FDD vs TDD : LTE Part-2

LTE-TDD

1- Paired spectrum : Does not require paired spectrum as both transmit and receive occur on the same channel.

2- Hardware cost : Lower cost as no diplexer is needed to isolate the transmitter and receiver. As cost of the UEs is of major importance because of the vast numbers that are produced, this is a key aspect.

3- Channel reciprocity : Channel propagation is the same in both directions which enables transmit and receive to use on set of parameters

4- UL / DL asymmetry : It is possible to dynamically change the UL and DL capacity ratio to match demand

5- Guard period / guard band : Guard period required to ensure uplink and downlink transmissions do not clash. Large guard period will limit capacity. Larger guard period normally required if distances are increased to accommodate larger propagation times.

6- Discontinuous transmission : Discontinuous transmission is required to allow both uplink and downlink transmissions. This can degrade the performance of the RF power amplifier in the transmitter.

7- Cross slot interference : Base stations need to be synchronised with respect to the uplink and downlink transmission times. If neighbouring base stations use different uplink and downlink assignments and share the same channel, then interference may occur between cells.

LTE-FDD

1- Paired spectrum : Requires paired spectrum with sufficient frequency separation to allow simultaneous transmission and reception

2- Hardware cost : Diplexer is needed and cost is higher.

3- Channel reciprocity : Channel characteristics different in both directions as a result of the use of different frequencies

4- UL / DL capacity : Determined by frequency allocation set out by the regulatory authorities. It is therefore not possible to make dynamic changes to match capacity. Regulatory changes would normally be required and capacity is normally allocated so that it is the

5- Guard period / guard band : Guard band required to provide sufficient isolation between uplink and downlink. Large guard band does not impact capacity.

6- Discontinuous transmission : Continuous transmission is required.

7- Cross slot interference : Not applicable

Source : http://goo.gl/T3Elwq

FDD vs TDD : LTE Part-1

LTE has been defined to accommodate both paired spectrum for Frequency Division Duplex, FDD and unpaired spectrum for Time Division Duplex, TDD operation.

Both LTE TDD and LTE FDD are deployed as each form of the LTE standard has its own advantages and disadvantages and decisions can be made about which format to adopt dependent upon the particular application.

FDD is far more widely implemented because of prior frequency spectrum assignments and earlier technologies.

Yet as spectrum becomes more costly and scarce, TDD will become more widely adopted as spectrum is reallocated and repurposed.

FDD can cover a larger area with the fixed DL/UL on different frequencies, but TDD can provide more DL capacity with the flexible DL/UL ratio.

TDD makes it relatively easy to dynamically change the capacity ratio between UL and DL to reallocate time slots, which makes it well suited for today’s DL-heavy traffic pattern. In most instances, network operators will desire more DL capacity than UL since users more frequently download content like video and web pages than upload content they’ve created.


Not only LTE, but every communication system (except very recent few ones based on full duplexing) requires pairing the transmission and the reception over orthogonal dimensions (frequency or time). Otherwise, the high powered Tx will saturate the neighboring Rx and probably damage it.

That means vendor could combine TDD cell and FDD cell into one base station. Actually most of vendors provide this.
However to choise TDD of FDD depends on business and traffic model. Generally TDD is cheaper than FDD, not only spectrum prices but also equipments price(because the TX&RX antenna could be one， but needs a GPS， so I guess TDD is cheaper than FDD)。Think that FDD have more distant coverages and capacities while TDD is flexible to adjust the ratio of DL/UL，operator like to use TDD as the supplement of FDD. Only seldom operators select TDD alone to build a RAN, most operator select to combine TDD and FDD。

Source : http://goo.gl/T3Elwq

Type matching and Dynamic Range in GNU Radio

GNU Radio checks if input and output types match by checking their size. If you happen to connect up ports with different types but the same size, you will most definitely get data junk.

Be careful with dynamic ranges. When you're using float or complex data types, you have a larger range than you'll ever need concerning the machine, but some sinks and sources have specific ranges you need to stick to. For example, audio sinks require samples within +-1 and will clip anything outside this interval. The USRP sink on the other hand needs samples in the +-32767 range (signed 16 bit values) because that's the dynamic range of the DAC.

Source : GNU Radio webpage