OFDM V/s OFDMA

OFDM stands for Orthogonal Frequency Division Multiplexing . OFDM is one of the feature of Wimax 802.16d standard developed for fixed wimax.

OFDM is used in point to point connectvity . For two way data transmission OFDM is used with TDD (Time Dicision Duplexing)

OFDMA stands for Orthogonal Frequency Division Multiple Access. OFDMA is point to multipoint version of OFDM . OFDM is multiplexing and OFDMA is multiple access where multiple CPE's can be connected with same base station.

In OFDMA fixed wimax point to multipoint connectivity , uplink transmission ( CPE to base station ) can be OFDM because one CPE can be conneced to one base station only , but in downlink transmission ( Base station to CPE) is OFDMA as one base station is connected to many CPE's.

Wi-Fi and Wi-Max

Wi-Fi stands for wireless fiedility and Wi-Max stands for World Wide Interoperability for Microwave Access.

Wi-Fi and Wi-Max are the technologies to access internet over the wireless.

Wi-Fi is wireless LAN technology that operate over the unlicensed frequency band . Wi-Fi provides connectivity within the campus i.e in LAN local area network.

Standards developed for Wi-Fi are 802.11a , 802.11b , 802.11g , 802.11n , 802.11ac.


Wi-Max in wireless WAN technology that can operate over licensed and licensed frequency band . the throughput per user can be much higher than Wi-Fi .

Wi-Max can be used to connect two or more Wi-Fi hotspots . It can be used to provide backhaul to the Wi-Fi access point.

802.16 d and 802.16 e are the standards developed for Wimax. 802.16d is for fixed Wi-Max and 802.16e is for mobile Wi-Max.

Bandwidth , Modulation and Data Rate.

Bandwidth is the range of frequency or band of frequency i.e the difference between the highest and the lowest frequency over which the data is to be transmitted.

Data rate is the rate at which data is transmitted from one point to another.


Modulation is the alteration in the characteristic of carrier frequency as per the baseband frequency , In digital communicationthe data rate depends on the selected modulation level.

The data rate will be lowest at BPSK and it will increase as we move towards the higher modulation level QAM64 , QAM 128 ...but the coverage range at same transmit power will decrease.

Larger channel bandwidth , higher modulation level and greater transmit power yields higher data rate.

BPSK   --- 1 bit / symbol
QPSK ---- 2 bit/ symbol
QAM 16  -- 4bit/symbol
QAM64 ---  6 bit/symbol.
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Point to Multipoint Wireless Communication

Point to multipoint as the name suggests , a single point can be connected to many other points.

IN WAN point to multipont connectivity , the centre point is known as the base station and in LAN connectivity it is known as the access point.

Here I will discuss about the WAN connectivity . At microwave frequencies , 60% of the first Fresnel zone must be clear for good signal strength.

The base station is the center point so it has to be configured (frequency , channel bandwidth , IP etc) first of all and installed at the height and azimuth as per the survey report.

Other devices known as CPEs are to be configured afterwards and should be with in the coverage range as per the survey report .

In point to multipoint connectivity the base station use to have the sector antenna of azimuth 30deg , 60 deg , 90 deg , 120 deg and Omnidirectional antenna , so the CPE must be within the coverage ange of the base station for which it is configured.

The range ( link distance ) depends on the antenna parameters.

Point to Point Wireless Communication.

Point to point communication means one point can be connected to only one another point.  At microwave frequencies at least 60% of the first Fresnel zone must be clear for good signal strength .

The device has to selected according to the link distance and data rate that is required.

As per the device specification if it is FDD we require two different frequencies for one point to point link , we have to configure the device  ( Radio ) with one uplink and one downlink frequency ,  It has to be configured on both the devices ( Radios ) so that communication can take place over those frequencies.

Another configuration is SSID ( System identifier) , SSID must be same on both the radios , IP must be configured so that reachability can be checked and also so that we can access the radio and tehre are some other configuration that varied from radio to radio.

If the device is TDD then same frequency must be configured on both the devices and both the radios must have same SSID, same channel channel bandwidth , rest of the confiduration depends on radio to radio .
Again the radio must be provided with IP so that it can accessed and reachability can be checked .

The range ( Distance of point to point link ) and data rate depends on antenna parameters.

Guard Band And Guard Time

In wireless communication the data is transmitted over the frequencies. For data transfer over the air we need to think about the interference .

Guard Band :  In FDD different frequencies are used for transmitting and receiving the  data . Frequency gap is required between the uplink and downlink frequencies to avoid the interference between the uplink and downlink transmission , this frequency gap is known as the guard band i.e the frequency band between the uplink and downlink frequency to avoid interference is guard band.

Guard Time : in TDD , The data transmits as well as receives at same frequency but not at same time . For one particular period of time  the device only receives the data and for another period of time it only transmits the data . The time gap between the transmitting time and receiving time is guard time i.e as soon as the device stop transmitting the data it waits for some time (Guard time) before it starts receiving the data.

Simplex And Dupex in Telecommunication

In telecommunication Simplex in term used for one way communication i.e the device can either transmit only or receive only means it has the ability to either transmit only or receive only.

Half duplex is the term used for two way communication i.e the device can transmit as well as receive but only one task can be done at a time i.e it has has the ability to transmit and receive but only one task can be done at a time.

Full duplex is a term used for two way communication where the device can transmit as well as receive at time simultaneously.


Now a days two types of duplex techniques are most prevailing in telecommunication. These are FDD and TDD.

In FDD the device can transmit as well as receive simultaneously at a time over different frequencies , In FDD the frequency used for receiving is different for frequency used in transmitting the data.

In TDD the device receive for a particular period of time and transmit for another period of time at same frequency, and this process continues to form a complete duplex.

NOTE :  In FDD  frequency for transmitting and receiving the data is different and  In TDD frequency for transmitting and receiving the data is same.

TELECOMMUNICATION

There was a time when we cannot talk to to the loved ones who are very far away from us , But as the time passed . technocrats worked to resolve the problems , many things come to existence , letter , messenger , fixed line telephone , mobile phones ,, internet ...
Earlier the aim was that at least we can talk to the person far away from us.

As the earliest requirement was only about talking to each other , the bandwidth required was very less , But with time we start feel it very less and we want to see the person far from us then we feel the requirement of data transfer and requirement keep on growing everyday.

technocrats fulfilled the requirement to much extent but  still the requirements are increasing day by day. They are working to optimize wired as well as wireless technologies so that maximum data can be sent to the much longer distance using minimum power and. compressed frequency band .