What's the difference between "dB", "dBm", "dBi" and "dbc"

 Reference : http://www.dslreports.com/faq/14091

Other similar links :

http://zajark.blog.com/2011/04/24/db-dbi-dbd-dbc-dbm-dbw-interpretation/

http://www.hllye.com/news/antenna/what-is-dbidbddbdbmdbc.html



I keep seeing people using the terms "dB", "dBm", and "dBi" interchangeably, when they actually mean very different things. So, here's a little background on the correct usage of the terms. Sorry if this is covered in the FAQs and links, but from the posts here, I don't think people have read them thoroughly.

A dB is a RELATIVE measure of two different POWER levels. There's also dB relative to VOLTAGE levels, but I won't go into those, as we're mostly concerned with POWER levels in our discussions here. 3dB is twice (or half) as much, 6dB is four times, 10dB is ten times, and so on. The formula for calculating gain or loss in dB is: 10log P1/P2. It's used for stating the gain or loss of one device (P1) IN RELATION to another (P2). Thus, I can say that an amplifier has 30 dB of gain, or I have 6dB total feedline loss. I CANNOT say, My amp puts out 30 dB, or I have a 24dB antenna, as you must state what you're referencing it to, which is where the subscript comes in. The dB by itself is not an absolute number, but a ratio.

For amplifiers, a common reference unit is the dBm, with 0dBm being equal to 1 milliwatt. Thus, an amp with an output of 30dBm puts out 1 Watt. How much gain it has is a different matter entirely, and you can have two different amps, each with an output of 30dBm (1Watt), that have different gains, and require different levels of drive power to achieve their outputs. You can also have two different amps with the same gain that have different output powers.

There's also dBW (Referenced to 1 WATT), but you generally only use those when dealing with Big Stuff, as 30dBW is 1000w, and way beyond what we deal with here!

For antennas, a common reference unit is the dBi, which states the gain of an antenna as referenced to an ISOTROPIC source. An Isotropic source is the perfect omnidirectional radiator, a true Point Source, and does not exist in nature. It's useful for comparing antennas, as since its theoretical, its always the same. It's also 2.41 dB BIGGER than the next common unit of antenna gain, the dBd, and makes your antennas sound better in advertising. The dBd is the amount of gain an antenna has referenced to a DIPOLE antenna. A simple dipole antenna has a gain of 2.41dBi, and a gain of 0dBd, since we're comparing it to itself. If I say I have a 24dB antenna, it means nothing, as I haven't told you what I referenced it to. It could be a 26.41dBi antenna (24dBd), or a 21.59dBi (also 24dBd!) antenna, depending on what my original reference was. The difference is 4.81dB, a significant amount. Most antenna manufacturers have gotten away from playing this game, but the reference will be different in different fields.

Commercial antennas tend to be rated in dBi, as the people buying them understand it, and Amateur Radio antennas tend to be dBd, as Hams are very familiar with dipoles. Sorry to go on for so long, but as an Engineer, it bugs me a bit to see things like this!

Role of preamble in 802.11a packet

Conventional Packet Oriented Networks Like IEEE 802.11a Precede Each Data Transmission with a PHY Layer Preamble and Header



The
purposes of the PHY preamble are:
• Signal detection
• Antenna diversity selection
• Setting AGC
• Frequency offset estimation
• Timing synchronization
• Channel estimation

Note that the address of the intended recipient is not in the PHY preamble.It is actually in the packet data and is interpreted at the MAC layer.

Each packet is addressed to a single recipient.

However, the randomized backoff period of the CSMA multiplexing scheme is idle time and therefore represents an inefficiency. The PHY preamble is also network overhead and further reduces efficiency, particularly for shorter packets.

The typical real-world efficiency of an 802.11a system is approximately 50 percent. In other words, for a network with a nominal data rate of 54 Mbps, the typical throughput is about 25 – 30 Mbps. Some of the inefficiencies can be mitigated by abandoning the CSMA multiplexing scheme and adopting a scheduled approach to packet transmission. Indeed, subsequent versions of the 802.11 protocol include this feature. Inefficiencies due to dedicated ACK packets can also be reduced by acknowledging packets in groups rather than individually.

In spite of potential improvements, it remains difficult to drive packet-oriented network efficiency much beyond 65 to 70 percent.

Removing any row containing NaN in MATLAB

X(any(isnan(X),2),:) = [];

>> X = [ 1 2 3; 3 4 5; NaN 3 NaN]

X =

     1     2     3
     3     4     5
   NaN     3   NaN

>> X(any(isnan(X),2),:) = []

X =

     1     2     3
     3     4     5

One can make a function for it also

function X = exciseRows(X)
X(any(isnan(X),2),:) = [];

How is it possible to achieve a dynamic range of 112dB with a resolution of just 14 bits?

It is correct that a 14-bit AD-Converter exhibits an effective signal to noise ratio of only 75dB. However, this value comprises for the whole bandwidth of 32MHz. By filtering appropriately and therefore reducing the bandwidth B, a process gain of P = 10*Log10(32MHz/B) can be achieved. With B=2400Hz, the process gain results in ca. 41dB, therefore the dynamic range is increased to 75 + 41 = 116dB.

Difference between Digital and Analog Reciever

Most of today’s conventional receivers are based on the tripple conversion concept. More often, DSP’s are placed in the third IF stage.  

The most important performance criteria like intermodulation,reciprocal mixing, blocking and the selectivity of roofing filter can no longer be influenced by the DSP, because these parameters are already determined in the preceding analog stages.


Due to the large number of components it is difficult to achieve tight performance parameters during mass production.

                                    

In the digital receiver the full signal spectrum is fed directly to a high resolution A/D converter. All signal processing is made digitally, which is ideal as it relates to distortions and noise. Since algorithms are determined by software, uniform performance parameters are obtained. Another benefit of this technology is the high degree of functional flexibility. New functions can be added by simple software download.





Source : http://adat.ch/p1e_digitalrx.html

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