Follow Up : Multiple Access Techniques

Now the number of connecting devices exploding!! So the capacity of the systems should grow in the same proportions!! Here comes another savior technique :  Multiple access..
These schemes are used to allow a number of mobile users to share simultaneously a finite amount of radio spectrum. The sharing of spectrum is essential as the available spectrum is not going to increase with the number of devices! High capacity can be achieved by simultaneously allocating the available bandwidth/channels to multiple users.

Lets start withe basics! The duplexing techniques. There are two ways 
1. Time Division Duplexing : Here multiple users share single radio channel by taking turns in the time domain. Individual users are allowed to access the channel in assigned time slots, and each duplex channel has both a forward time slot and a reverse time slot to facilitate bidirectional communication.   

2. Frequency Division Duplexing : Here every user will have two distinct bands of frequencies. The forward band provides traffic from the base station to the mobile, and the reverse band provides traffic from the mobile to the base station.

Now the "Multiple Access".. The following diagram shows the multiplexing techniques.

1. FDMA : Frequency Division Multiple Access
Individual channels are assigned to every user.

2. TDMA :  Time Division Multiple Access
First the whole radio spectrum into slots, one user in each slot is allowed to                  
transmit or receive at any given time.

3. CDMA : Code Division Multiple Access
This is a type of spread spectrum techniques.All the users can use a wide band for entire time but each has a different psuedo-random code assigned.

I hope now some of the terms I mentioned while writing about 3G and 4G standards make more sense.

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The long term Evolution

So after looking at the first three generations, here we are. I do not want to call LTE as a 4G standard yet as according to International Telecommunications Union-Radio communications sector (ITU-R) for any 4G system, enhanced peak data rates to support advanced services and applications (100 Mbit/s for high and 1 Gbit/s for low mobility )..Now thats something really huge..From 10s of Mbps we are going to 1Gbps!! I am not quite sure if any of the service provider actually provides these data rates. Well it is advertised as 4G though..There were couple of other standards beyond the third generation such as HSPA+ and WiMAX.  So the operators had to choose between these three. For now we will be focusing only on LTE, the Long Term Evolution. I really like the name given to this standard, it actually suggests that atleast this generation will be able to cater our needs for a longer time. As we have seen in the previous posts, the wireless communication industry has seen rapid growth over the years. The forecasts say that 3GPP LTE will enable the growth of cellular systems and might be able to meet the need till 2017. Now thats a long time considering the growth patterns seen in last 5 years. The number of connected devices will reach 15 billions by 2015!! So we will need robust systems to support these big number of users and speed and coverage will be the decisive factors in competitors..

It is said that "Necessity id the mother of inventions"..Well same goes for the development of LTE. There were some important driving factors in the development of LTE.

1. Ever increasing demand of higher data rates.
2. Increasing number of connected devices
3. Flexibility for the operators (LTE supports flexible bandwidths)
4. Availability of smart phones

There are several requirements for LTE published by ITU. (I am using the official ITU document for reference)

1. Peak data rate : 100 Mbps DL/ 50 Mbps UL within 20 MHz bandwidth 
(DL : 5bps/Hz, UL : 2.5bps/Hz, 2x2 MIMO default)
2. Co-existance with 2G and 3G systems.
3. Latency : upto 100ms
3. Mobility : Optimized for 0 ~ 15 km/h, 15 ~ 120 km/h supported with high     performance.Can support upto 350km/h.
4. Spectrum usage:Flexible.Permissible channel bandwidths:1.4, 3, 5,10, 20 MHz
5. Supported duplexing : FDD, TDD

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