FM Demodulation

    THE PHASE LOCKED LOOP (PLL)  
     
In the last section, the PLL was mentioned as a way to demodulate an FM signal. In addition to FM demodulation, PLLs are used in a wide variety of communications applications, which include TV receivers, tone decoders, telemetry receivers, modems and data synchronizers, to name a few. Many of these applications are covered in electronic communications courses. In fact, entire books have been written on the finer points of PLL operation, analysis, and applications. The approach in this section is intended only to present the basic concept and give you an intuitive idea of how PLLs work and how they are used in FM demodulation. A specific PLL integrated circuit is also introduced.


    The Basic PLL Concept                     
    
The phase locked loop (PLL) is a feedback circuit consisting of a phase detector, a low-pas filter, and a voltage-controlled oscillator (VCO) Some PLLs also includes an amplifier in the loop. A basic block diagram is shown in figure 5.16.

Basic PLL block diagram

The general operation of a PLL is as follows. The phase detector compares the phase difference between the incoming signal, Vi, and the VCO signal, Vo.

When the frequency of the incoming signal, fi, is the difference from that of the VCO frequency, fo, the phase angle between the two signals is also different. The output of the phase detector and the filter is proportional to the phase difference of the two signals. This proportional voltage is fed to the VCO, forcing its frequency to move toward the frequency of the incoming signal until the two frequencies are equal. At this point, the PLL is locked onto the incoming frequency. If fi change, the phase difference also changes, forcing the VCO to track the incoming frequency.
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References
 
" FM Demodulation " !

Mobile Tracking using the Cell ID

Cell ID location tracking system is a great way to know where someone is within an accuracy of about 1km but it is not a substitute for GPS tracking systems. The Cell ID system is only accurate enough to answer the question "Which part of town are you in" and not " Which street are you in". For the majority of users this is sufficient to prove if someone was lying about their location.  

Mobile Tracking using the Cell ID

    What is Cell ID?     

A GSM Cell ID (CID) is a generally unique number used to identify each Base transceiver station (BTS) or sector of a BTS within a Location area code (LAC) if not within a GSM network.

    How Cell ID technique works?      

A network provider has several hundred radio base stations ( cells) that make up its network. Each base station is a "cell" which covers a physical area and these cells connect together to make up the entire network. The size of cells depends on the density of base station installed in a given area. Cell sizes can vary from 100m in cities to 3K in rural areas .A mobile phone connects to the network using the cell in which it is located.. As the mobile moves, it leaves one cell and joins another. The new cell then takes responsibility for connecting the phone to the network.
Every time the cell that "owns" the mobile changes, you can get the cell id and cell name ( if available) . You can tell which cell controls the mobile by the name or by resolving the cell id against a database of cell id to locations.

Cell ID location tracking is based on information that is freely available from the airwaves, and the location information is the form of a CELL ID, and the name of the CELL. The name of the cell is usually the name of a part of town, and so this is good enough to know where someone is. 

Source:       
               FlexiSPY
               Wikipedia   
               GSM Modem

" Mobile Tracking using the Cell ID " !

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Code Division Multiple Access (CDMA)

    CDMA         

Short for Code-Division Multiple Access, a digital cellular technology that uses spread-spectrum techniques. Unlike competing systems, such as GSM, that use TDMA, CDMA does not assign a specific frequency to each user. Instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence. CDMA consistently provides better capacity for voice and data communications than other commercial mobile technologies, allowing more subscribers to connect at any given time, and it is the common platform on which 3G technologies are built.

Code Division Multiple Access (CDMA)

CDMA users share a common frequency channel. All users are on the same frequency at the same time. However, each pair of users is assigned a special code that reduces interference while increasing privacy.

    Advantages of CDMA

Most efficient use of available bandwidth the foremost advantage is the large band width and its equivalent effect utilization for sending and receiving the data.

No separate time Access divisions
In CDMA technology there is no time limitation slot division for using the band width and sending and receiving the data like in TDMA.

Power and bandwidth frequency
Power frequency remains constant in both sending and receiving and uniform bandwidth keeps the transmission faster and in time.

Resource allocation
Another advantage of CDMA is the flexible resource allocation which means that a certain kind of pin code is provide to the active users to communicate with each other. Fixed time and bandwidth keeps the simultaneous exchange of message at both sender and receivers end and specific number of packets with limited length keeps the extra traffic out of the channel.

    Disadvantages of CDMA     

The main disadvantage of CDMA is that it can’t be used for the multi channel or multiple transmission paths. All the communication is performed over a single path. This make it fixed for singular purposes which is main drawback of CDMA technology.

    Space Division Multiple Access (SDMA)  

Space Division Multiple Access (SDMA) - A variation of TDMA and CDMA that potentially will be used in high-bandwidth, third-generation wireless products.

SDMA is a channel access method based on creating parallel spatial pipes next to higher capacity pipes through spatial multiplexing and/or diversity, by which it is able to offer superior performance in radio multiple access communication systems. In traditional mobile cellular network systems, the base station has no information on the position of the mobile units within the cell and radiates the signal in all directions within the cell in order to provide radio coverage. This results in wasting power on transmissions when there are no mobile units to reach, in addition to causing interference for adjacent cells using the same frequency, so called co-channel cells. Likewise, in reception, the antenna receives signals coming from all directions including noise and interference signals. By using smart antenna technology and differing spatial locations of mobile units within the cell, space-division multiple access techniques offer attractive performance enhancements. The radiation pattern of the base station, both in transmission and reception, is adapted to each user to obtain highest gain in the direction of that user. This is often done using phased array techniques.

     Comparison of Multiple Access Systems
 

Comparison of Multiple Access Systems

This table summarizes some of the technical aspects of the multiple access technologies.

The technology used determines the channel's capacity. TDMA triples the capacity of FDMA, but CDMA capacity can be up to seven times that of TDMA.

References:
                   WiFi Notes
                   Wikipedia
                   CDMA University
                   WEBOPEDIA
                   How Stuff Works


" Code Division Multiple Access (CDMA) " !

Frequency Division Multiple Access (FDMA)

    FDMA                           

Frequency Division Multiple Access (FDMA)

Frequency Division Multiple Access or FDMA is a channel access method used in multiple-access protocols as a channelization protocol. FDMA gives users an individual allocation of one or several frequency bands, or channels. It is particularly commonplace in satellite communication. FDMA, like other Multiple Access systems, coordinates access between multiple users.

Each FDMA subscriber is assigned a specific frequency channel. No one else in the same cell or a neighboring cell can use the frequency channel while it is assigned to a user. This reduces interference, but severely limits the number of users.

    Advantage of FDMA                         
 
* FDMA technique doesn't need any base-control station.
* Data that transfeered between each station to another during the transmission process will not be lost.
* After the transmission of data,the effect ot the delay distortion will be so small and it can be ignored.
* There is no need for network timing.
* The channel operations in FDMA are simple.
* In FDMA,the reduction of the information bit rate has a good effect on the capacity.
* No need for any equalization.
* Because of the transmission is continuous,there is almost no need for bits that are responsible for synchronization.
* Simplicity in FDMA algorithms.

    Disadvantages of FDMA                   

* In the FDMA technique,it is impossible for the stations to receive data from more than one transmission source.
* One of most important thing in communication systems is the maximum data rate which is small and fixed for every channel in FDMA.
* Because of the guard bands,the capacity of the FDMA will be decreased.
* Increasing the cost of FDMA,since the band filters are narrow that can't be recognized by VLSI.
* FDMA requires special filters to avoid any interference between the narrow channels.


    TDMA                        
    CDMA                       
    SDMA                        

Reference:

    * Wikipedia
    * CDMA University
    * WEBOPEDIA
    * How Stuff Works   

" Frequency Division Multiple Access (FDMA) " !

Multiple Access Systems

    Why Multiple Access?  
 
Multiple Access systems coordinates access between multiple users.Wireless telecommunications has dramatically increased in popularity, resulting in the need for technologies that allow multiple users to share the same Bandwidth.
The following Multiple Access Systems have very different approaches to the bandwidth problem. 

Time Division Multiple Access (TDMA)

     TDMA                       

Short for Time Division Multiple Access, a technology for delivering digital wireless service using time-division multiplexing (TDM). TDMA works by dividing a radio frequency into time slots and then allocating slots to multiple calls. In this way, a single frequency can support multiple, simultaneous data channels.
The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only a part of its channel capacity. TDMA is used in the digital 2G cellular systems such as Global System for Mobile Communications (GSM), IS-136, Personal Digital Cellular (PDC) and iDEN, and in the Digital Enhanced Cordless Telecommunications (DECT) standard for portable phones. It is also used extensively in satellite systems, combat-net radio systems, and PON networks for upstream traffic from premises to the operator.
TDMA is a type of Time-division multiplexing, with the special point that instead of having one transmitter connected to one receiver, there are multiple transmitters. In the case of the uplink from a mobile phone to a base station this becomes particularly difficult because the mobile phone can move around and vary the timing advance required to make its transmission match the gap in transmission from its peers.
 
    Advantages of TDMA                       

The radio part of the mobile only needs to listen and broadcast for its own time slot. For the rest of the time, the mobile can carry out measurements on the network, detecting surrounding transmitters on different frequencies. This allows safe inter frequency handovers, something which is difficult in CDMA systems, not supported at all in IS-95 and supported through complex system additions in Universal Mobile Telecommunications System (UMTS). This in turn allows for co-existence of microcell layers with macrocell layers.


    Disadvantages of TDMA                 

TDMA systems create interference at a frequency which is directly connected to the time slot length. This is the buzz which can sometimes be heard if a TDMA phone is left next to a radio or speakers.Another disadvantage is that the "dead time" between time slots limits the potential bandwidth of a TDMA channel. These are implemented in part because of the difficulty in ensuring that different terminals transmit at exactly the times required. Handsets that are moving will need to constantly adjust their timings to ensure their transmission is received at precisely the right time, because as they move further from the base station, their signal will take longer to arrive. This also means that the major TDMA systems have hard limits on cell sizes in terms of range, though in practice the power levels required to receive and transmit over distances greater than the supported range would be mostly impractical anyway.


    FDMA                        
    CDMA                       
    SDMA                        

Reference:
    
    * Wikipedia 
    * CDMA University
    * WEBOPEDIA
    * How Stuff Works


" Multiple Access Systems " !