3G-Mobile

3G is the third generation of tele standards and technology for mobile networking, superseding 2.5G. It is based on the International Telecommunication Union (ITU) family of standards under the IMT-2000.
3G networks enable network operators to offer users a wider range of more advanced services while achieving greater network capacity through improved spectral efficiency. Services include wide-area wireless voice telephony, video calls, and broadband wireless data, all in a mobile environment. Additional features also include HSPA data transmission capabilities able to deliver speeds up to 14.4 Mbit/s on the downlink and 5.8 Mbit/s on the uplink.
Unlike IEEE 802.11 networks, which are commonly called Wi-Fi or WLAN networks, 3G networks are wide-area cellular telephone networks that evolved to incorporate high-speed Internet access and video telephony. IEEE 802.11 networks are short range, high-bandwidth networks primarily developed for data.

Implementation and history

The first pre-commercial 3G network was launched by NTT DoCoMo in Japan branded FOMA, in May 2001 on a pre-release of W-CDMAtechnology. The first commercial launch of 3G was also by NTT DoCoMo in Japan on October 1, 2001. The second network to go commercially live was by SK Telecom in South Korea on the 1xEV-DO technology in January 2002. By May 2002 the second South Korean 3G network was by KTF on EV-DO and thus the Koreans were the first to see competition among 3G operators.
The first European pre-commercial network was at the Isle of Man by Manx Telecom, the operator then owned by British Telecom, and the first commercial network in Europe was opened for business by Telenor in December 2001 with no commercial handsets and thus no paying customers. These were both on the W-CDMA technology.
The first commercial United States 3G network was by Monet Mobile Networks, on CDMA2000 1x EV-DO technology, but this network provider later shut down operations. The second 3G network operator in the USA was Verizon Wireless in October 2003 also on CDMA2000 1x EV-DO, and this network has grown strongly since then.
The first pre-commercial demonstration network in the southern hemisphere was built in Adelaide, South Australia by m.Net Corporation in February 2002 using UMTS on 2100 MHz. This was a demonstration network for the 2002 IT World Congress. The first commercial 3G network was launched by Hutchison Telecommunications branded as Three in April 2003.
In December 2007, 190 3G networks were operating in 40 countries and 154 HSDPA networks were operating in 71 countries, according to the Global Mobile Suppliers Association (GSA). In Asia, Europe, Canada and the USA, telecommunication companies use W-CDMAtechnology with the support of around 100 terminal designs to operate 3G mobile networks.
In Europe, mass market commercial 3G services were introduced starting in March 2003 by 3 (Part of Hutchison Whampoa) in the UK and Italy. The European Union Council suggested that the 3G operators should cover 80% of the European national populations by the end of 2005.
Roll-out of 3G networks was delayed in some countries by the enormous costs of additional spectrum licensing fees. (See Telecoms crash.) In many countries, 3G networks do not use the same radio frequencies as 2G, so mobile operators must build entirely new networks and license entirely new frequencies; an exception is the United States where carriers operate 3G service in the same frequencies as other services. The license fees in some European countries were particularly high, bolstered by government auctions of a limited number of licenses and sealed bid auctions, and initial excitement over 3G's potential. Other delays were due to the expenses of upgrading equipment for the new systems.
By June 2007 the 200 millionth 3G subscriber had been connected. Out of 3 billion mobile phone subscriptions worldwide this is only 6.7%. In the countries where 3G was launched first - Japan and South Korea - 3G penetration is over 70%[2]. In Europe the leading country is Italy with a third of its subscribers migrated to 3G. Other leading countries by 3G migration include UK, Austria, Australia and Singapore at the 20% migration level. A confusing statistic is counting CDMA 2000 1x RTT customers as if they were 3G customers. If using this oft-disputed[by whom?] definition, then the total 3G subscriber base would be 475 million at June 2007 and 15.8% of all subscribers worldwide.
Still several major countries such as Indonesia have not awarded 3G licenses and customers await 3G services. China delayed its decisions on 3G for many years, partly hoping to have the Chinese 3G standard, TD-SCDMA, to mature for commercial production. Chinaannounced in May 2008, that the telecoms sector was re-organized and three 3G networks would be allocated so that the largest mobile operator, China Mobile, would retain its GSM customer base and launch 3G onto the Chinese standard, TD-SCDMA. China Unicom would retain its GSM customer base but relinquish its CDMA2000 customer base, and launch 3G on the globally leading WCDMA (UMTS) standard. The CDMA2000 customers of China Unicom would go to China Telecom, which would then launch 3G on the CDMA 1x EV-DO standard. This meant that China would have all three main cellular technology 3G standards in commercial use. Finally in January 2009, Ministry of industry and Information Technology of China has awarded licenses of all three standards，TD-SCDMA to China Mobile, WCDMA to China Unicom and CDMA2000 to China Telecom.
In November 2008, Turkey has auctioned four IMT 2000/UMTS standard 3G licenses with 45, 40, 35 and 25 MHz top frequencies. Turkcellhas won the 45MHz band with its €358 million offer followed by Vodafone and Avea leasing the 40 and 35MHz frequencies respectively for 20 years. The 25MHz top frequency license remains to be auctioned.
The first African use of 3G technology was a 3G videocall made in Johannesburg on the Vodacom network in November 2004. The first commercial launch of 3G in Africa was by EMTEL in Mauritius on the W-CDMA standard. In north African Morocco in late March 2006, a 3G service was provided by the new company Wana.
Rogers Wireless began implementing 3G HSDPA services in eastern Canada early 2007 in the form of Rogers Vision. Fido Solutions and Rogers Wireless now offer 3G service in most urban centres.

UMTS Terminals

The technical complexities of a 3G phone or handset depends on its need to roam onto legacy 2G networks. In the first country, Japan, there was no need to include roaming capabilities to older networks such as GSM, so 3G phones were small and lightweight. In most other countries, the manufacturers and network operators wanted multi-mode 3G phones which would operate on 3G and 2G networks (e.g., W-CDMA and GSM), which added to the complexity, size, weight, and cost of the handset. As a result, early European W-CDMA phones were significantly larger and heavier than comparable Japanese W-CDMA phones.
Japan's Vodafone KK experienced a great deal of trouble with these differences when its UK-based parent, Vodafone, insisted the Japanese subsidiary use standard Vodafone handsets. Japanese customers who were accustomed to smaller handsets were suddenly required to switch to European handsets that were much bulkier and considered unfashionable by Japanese consumers. During this conversion, Vodafone KK lost 6 customers for every 4 that migrated to 3G. Soon thereafter, Vodafone sold the subsidiary which is now known as SoftBank Mobile.
The general trend to smaller and smaller phones seems to have paused, perhaps even turned, with the capability of large-screen phones to provide more video, gaming and internet use on the 3G networks.

Speed

The ITU has not provided a clear definition of the speeds users can expect from 3G equipment or providers. Thus users sold 3G service may not be able to point to a standard and say that the speeds it specifies are not being met. While stating in commentary that "it is expected that IMT-2000 will provide higher transmission rates: a minimum speed of 2Mbit/s and maximum of 14.4Mbit/s for stationary users, and 348 kbit/s in a moving vehicle,"[3] the ITU does not actually clearly specify minimum or average speeds or what modes of the interfaces qualify as 3G, so various speeds are sold as 3G intended to meet customers expectations of broadband speed. It is often suggested by industry sources that 3G can be expected to provide 384 kbit/s at or below pedestrian speeds, but only 128 kbit/s in a moving car. While EDGE is part of the 3G standard, some phones report EDGE and 3G network availability as separate things.

Network standardization

The International Telecommunication Union (ITU) defined the demands for 3G mobile networks with the IMT-2000 standard. An organization called 3rd Generation Partnership Project (3GPP) has continued that work by defining a mobile system that fulfills the IMT-2000 standard. This system is called Universal Mobile Telecommunications System (UMTS).

IMT-2000 standards and radio interfaces International Telecommunications Union (ITU): IMT-2000 consists of six radio interfaces
§ W-CDMA also known as UMTS
§ CDMA2000
§ TD-CDMA / TD-SCDMA
§ UWC (often implemented with EDGE)
§ DECT
§ Mobile WiMAX[4]


Advantages of a layered network architecture

Unlike GSM, UMTS is based on layered services. At the top is the services layer, which provides fast deployment of services and centralized location. In the middle is the control layer, which helps upgrading procedures and allows the capacity of the network to be dynamically allocated. At the bottom is the connectivity layer where any transmission technology can be used and the voice traffic will transfer over ATM/AAL2 or IP/RTP.

3G evolution (pre-4G)

The standardization of 3G evolution is progressing in both 3GPP and 3GPP2. The corresponding specifications of 3GPP and 3GPP2 evolutions are named as LTE and UMB, respectively. Development on UMB has been cancelled by Qualcomm as of November 2008. 3G evolution uses partly beyond 3G technologies to enhance the performance and to make a smooth migration path. There are several different paths from 2G to 3G. In Europe the main path starts from GSM when GPRS is added to a system. From this point it is possible to go to the UMTS system. In North America the system evolution will start from Time division multiple access (TDMA), change to Enhanced Data Rates for GSM Evolution (EDGE) and then to UMTS.
In Japan, two 3G standards are used: W-CDMA used by NTT DoCoMo (FOMA, compatible with UMTS) and SoftBank Mobile (UMTS), andCDMA2000, used by KDDI. Transition for market purposes to 3G was completed in Japan in 2006.
The first introduction of 3G (UMTS/HSDPA) technology in the Caribbean (2007) was done by SETAR in Aruba in December 2007. The Implementation phase of this network was carried out by Alcatel-Lucent. SETAR had also implemented a 3G network based on CDMA 1X EV-DO in April 2007.
Not just broadband internet can be exploted from multi megabit speeds. Video calling and VOIP. HSDPA (High Speed Data Packet Access) has capabilities of bringing 14.4 Mbit/s downstream, this is faster than most standard lines, and even some in cities in well developed areas. Not to mention capabilities of 5.8Mbit/s uplink that is more than ten times standard ADSL, And almost seven times the leading cable provider; Virgin Media.
There are now around 400 3G and HSDPA networks around the world in a quarter of the worlds countries. The migration of global subscribers to 3G has passed 15%, and in countries where 3G has been launched, the migration rate is over 35% by the end of 2008. Many operators have launched low cost or fixed rate data plans for 3G data use which has increased usage and lowered costs. At the launch of 3.5G HSDPA, in many markets this technology is provided as a portable "broadband" modem connection for laptop and even desktop computer users and priced at the low end of broadband pricing. 3G data is however expensive when roaming, with the average cost per megabyte is still in the £5.00/mb range. It would be hard to use many megabytes due to the undeveloped speeds that many networks provide.
In the UK the mobile network 3 (Three) boasts that 90% of the UKs population is covered with 3G, and 99% with the standard talk and text network (2G/2.5G/EDGE)
As anticipated, if stationary, or walking slowly you can expect a minimum of 2Mbit/s. but if in a car doing average city speeds, this falls to 348kbit/s. 3G networks in Britain offer a variety of packages. Going up from 1.8Mbit/s on networks such as T-Mobile and right up as far as 7.2Mbit/s; the same speed as a fixed line within a few hundred metres from its exchange is possible in urban areas of London taking the whole concept of fast easy mobile broadband up to a whole new level. The packages they offer however cannot give you that sustained 7.2Mbit/s, a typical 3GB (3072megabytes) plan costs between £15 and £20 a month. Three is offering 15GB for a record breaking £30 a month, or £15 if you have a contract with them already. Three however does not give such headline speeds as Vodafone.
3G is still in its early years, high prices are to be anticipated because of high fees for frequency licencing and the sheer cost of employing dozens and dozens of teams of engineers to implement a nationwide network and then to maintain it. Canada, for example, boasts some of the highest data access fees in the world for subscribers. Without a data contract 1KB of data is charged at $0.05, translating to $50 per megabyte used on Canada's GSM providers Rogers and Fido.
A 4g network is in the pipe line, capable of speeds of 100Mbit/s while moving and 1Gbit/s stationary. This however will not see the light of day until at least four, or even eight years time when they have the right equipment to use it. By that stage, bandwidth will be all around us to take advantage of.

Evolution from 2G to 3G

2G networks were built mainly for voice data and slow transmission. Due to rapid changes in user expectation, they do not meet today's wireless needs. Evolution from 2G to 3G can be sub-divided into following phases:
§ 2G to 2.5G
§ 2.5G to 2.75G
§ 2.75G to 3G
From 2G to 2.5G (GPRS)

The first major step in the evolution to 3G occurred with the introduction of General Packet Radio Service (GPRS). So the cellular services combined with GPRS became 2.5G.
GPRS could provide data rates from 56 kbit/s up to 114 kbit/s. It can be used for services such as Wireless Application Protocol (WAP) access, Short Message Service (SMS), Multimedia Messaging Service (MMS), and for Internet communication services such as email and World Wide Web access. GPRS data transfer is typically charged per megabyte of traffic transferred, while data communication via traditional circuit switching is billed per minute of connection time, independent of whether the user actually is utilizing the capacity or is in an idle state.
GPRS is a best-effort packet switched service, as opposed to circuit switching, where a certain Quality of Service (QoS) is guaranteed during the connection for non-mobile users. It provides moderate speed data transfer, by using unused Time division multiple access (TDMA) channels. Originally there was some thought to extend GPRS to cover other standards, but instead those networks are being converted to use the GSM standard, so that GSM is the only kind of network where GPRS is in use. GPRS is integrated into GSM Release 97 and newer releases. It was originally standardized by European Telecommunications Standards Institute (ETSI), but now by the 3rd Generation Partnership Project (3GPP).

From 2.5G to 2.75G

GPRS networks evolved to EDGE networks with the introduction of 8PSK encoding. Enhanced Data rates for GSM Evolution (EDGE), Enhanced GPRS (EGPRS), or IMT Single Carrier (IMT-SC) is a backward-compatible digital mobile phone technology that allows improved data transmission rates, as an extension on top of standard GSM. EDGE can be considered a 3G radio technology and is part of ITU's 3G definition, but is most frequently referred to as 2.75G. EDGE was deployed on GSM networks beginning in 2003—initially by Cingular (now AT&T) in the United States.
EDGE is standardized by 3GPP as part of the GSM family, and it is an upgrade that provides a potential three-fold increase in capacity of GSM/GPRS networks. The specification achieves higher data-rates by switching to more sophisticated methods of coding (8PSK), within existing GSM timeslots.
EDGE can be used for any packet switched application, such as an Internet, video and other multimedia.

From 2.75G to 3G

From EDGE networks the introduction of UMTS networks and technology is called pure 3G. 3G Bandwidth 5 MHz

Migrating from GPRS to UMTS

From GPRS network, the following network elements can be reused:
§ Home location register (HLR)
§ Visitor location register (VLR)
§ Equipment identity register (EIR)
§ Mobile switching centre (MSC) (vendor dependent)
§ Authentication centre (AUC)
§ Serving GPRS Support Node (SGSN) (vendor dependent)
§ Gateway GPRS Support Node (GGSN)
From Global Service for Mobile (GSM) communication radio network, the following elements cannot be reused
§ Base station controller (BSC)
§ Base transceiver station (BTS)
They can remain in the network and be used in dual network operation where 2G and 3G networks co-exist while network migration and new 3G terminals become available for use in the network.
The UMTS network introduces new network elements that function as specified by 3GPP:
§ Node B (base station)
§ Radio Network Controller (RNC)
§ Media Gateway (MGW)
The functionality of MSC and SGSN changes when going to UMTS. In a GSM system the MSC handles all the circuit switched operations like connecting A- and B-subscriber through the network. SGSN handles all the packet switched operations and transfers all the data in the network. In UMTS the Media gateway (MGW) take care of all data transfer in both circuit and packet switched networks. MSC and SGSN control MGW operations. The nodes are renamed to MSC-server and GSN-server.

Security

3G networks offer a greater degree of security than 2G predecessors. By allowing the UE to authenticate the network it is attaching to, the user can be sure the network is the intended one and not an impersonator. 3G networks use the KASUMI block crypto instead of the olderA5/1 stream cipher. However, a number of serious weaknesses in the KASUMI cipher have been identified.
In addition to the 3G network infrastructure security, end to end security is offered when application frameworks such as IMS are accessed, although this is not strictly a 3G property.

Issues

Although 3G was successfully introduced to users across the world, some issues are debated by 3G providers and users:
§ Expensive input fees for the 3G service licenses & agreements
§ Numerous differences in the licensing terms
§ Large amount of debt currently sustained by many telecommunication companies, which makes it a challenge to build the necessary infrastructure for 3G
§ Lack of member state support for financially troubled operators
§ Expense of 3G phones
§ Lack of buy-in by 2G mobile users for the new 3G wireless services
§ Lack of coverage, because it is still a new service
§ High prices of 3G mobile services in some countries, including Internet access (see flat rate)
§ Current lack of user need for 3G voice and data services in a hand-held devices

See Also
§ 3GP
§ DigRF V3
§ IP Multimedia Subsystem
§ Spectral efficiency
§ WiBro
§ Wireless modem

Further Reading

§ Ahonen, M-Profits Making Money with 3G (Wiley, 2002), first business book about 3G, ISBN 978-0470847756
§ Ahonen, Kasper and Melkko, 3G Marketing (Wiley, 2004), first marketing book for 3G, ISBN 978-0470851005
§ Ahonen and Barrett (editors), Services for UMTS (Wiley, 2002) first book on the services for 3G, ISBN 978-0471485506
§ Holma and Toskala (editors), WCDMA for UMTS, (Wiley, 2000) first book dedicated to 3G technology, ISBN 978-0471720515
§ Kreher and Ruedebusch, UMTS Signaling: UMTS Interfaces, Protocols, Message Flows and Procedures Analyzed and Explained(Wiley 2007), ISBN 978-0470065334
§ Laiho, Wacker and Novosad, Radio Network Planning and Optimization for UMTS (Wiley, 2002) first book on radio network planning for 3G, ISBN 978-0470015759

References

1. ^ Clint Smith, Daniel Collins. "3G Wireless Networks", page 136. 2000.
2. ^ http://www.plus8star.com/?p=123 Plus 8 Star presentation, "Is 3G a Dog or a Demon - Hints from 7 years of 3G Hype in Asia"
3. ^ "Cellular Standards for the Third Generation". ITU. 2005-12-01.

CDMA

(CDMA)

Code division multiple access (CDMA) is a channel access method utilized by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOneand CDMA2000 (which are often referred to as simply "CDMA"), this uses CDMA as an underlying channel access method. One of the basic concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a single communication channel. This allows several users to share abandwidth of frequencies. This concept is called multiplexing. CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of"spread-spectrum" signaling, since the modulated coded signal has a much higher data bandwidth than the data being communicated. An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different languages (code division). CDMA is analogous to the last example where people speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can understand each other.

Uses

One of the early applications for code division multiplexing—predating, and distinct from cdmaOne—is in GPS. The Qualcomm standard IS-2000, known as CDMA2000. This standard is used by several mobile phone companies, including theGlobalstar satellite phone network. CDMA has been used in the OmniTRACS satellite system for transportation logistics.

Steps in CDMA Modulation

CDMA is a spread spectrum multiple access technique. A spread spectrum technique is one which spreads the bandwidth of the data uniformly for the same transmitted power. Spreading code is a pseudo-random code which has a narrow Ambiguity function unlike other narrow pulse codes. In CDMA a locally generated code runs at a much higher rate than the data to be transmitted. Data for transmission is simply logically XOR (exclusive OR) added with the faster code. The figure shows how spread spectrum signal is generated. The data signal with pulse duration of T_b is XOR added with the code signal with pulse duration of T_c. (Note: bandwidth is proportional to 1 / T whereT = bit time) Therefore, the bandwidth of the data signal is 1 / T_b and the bandwidth of the spread spectrum signal is 1 / T_c. Since T_c is much smaller than T_b, the bandwidth of the spread spectrum signal is much larger than the bandwidth of the original signal. The ratioT_b / T_c is called spreading factor or processing gain and determines to certain extent the upper limit of total number of users supported simultaneously by a base station

Each user in a CDMA system uses a different code to modulate their signal. Choosing the codes used to modulate the signal is very important in the performance of CDMA systems. The best performance will occur when there is good separation between the signal of a desired user and the signals of other users. The separation of the signals is made by correlating the received signal with the locally generated code of the desired user. If the signal matches the desired user's code then the correlation function will be high and the system can extract that signal. If the desired user's code has nothing in common with the signal the correlation should be as close to zero as possible (thus eliminating the signal); this is referred to as cross correlation. If the code is correlated with the signal at any time offset other than zero, the correlation should be as close to zero as possible. This is referred to as auto-correlation and is used to reject multi-path interference. 

In general, CDMA belongs to two basic categories: synchronous (orthogonal codes) and asynchronous (pseudorandom codes).

Code Division Multiplexing (Synchronous CDMA)

Synchronous CDMA exploits mathematical properties of orthogonality between vectors representing the data strings. For example, binary string "1011" is represented by the vector (1, 0, 1, 1). Vectors can be multiplied by taking their dot product, by summing the products of their respective components. If the dot product is zero, the two vectors are said to be orthogonal to each other.

Each user in synchronous CDMA uses an orthogonal codes to modulate their signal. An example of four mutually orthogonal digital signals is shown in the figure. Orthogonal codes have a cross-correlation equal to zero; in other words, they do not interfere with each other. In the case of IS-95 64 bit Walsh codes are used to encode the signal to separate different users. Since each of the 64 Walsh codes are orthogonal to one another, the signals are channelized into 64 orthogonal signals. The following example demonstrates how each users signal can be encoded and decoded.

ExampleS

Start with a set of vectors that are mutually orthogonal. (Although mutual orthogonality is the only condition, these vectors are usually constructed for ease of decoding, for example columns or rows from Walsh matrices.) An example of orthogonal functions is shown in the picture on the left. These vectors will be assigned to individual users and are called the "code", "chipping code" or "chip code". In the interest of brevity, the rest of this example uses codes (v) with only 2 digits.

Each user is associated with a different code, say v. If the data to be transmitted is a digital zero, then the actual bits transmitted will be –v, and if the data to be transmitted is a digital one, then the actual bits transmitted will be v. For example, if v=(1,–1), and the data that the user wishes to transmit is (1, 0, 1, 1) this would correspond to (v, –v, v, v) which is then constructed in binary as ((1,–1),(–1,1),(1,–1),(1,–1)). For the purposes of this article, we call this constructed vector the transmitted vector.

Each sender has a different, unique vector v chosen from that set, but the construction method of the transmitted vector is identical.

Now, due to physical properties of interference, if two signals at a point are in phase, they add to give twice the amplitude of each signal, but if they are out of phase, they "subtract" and give a signal that is the difference of the amplitudes. Digitally, this behaviour can be modelled by the addition of the transmission vectors, component by component. If sender has code (1,–1) and data (1,0,1,1), and sender1 has code (1,1) and data (0,0,1,1), and both senders transmit simultaneously, then this table describes the coding steps:

Because signal0 and signal1 are transmitted at the same time into the air, they add to produce the raw signal:

(1,–1,–1,1,1,–1,1,–1) + (–1,–1,–1,–1,1,1,1,1) = (0,–2,–2,0,2,0,2,0)

This raw signal is called an interference pattern. The receiver then extracts an intelligible signal for any known sender by combining the sender's code with the interference pattern, the receiver combines it with the codes of the senders. The following table explains how this works and shows that the signals do not interfer with one another:

Further, after decoding, all values greater than 0 are interpreted as 1 while all values less than zero are interpreted as 0. For example, after decoding, data0 is (2,–2,2,2), but the receiver interprets this as (1,0,1,1).

We can also consider what would happen if a receiver tries to decode a signal when the user has not sent any information. Assume signal0=(1,-1,-1,1,1,-1,1,-1) is transmitted alone. The following table shows the decode at the receiver:

When the receiver attempts to decode the signal using sender1’s code, the data is all zeros, therefore the cross correlation is equal to zero and it is clear that sender1 did not transmit any data.

Asynchronous CDMA

The previous example of orthogonal Walsh sequences describes how 2 users can be multiplexed together in a synchronous system, a technique that is commonly referred to as Code Division Multiplexing (CDM). The set of 4 Walsh sequences shown in the figure will afford up to 4 users, and in general, an NxN Walsh matrix can be used to multiplex N users. Multiplexing requires all of the users to be coordinated so that each transmits their assigned sequence v (or the complement, -v) starting at exactly the same time. Thus, this technique finds use in base-to-mobile links, where all of the transmissions originate from the same transmitter and can be perfectly coordinated.

On the other hand, the mobile-to-base links cannot be precisely coordinated, particularly due to the mobility of the handsets, and require a somewhat different approach. Since it is not mathematically possible to create signature sequences that are orthogonal for arbitrarily random starting points, unique "pseudo-random" or "pseudo-noise" (PN) sequences are used in Asynchronous CDMA systems. A PN code is a binary sequence that appears random but can be reproduced in a deterministic manner by intended receivers. These PN codes are used to encode and decode a users signal in Asynchronous CDMA in the same manner as the orthogonal codes in synchrous CDMA (shown in the example above). These PN sequences are statistically uncorrelated, and the sum of a large number of PN sequences results in Multiple Access Interference (MAI) that is approximated by a Gaussian noise process (following the "central limit theorem" in statistics). If all of the users are received with the same power level, then the variance (e.g., the noise power) of the MAI increases in direct proportion to the number of users. In other words, unlike synchronous CDMA, the signals of other users will appear as noise to the signal of interest and interfere slightly with the desired signal in proportion to number of users.

All forms of CDMA use spread spectrum process gain to allow receivers to partially discriminate against unwanted signals. Signals encoded with the specified PN sequence (code) are received, while signals with different codes (or the same code but a different timing offset) appear as wideband noise reduced by the process gain.

Since each user generates MAI, controlling the signal strength is an important issue with CDMA transmitters. A CDM (Synchronous CDMA), TDMA or FDMA receiver can in theory completely reject arbitrarily strong signals using different codes, time slots or frequency channels due to the orthogonality of these systems. This is not true for Asynchronous CDMA; rejection of unwanted signals is only partial. If any or all of the unwanted signals are much stronger than the desired signal, they will overwhelm it. This leads to a general requirement in any Asynchronous CDMA system to approximately match the various signal power levels as seen at the receiver. In CDMA cellular, the base station uses a fast closed-loop power control scheme to tightly control each mobile's transmit power. See Near-far problem for further information on this problem.

Advantages of Asynchronous CDMA over other techniques

1. Efficient Practical utilization of Fixed Frequency Spectrum

Asynchronous CDMA's main advantage over CDM (Synchronous CDMA), TDMA and FDMA is that it can use the spectrum more efficiently in mobile telephony applications. (In theory, CDMA, TDMA and FDMA have exactly the same spectral efficiency but practically, each has its own challenges - power control in the case of CDMA, timing in the case of TDMA, and frequency generation/filtering in the case of FDMA.) TDMA systems must carefully synchronize the transmission times of all the users to ensure that they are received in the correct timeslot and do not cause interference. Since this cannot be perfectly controlled in a mobile environment, each timeslot must have a guard-time, which reduces the probability that users will interfere, but decreases the spectral efficiency. Similarly, FDMA systems must use a guard-band between adjacent channels, due to the random doppler shift of the signal spectrum which occurs due to the user's mobility. The guard-bands will reduce the probability that adjacent channels will interfere, but decrease the utilization of the spectrum.

2. Flexible Allocation of Resources

Asynchronous CDMA offers a key advantage in the flexible allocation of resources i.e. allocation of a PN codes to active users. In the case of CDM, TDMA and FDMA the number of simultaneous orthogonal codes, time slots and frequency slots respectively is fixed hence the capacity interms of number of simultaneous users is limited. There are a fixed number of orthogonal codes, timeslots or frequency bands that can be allocated for CDM, TDMA and FDMA systems, which remain underutilized due to the bursty nature of telephony and packetized data transmissions. There is no strict limit to the number of users that can be supported in an Asynchronous CDMA system, only a practical limit governed by the desired bit error probability, since the SIR (Signal to Interference Ratio) varies inversely with the number of users. In a bursty traffic environment like mobile telephony, the advantage afforded by Asynchronous CDMA is that the performance (bit error rate) is allowed to fluctuate randomly, with an average value determined by the number of users times the percentage of utilization. Suppose there are 2N users that only talk half of the time, then 2N users can be accommodated with the sameaverage bit error probability as N users that talk all of the time. The key difference here is that the bit error probability for N users talking all of the time is constant, whereas it is a random quantity (with the same mean) for 2N users talking half of the time.

In other words, Asynchronous CDMA is ideally suited to a mobile network where large numbers of transmitters each generate a relatively small amount of traffic at irregular intervals. CDM (Synchronous CDMA), TDMA and FDMA systems cannot recover the underutilized resources inherent to bursty traffic due to the fixed number of orthogonal codes, time slots or frequency channels that can be assigned to individual transmitters. For instance, if there are N time slots in a TDMA system and 2N users that talk half of the time, then half of the time there will be more than N users needing to use more than N timeslots. Furthermore, it would require significant overhead to continually allocate and deallocate the orthogonal code, time-slot or frequency channel resources. By comparison, Asynchronous CDMA transmitters simply send when they have something to say, and go off the air when they don't, keeping the same PN signature sequence as long as they are connected to the system.

3. Privacy protection in Spread Spectrum CDMA due to anti-jamming capabilities of PN sequences

Spread Spectrum Characteristics of CDMA

Most modulation schemes try to minimize the bandwidth of this signal since bandwidth is a limited resource. However, spread spectrum techniques use a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth. One of the initial reasons for doing this was military applications including guidance and communication systems. These systems were designed using spread spectrum because of its security and resistance to jamming. Asynchronous CDMA has some level of privacy built in because the signal is spread using a pseudorandom code; this code makes the spread spectrum signals appear random or have noise-like properties. A receiver cannot demodulate this transmission without knowledge of the pseudorandom sequence used to encode the data. CDMA is also resistant to jamming. A jamming signal only has a finite amount of power available to jam the signal. The jammer can either spread its energy over the entire bandwidth of the signal or jam only part of the entire signal. CDMA can also effectively reject narrowband interference. Since narrowband interference affects only a small portion of the spread spectrum signal, it can easily be removed through notch filtering without much loss of information. Convolution encoding and interleavingcan be used to assist in recovering this lost data. CDMA signals are also resistant to multipath fading. Since the spread spectrum signal occupies a large bandwidth only a small portion of this will undergo fading due to multipath at any given time. Like the narrowband interference this will result in only a small loss of data and can be overcome.

Another reason CDMA is resistant to multipath interference is because the delayed versions of the transmitted pseudorandom codes will have poor correlation with the original pseudorandom code, and will thus appear as another user, which is ignored at the receiver. In other words, as long as the multipath channel induces at least one chip of delay, the multipath signals will arrive at the receiver such that they are shifted in time by at least one chip from the intended signal. The correlation properties of the pseudorandom codes are such that this slight delay causes the multipath to appear uncorrelated with the intended signal, and it is thus ignored.

Some CDMA devices use a rake receiver, which exploits multipath delay components to improve the performance of the system. A rake receiver combines the information from several correlators, each one tuned to a different path delay, producing a stronger version of the signal than a simple receiver with a single correlator tuned to the path delay of the strongest signal. 

Frequency reuse is the ability to reuse the same radio channel frequency at other cell sites within a cellular system. In the FDMA and TDMA systems frequency planning is an important consideration. The frequencies used in different cells need to be planned carefully in order to ensure that the signals from different cells do not interfere with each other. In a CDMA system the same frequency can be used in every cell because channelization is done using the pseudorandom codes. Reusing the same frequency in every cell eliminates the need for frequency planning in a CDMA system; however, planning of the different pseudorandom sequences must be done to ensure that the received signal from one cell does not correlate with the signal from a nearby cell. 

Since adjacent cells use the same frequencies, CDMA systems have the ability to perform soft handoffs. Soft handoffs allow the mobile telephone to communicate simultaneously with two or more cells. The best signal quality is selected until the handoff is complete. This is different from hard handoffs utilized in other cellular systems. In a hard handoff situation, as the mobile telephone approaches a handoff, signal strength may vary abruptly. In contrast, CDMA systems use the soft handoff, which is undetectable and provides a more reliable and higher quality signal. 

References

Viterbi, Andrew J. (1995). CDMA: Principles of Spread Spectrum Communication (1st ed.). Prentice Hall PTR. ISBN 0201633744.. Telecom Resources. undated. Retrieved on 2006-04-09.

Lohninger, Hans (2005-12-17). "Direct Sequence CDMA Simulation". Learning by Simulations. Retrieved on 2006-04-09.. Retrieved on 2008-04-29.