channel estimation information can be collected with the signal stream.W-CDMA does not need base station timing synchronization, whereas base station timing synchronization in cdma2000 can provide decreased latency and a reduced chance of dropping calls during soft handoff.
Table 4. Comparison of W-CDMA and cdma2000
TECHNOLOGY W-CDMA CDMA2000
Chip rate 4.096 MCps 3.6864 MCps
Forward page link pilot structure Dedicated Pilot with TDM Common Pilot with CDM
Base station Timing Synchronization Asynchronous Synchronous
Forward page link modes A multi carrier mode capable of overlay onto IS-95 carriers
4.3.2 Nortel W-CDMA trial system
Figure (4) illustrates the Nortel W-CDMA trial system. This system operates at 1920-1940 MHz (uplink) and 2110-2130 MHz (downlink). The base transceiver station (BTS) consists of six power amplifiers. The power of every sector in the BTS is 20W. A sector is allocated one or two 5 MHz carriers. The mobile communication center (MCC) consists if a radio network controller (RNC) and an ATM switch. The RNC can connect up to three BTSs over the A-bis interface. The MCC ATM switch supports ISDN and OC-3 interfaces.
figure (4)
Nortel W-CDMA trial system.
The trial system connects to a Nortel GSM network and PSTN through a Meridian CTI page link (ISDN interface to MSC). The MCC connects to the IP network through an ATM backbone. The MSs used in this system were developed by Panasonic. The terminals can support an 8.8 Kbps voice handset, a 64 Kbps PCMCIA card, or a video phone. The services supported by this system include 8.8 Kbps voice, 64 Kbps circuit-switched data, and 384 Kbps packet-switched data.
5. Conclusion.
After successful trial periods and huge investments in network infrastructure, many mobile operators worldwide have deployed 3G wireless networks. Broadband connectivity up to 2Mbps, terminal mobility and deployment in 2000, along with substantially wider and enhanced range of services than 2G systems have been among the major features of the ITU 3G standard. Unlike the exemplary 2G story 3G had to face hard economic situation worldwide, which affected the telecommunication sector and created a vicious circle in which telecom manufacturers could not afford to invest in new mobile systems research and operators could not afford the risk of immature technologies. Moreover, the successful 2G voice and SMS contrasted with the lack of new 3G application, apart from the mobile internet, and the expensive and time consuming spectrum license auctions further delayed 3G systems deployment. This situation gave an opportunity for emerging license-free wireless LAN and personal area network (Bluetooth) technologies to be deployed, capturing a fast growing market with minimum initial investments and operational cost.
The search is on, for the breakthrough steps that will integrate all diverse wireless technologies, along with new truly broadband wireless technologies, in the ( r )evolutionary path towards the emerging 3G+ and 4G heterogeneous networking environment. The vision of 3G+/4G wireless mobile systems will be the provision of broadband access, seamless global roaming, and internet/data/voice everywhere, utilizing for each the most operated always best connecting technology.
TDD-CDMA for fourth generation wireless communications will use an evolutionary time division duplex mode (TDD) of CDMA based path to 3.5G/4G systems.
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