Archive for June, 2010


Advantages of Time Division Duplex (TDD) for Broadband Wireless in Last Mile Applications

WiMAX uses either of two types of duplex methods to separate uplink (UL) and downlink (DL) communication signals: Time Division Duplex (TDD) and Frequency Division Duplex (FDD). Both methods have clear advantages depending on the application, and the type of  Wireless architecture.


There are two main techniques for dividing forward and reverse communication channels on the same physical transmission medium:

  1. Time Division Duplex (TDD)
  2. Frequency Division Duplex (FDD)

Time Division Duplex (TDD)

Using the TDD method, a single frequency channel is assigned to both the transmitter and the receiver. Both the uplink (UL) and downlink (DL) traffic use the same frequency f0 but at different times.

In effect, TDD divides the data stream into frames and, within each frame, assigns different time slots to the forward and reverse transmissions. This allows both types of transmissions to share the same transmission medium (i.e., the same radio frequency), while using only the part of the bandwidth required by each type of traffic .

Several inferences can be drawn from this description:

  1. Since the TDD scheme can allocate dynamically the amount of time slots assigned to each direction — transmit and receive– an operator can define the percentage of UL versus DL traffic. This is especially important for Internet-type traffic (the ratio for UL/DL is no longer constrained to a fixed 50/50).
  2. A guard band is not required to separate the UL and DL channels, because they both use the same frequency – hence, there is no loss in spectrum. A guard period, though, is necessary for synchronization purposes and to accommodate the turnaround time and the round trip delay whenever switching transmission from DL to UL, and vice versa.
  3. Because the UL/DL allocation is dynamic, there is very little waste of spectrum for asymmetric operations, i.e., last-mile applications, where typically the UL traffic is a fraction of the DL traffic. (Some spectrum is still lost for the guard periods, but this is negligible compared to the total length of data in a time slot).

Frequency Division Duplex (FDD)

Using the FDD method, a distinct frequency channel is assigned to both the transmitter and the receiver. At any particular instant in time, uplink (UL) traffic uses a frequency f0 that is different from the frequency f1 used by the downlink (DL) traffic.

The Base Station Unit (BSU) may receive uplink traffic while it simultaneously transmits on the downlink. To avoid the high design costs that FDD imposes on Subscriber Units (SU), WiMAX SUs use a hybrid duplex method called HFDD (half-duplex FDD). HFDD is very similar to TDD. An HFDD device transmits and receives at different times like a TDD device. The difference is that it also uses different frequencies for transmit and receive to communicate with an FDD Base Station. As a result, HFDD Subscriber Units offer only half the throughput capacity of a full duplex FDD Subscriber Unit.

FDD is typically used in applications that require an equal up- and downlink bandwidth, as all TDM voice applications do. Therefore, regulatory agencies grant up- and downlink channels of equal capacity for FDD-based systems.

Several inferences can be drawn from this description:

  1. Due to the symmetric nature of FDD transmission channels, and the FDD legacy as duplex method of choice for TDM voice applications, FDD transmission channels are always of equal size (50% for UL and 50% for DL). In applications such as Internet access, which can be very asymmetric in nature, a large percentage of the available UL bandwidth remains unused and is, therefore, wasted.
  2. A guard band about two times the size of the UL or DL channel is required to separate the UL and DL channels.  This amounts to an additional 50% loss in spectrum.
  3. Once the channel bandwidth is granted by the regulator, the UL/DL allocation cannot be changed. This leads to unused spectrum for asymmetric operations, i.e., for last-mile applications, where typically the UL traffic is a fraction of the DL traffic.

WiMAX and the Last Mile

Based on the characteristics of the two duplex methods (TDD and FDD), as analyzed in the previous section, it is clear that the TDD method provides the best network equipment for last-mile access service. However, this does not imply that FDD should be discarded completely. FDD has its place in the network, making a much stronger case for symmetric-type traffic, such as that found in a cellular/T1 backhaul. In this case, TDD tends to waste bandwidth during switchover from transmit to receive, has greater inherent latency, and may require more power-hungry circuitry.

  1. For last-mile applications, WiMAX uses a lower frequency range – 2 GHz to 11 GHz (similar to Wi-Fi). This allows Non Line of Sight (NLOS) connections, because lower-wavelength transmissions are not as easily disrupted by physical obstructions. They are better able to diffract, or bend, around obstacles. TDD is better here because of the asymmetry of traffic.
  2. For cellular/T1 backhaul applications, a line-of-sight (LOS) connection is stronger and more stable, so it is able to send much more data with fewer errors. LOS transmissions use higher frequencies, with ranges reaching a possible 66 GHz. At higher frequencies, there is less interference and much more bandwidth. FDD is better in this application because of the symmetry of traffic.

Additional Considerations

The choice of TDD or FDD may be dictated by the regulatory agency. Each country and/or regulatory body can specify if one or more duplex methods are permissible in a given frequency band. The operator must first determine if the frequency band requires a specific duplex method or accepts either.

The determining factor for the operator making a duplex choice should be determined by customers (audience) and the applications that customers expect of the service.  With the heavy desire for WiMAX Forum Certified equipment to enable broader deployment of last-mile access systems, there are excellent reasons to select the TDD method for this application.

A comparison of technical issues regarding FDD versus TDD is shown below.

TDD is ideally suited to the transport of asymmetric traffic, as is typical with Internet access, and it allows service providers to define accordingly the percentage of bandwidth allocated to each direction. In addition, TDD makes more efficient use of spectrum, allowing network operators to achieve greater returns on their investments in infrastructure. As for FDD, it is the scheme of choice when traffic is symmetric, as in carrier backhaul and enterprise data transfer applications.

Therefore, in the near term – as WiMAX is adopted for last-mile applications – network operators are advised to choose TDD in order to achieve the flexibility required for managing divergent traffic patterns.

Comparison of FDD versus TDD

Guard Band: FDD requires a guard band to separate the DL and UL channels. In MMDS, two RF channels are used to separate the UL & DL channels, which amount to a substantial loss in spectrum. FDD requires no guard bands.

Guard Time: for FDD no guard time is required at the end of DL transmission. However, guard time is required at the end of UL transmission because typically the SUs are HFDD units that need to turn around from Tx to Rx to receive the new BSU schedule information for the next downlink.
Frequency Plan and Reuse: FDD – The adjacent channel interference is much lower than in a TDD scheme, whereas for TDD Frequency planning is required only for one channel. If all TDD-based systems are synchronized to GPS, using the same frame size and DL/UL partitioning can mitigate interference.

Hardware Cost: FDD requires one transmitter and a separate receiver. Further a diplexer and shields are required to isolate the DL and UL, whereas in TDD the transmitter and receiver use the same filters, mixers etc the cost of a TDD scheme is substantially less than an FDD scheme.

Dynamic Bandwidth Allocation: In FDD once the channel bandwidth is granted by the regulator the UL/DL allocation cannot be modified. This leads to unused spectrum for asymmetric operations such as Internet traffic. In TDD the cell interference is not a problem, adaptive UL/DL allocation allows dynamic bandwidth allocation for UL and DL traffic. This is especially important for Internet traffic.

Latency: The average FDD latency in a PMP system is 1 frame and the best case latency is about 0.5 frame, in TDD the average TDD latency in a PMP system is 2 frames and the best case latency is about 1 frame.

Adaptive Antenna System/ Multiple Input-Multiple Output (AAS/ MIMO) advantages: For closed loop beam forming, FDD requires the SU to provide the channel response for the DL direction. This increases the latency and reduces the performance of the beam former. TDD allows the BSU to estimate the DL channel as both DL and UL are operating on the same frequency. The performance of the beam former is therefore better.

Categories: WiMAX

Mesh Network Engineer’s Blog

Why did I name my blog as a Mesh network Engineer’s blog ? It has nothing to do with Mesh networks or in fact about any network in general. But my opinion that we are all meshed together in a network, and the network is now called a cloud. Like how Frigyes Karinthy thought of the idea that we are all separated by six degrees from each other on the human web, we in today’s world are meshed together by all the social networks and cloud applications. Hence my usage of this name.

More Insight into six degrees of separation:

American playwright John Guare wrote a play in 1990 and later released a film in 1993 that popularized it. It is Guare’s most widely-known work.The play ruminates upon the idea that any two individuals are connected by at most five others. As one of the characters states:
I read somewhere that everybody on this planet is separated by only six other people. Six degrees of separation between us and everyone else on this planet. The President of the United States, a gondolier in Venice, just fill in the names. I find it A) extremely comforting that we’re so close, and B) like Chinese water torture that we’re so close because you have to find the right six people to make the right connection… I am bound to everyone on this planet by a trail of six people.
Guare, in interviews, attributed his awareness of the “six degrees” to Marconi. Although this idea had been circulating in various forms for decades, it is Guare’s piece that is most responsible for popularizing the phrase “six degrees of separation.” Following Guare’s lead, many future television and film sources would later incorporate the notion into their stories.
Categories: Broadband

3G to 4G – Differences & Challenges

Walking to work with rain pattering on my face, I was thinking – why is everybody scrambling to get 4G networks rolled out ? And why is everybody beating their chest like a quintessential Gorilla in the media – proclaiming the best network by standards of coverage or speed. Why is there a media blitz about coverage maps, speeds and ‘like’ 4G experiences ? Is HSPA+ like 4G ? Well some operators have touted HSPA+ as ‘4G’  in the media. I am a Telecom engineer by profession and these buzz words confuse me between fact and reality, and I can only guess what it does to a layman.

During my last trip to India, last year an older gentleman asked me if I can spec out a good phone for him to buy. He wanted to buy a 3G phone as he ‘overheard’  that they can perform better and access the internet faster. I had a hard time explaining to him that it is not the phone but the network that needs to be enabled first – we just started rolling out a 3G networks in India !

In the USA here we are working on deploying the 4G networks. I come back to my original question – but why ? Is there one killer app that is a bandwidth hog or one device that needs this broadband level bandwidth ? There is no straight answer for this – we have created bubbles around us at home and work with WiFi and enabled devices around them. It would be great to have it surround us and envelope us to make it a bigger bubble, with access to on-demand apps and services like Hulu, maps, food places with Zagat ratings etc. So what is changing our behavior ? Why are we demanding more bandwidth from the operators with flatter ARPU’s, and ever increasing demand for data. Are now a generation that depends on internet for most of our social media interaction or being in constant touch with each other. Working for operators has taught me this, the slew of smartphones with iPhone leading the pack has thrown the network capacity into a growth frenzy. The pipes as we know are filling faster than the operator can deploy, what used to be a single T1/E1 per site has migrated to IP/AAV and the average user. And in this preparation for the next wave of Smartphones and data demands, all carriers are rushing to introduce the next-generation – 4G.

As an operator what is changing with 4G ? Lets dwell on some history with 4G  – both the Radio Network as well as Core network is changing, with 3G the major difference was the Radio side, with minor tweaks applied to the Packet core side. With W-CDMA technologies the traditional GSM operators had to learn that the traffic channels were no longer limited to 200 KHz in size and that power control with network load (cell breathing) played a big role in coverage footprint.  With 4G we are back to the concept of channels or resource blocks of 180 KHz each.

Voice will remain circuit switched for some time as legacy networks continue to be widely used, but the future with LTE and beyond is voice over IP. That will bring new challenges for the core network, especially for quality of service. Voice likely will be given higher priority over most data because users have a lower tolerance of diminished quality for voice calls.

Categories: 3GPP, Broadband, LTE, WiMAX