Forward Traffic Channels are code channels and used to assign calls, usually voice and signaling traffic to the individual users.
CDMA stands for Code Division Multiple Access. It is a wireless technology used in trmission of signals from places with high Security and noise reduction. The principle of Spread Spectrum is used to work with CDMA. Spread signal is below the noise level and noise has no effect on the signal. CDMA is not a frequency specific to each user, instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence. A unique code is received by all mobile network users and is allowed continuous network access instead of intermittent or timed access.
Cellular system tracks mobile stations in order to maintain their communication links. The mobile station goes to neighbor cell and communication link switches from current cell to the neighbor cell which is called as soft handover.
P (gain) = 10log (W/R)
W is Spread Rate
R is Data Rate
For CDMA P (gain) = 10log (1228800/9600)
Actual processing gain = P (gain) - SNR
= 21 – 7 = 14dB
CDMA uses variable rate coder
The Voice Activity Factor of 0.4 is considered = -4dB.
CDMA has 100% frequency reuse. Use of same frequency in surrounding cells causes some additional interference.
In CDMA, frequency reuse efficiency is 0.67 (70% eff.) = -1.73dB
Frequency Division Multiple Access (FDMA) is one of the most common analogue multiple access methods. The frequency band is divided into channels of equal bandwidth so that each conversation is carried on a different frequency. Guard bands are used between the adjacent signal spectra to minimize crosstalk between the channels.
The power of closed loop control is used to compensate for the rapid Rayleigh discoloration. This time, the mobile trmitted power is controlled by the base station. For this purpose, the base station continuously monitors the reverse link signal quality. If the quality of the connection is poor then the base station increases the power. Similarly, if the quality of the link is very high then the mobile base station controller reduces the power. This is called as reverse link power control.
CDMA is a spread spectrum technique where multiple users to access the system at the same example in a cell, and of course on the same frequency. Therefore, discriminate the users on the reverse link (i.e. information from MS to the base station). It spreads information using codes that are unique to the mobile station in all the CDMA cellular systems. This code has an element that is the ESN. But it doesn’t use the ESN in the same format; instead, it uses an ESN swapped.
CDMA system uses a signal fast chip rate for spreading spectrum and it has a high time resolution. For this reason, CDMA is able to recognize by decomposing each of the path to reach with the time difference. From this, it receives a different paths of signal each one separately, by summing later, it can prevent signal degradation. This is called as the RAKE receiver.
Synchronization channel carries a single repeating message and trmits the synchronization configuration information and the system of the mobile station in the CDMA system.
Access channels are used by mobile stations to establish communications with the base station or to wer Paging Channel messages. The access channel is used for short signaling message exchanges such as call-ups, responses to pages, and registrations.
Time Division Multiple Access (TDMA) is a complex technology, because it requires a very accurate synchronization between the trmitter and the receiver. The TDMA is used in digital mobile radio systems. The individual mobile stations are assigned cyclically a frequency for exclusive use for the duration of a time interval.
Power control is the intelligent selection of trmit power in a communication system for achieving best performance within the system. The performance depends on context and there are chances to include optimizing metrics like link data rate, network capacity, geographic coverage, and range. A higher trmit power trlates into a higher signal power at the receiver.
In FDMA when the channel is not used, it is the channel bandwidth while rest simply is relatively narrow (30 KHz), known as System narrowband. Little or no equalization is needed. For broadcasting, time symbols are suitable analogue links. Framing for FDMA or synchronization bits are not needed for the tight filter streaming. It is required to minimize the combined interference of FDD.
In FDMA or TDMA cellular system, new communication establishes after breaking current communication at the moment doing handoff. Communication between MS and BS breaks at the moment switching frequency or timeslot which is known as hard handover.
Pilot channel is a reference channel using the mobile station to acquire the time and as a phase reference for coherent demodulation. It is continuously trmitted by each base station on each active CDMA frequency. Each mobile station tracks this signal continuously.
CDG is comprised of service providers, infrastructure manufacturers, device vendors, test equipment vendors, application developers, and content providers. Its members jointly define the technical requirements for the development of complementary systems CDMA2000 and 4G and interoperability with other emerging wireless technologies to increase the availability of wireless products and services to consumers and businesses worldwide.
Walsh Codes are most commonly used in CDMA applications orthogonal codes. These codes correspond to lines of a special square matrix called the Hadamard matrix. For a set of Walsh codes of length N, it consists of n lines to form a square matrix of n × n Walsh code. The IS-95 system uses 64 Walsh function matrix 6@The first line of this matrix contains a string of all zeros with each of the following lines containing different combinations of bit 0 and @Each line is orthogonal and equal representation for binary bits. When implemented with the CDMA system, each mobile user uses one of the 64 sequences of rows in the matrix as a spreading code, providing zero cross-correlation among all the other users.
The forward channel CDMA is the direction of the communication or mobile-to-cell downlink path.
The DS-CDMA system uses two types of spreading sequences − PN sequences and orthogonal codes. The PN sequence is generated by the pseudo-random noise generator which is simply a binary linear feedback shift register, consisting of XOR gates and a shift register. This PN generator has the ability to create a sequence identical for both the trmitter and the receiver, and while retaining the desirable properties of the noise randomness bit sequence.
The band used in CDMA is 824 MHz to 894 MHz (50 MHz + 20 MHz separation);
Frequency channel is divided into code channels; and
1.25 MHz of FDMA channel is divided into 64 code channels.
Reverse channel is consists of two channels which includes:−
Frequency hopping is a spread spectrum in which the propagation takes place by hopping in frequency over a wide band. The precise order in which the break occurs is determined by a hopping table generated by using a pseudo-random code sequence.
The factors deciding capacity are:−
Capacity in CDMA is soft, CDMA has all users on each frequency and users are separated by code. This me, CDMA operates in the presence of noise and interference. In addition, neighboring cells use the same frequencies, which me no re-use. So, CDMA capacity calculations should be very simple. No code channels in a cell, multiplied by no cells. But it is not that simple. Although not available code channels are 64, it may not be possible to use a single time, since the CDMA frequency is the same. Flexible capability me that all the code channels can be pursued at a time, but at the expense of quality.
Paging Channels’ main objective is to send out pages, that is, notifications of incoming calls, to the mobile stations. The base station uses these pages to trmit system overhead information and mobile station specific messages.
In FDMA or TDMA, radio resource is allocated not to interfere among neighbor cells:−
CDMA system is against this, since all users share the same frequency, the arrangement of the frequency is not an issue. This is in designing the system, which will be a very big advantage.
Similar to reverse link power control, forward link power control is also necessary to maintain the forward link quality to a specified level. This time, the mobile monitors the forward link quality and indicates to the base station to turn on or off, this power control has no effect on the near-far problem because all the signals are blurred together the same level of power when they get to the mobile. In short, there is no near-far problem in the forward link.
TDD is Time Division Duplex. A duplex method by which the Uplink and the Downlink trmissions are carried over the same frequency using synchronized time intervals. The carrier uses a 5 MHz band, although there is a low chip rate solution under study by the 3GPP (1.28 Mcps). The available frequency bands for TDD will be 1900-1920 MHz and 2010-2025 MHz.
Reverse traffic channels are used by individual users in their actual calls to trmit traffic from a single mobile station to one or more base stations.
There are four main interferences in CDMA as given below:−
Frequency Division Duplex is one of the multiple access methods in wireless technology; it uses following frequency bands −
Uplink: 1920 MHz - 1980 MHz and
Downlink: 2110 MHz - 2170 MHz.
Mobile Station Identities:−
Spread spectrum is a form of wireless communications in which the frequency of the trmitted signal is deliberately varied. This results in a much greater bandwidth than the signal would have, if its frequency were not varied. In other words, the trmitted signal bandwidth is greater than the minimal information bandwidth needed to successfully trmit the signal. Some function other than the information itself is being employed to determine the resultant trmitted bandwidth.
Following two types of spread spectrum techniques are used:−
In wireless communications, fading is the deviation of the signal attenuation affecting a certain propagation media. Discoloration may vary with time, the geographical position or frequency of the radio, which is often modeled as a random process. A fading channel is a communication channel experiencing fading. In wireless systems, fading can be due to multipath,and multi-path fading.
Forward channel consists of four channels which includes:−
Code Division Multiple Access system is very different from time and frequency multiplexing. In this system, a user has access to the whole bandwidth for the entire duration. The basic principle is that different CDMA codes are used to distinguish between different users. Forms generally used are direct sequence spread spectrum modulation (DS-CDMA), frequency hopping or mixed CDMA detection (JD CDMA). Here, a signal is generated which extends over a wide bandwidth. A code called spreading code is used to perform this action. Using a group of codes, which are orthogonal to each other, it is possible to select a signal with a given code in the presence of many other signals with different orthogonal codes.
The Reverse CDMA channel is the mobile-to-cell direction of the communication or uplink path.