How Stuff Works – 802.11n and Channel Bonding

January 25, 2009 in 802.11n, Channel Bonding | Tags: 802.11n, Channel Bonding

We already discussed how MIMO works.  Let’s look at another technical improvement currently utilized in 802.11n – channel bonding:

The graphic is fairly self explanatory – traditional 802.11 channels are either 20 MHz wide (OFDM) or 22 MHz wide (DSSS).  Channel bonding combines two adjacent channels, which effectively doubles the amount of available bandwidth.

One footnote to channel bonding is that it works best in the 5GHz frequency band, as there is only space for three traditional, non-overlapping channels in the 2.4GHz frequency band.   Therefore, there is only enough space for one bonded channel in that portion of the RF spectrum.

How Stuff Works – 802.11n and Spatial Multiplexing

February 1, 2009 in 802.11n, Spatial Multiplexing | Tags: 802.11n, MIMO, Spatial Multiplexing

This is the third post in my “How Stuff Works” series.  The first two posting discussed MIMO and channel bonding.  This post looks at another technical improvement that leads to greater speed in 802.11n networks – spatial multiplexing.

It is helpful to take a quick look at a classic 802.11 transmitter.

802.11 Classic Transmitter

In this scenario, only one data stream is sent from the transmitter to the receiver (represented by the orange line).

With spatial multiplexing, multiple data streams are transmitted at thesame time.  They are transmitted on the same channel, but by different antenna.  They are recombined at the receiver using MIMO signal processing.  This is represented in the diagram above with two spatial streams – an orange colored one and a navy blue colored one.

Spatial Multiplexing - Two Streams

Spatial multiplexing doubles, triples, or quadruples the data rate depending on the number of transmit antennas.   Remember, you may hear three numbers when referring to 802.11n or MIMO networks – the first is the number of transmit antenna, the second is the number of receive antenna, and the third is the number of spatial streams.   For example, a 3×3x2 system has two spatial streams.

How Stuff Works – 802.11n MIMO

January 24, 2009 in 802.11n, MIMO | Tags: 2x3 MIMO, 3x3 MIMO, 802.11n, MIMO,wireless

If you have been following my blog, you know that I have a poll regarding the greatest challenges to 802.11n deployment in the enterprise.  (Here’s your chance to rock the vote!)

One challenge for replacing desktop Ethernet with wireless is speed.  Perhaps the widest publicized enhancement to 802.11n is that of MIMO (“my-moe”) antennas, which stands for “Multiple Input, Multiple Output”.  How does MIMO work?

To answer that question, let’s look at how a classic 802.11 wireless transmitter operates:

802.11 Classic Transmitter

In this case, the signal is sent out of one antenna.  The signal is received by both antennas at the other end, but only one signal is processed and sent up to the MAC layer.   Antenna diversity helps in the fact that the best signal is the one that is processed, but remember that it is still a single antenna that processes the receives and processes the RF energy.

Let’s compare that to a MIMO antenna structure:

MIMO Signal Processing

In this case, we have three transmit antennas and three receive antennas (often referred to as 3×3 MIMO).  The black, green, and red lines above each represent their own signal.   With MIMO all three signals are received and processed up the stack.   This significantly improves the receiver’s “ability to hear” and it represented in the graph above by the orange line.

You may hear different implementations of MIMO such as 2×3 and 3×3.  The first number is the number of transmit antennas and the second number is the number of receive antennas.   If you hear 3x3x2, the last number refers to the number of spatial streams, which I will discuss in another post.

FDD LTE Networks vs TDD LTE Networks

FDD LTE Networks vs TDD LTE Networks

LTE (3GPP Long Term Evolution) seems to be the next generation in mobile phone technology as many providers arebeginning to augment their networks with LTE. As we know, mobile phone traffic is divided into two parts: an uplink and a downlink. In this regard, LTE supports two duplexing modes: FDD (Frequency Division Duplexing) and TDD (Time Division Duplexing). The main difference between FDD and TDD is in how they divide the single channel to provide paths for both uploading and downloading. FDD does this by dividing the frequency band allotted into two discrete smaller channels. On the other hand, TDD uses the entire channel but alternates between uploading and downloading.

Because of how FDD operates, it is classified as a full duplex system. This means that both the upload and download are always available. TDD is just half duplex as either upload or download can use the channel but not at the same time. However, because the time divisions are very small, this is not noticeable in applications like voice calls that need full duplex operation.

FDD and TDD LTE have their own strengths and weaknesses. FDD is generally better suited for applications like voice calls that have symmetric traffic. This is because traffic in both directions is always constant and TDD would be wasting bandwidth in constantly switching from one to the other. TDD shines in applications that have asymmetric traffic, an example of which is online browsing. When browsing the web, it is typical that download is much higher than upload; but when you are uploading videos, for example, the reverse is true. TDD can allocate more time for the part that requires more bandwidth, thereby balancing the load. With FDD, the bandwidth cannot be dynamically reallocated and the unused bandwidth is wasted.

Another advantage of FDD LTE appears when planning sites for base stations. Because FDD base stations use different frequencies for receiving and transmitting, they effectively do not hear each other and no special planning is needed. With TDD, special considerations need to be taken in order to prevent neighboring base stations from interfering with each other.

Summary:

1. FDD LTE uses frequency division, while TDD LTE uses time division
2. FDD LTE is full duplex, while TDD LTE is half duplex
3. FDD LTE is better for symmetric traffic, while TDD is better for asymmetric traffic
4. TDD LTE is better at reallocating traffic than FDD LTE
5. FDD LTE allows for easier planning than TDD LTE

Portfolio 2011 - Dec Q4

Portfolio 2011 - Dec Q4

2/2/2011 - 虎年以1531.82点封关。
1/4/2011 Q1 KLSE 收在 1555点 +23.18
30/6/2011 Q2 KLSE 收在 1579 点 +24
30/9/2011 Q3 KLSE 收在 1387.13点 -192

30/12/2011 Q4 KLSE 收在 1530.73点 +143.6

SUNWAY-WA 5211WA
YTL 4677

30/12/2011 持有股票
CIMB 1023
FAJAR 7047
SUNREIT 5176
GENTING SP G13
SUNWAY 5211

接下来的2012年

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