The Birth of TDD-LTE and FDD-LTE

The Birth of TDD-LTE and FDD-LTE

LTE-Advanced (Long Term Evolution-Advanced) is used on fourth generation (4G) in mobile phone technology as many providers are beginning to augment their networks with LTE. As known, mobile phone traffic is divided into two parts: an uplink and a downlink.

Definition of TDD and FDD

In communication systems, a user needs to exchange data with one or more parties through a shared resource – a common channel. Depending on whether the data is transmitted/received simultaneously, the following transmission techniques exist:

1. Simplex – One party transmits data and the other party receives data. No simultaneous transmission is possible – the
communication is one-way and only one frequency (channel) is used. Examples of simplex communication are traditional (non-interactive) radio and television.

2. Half Duplex – Each party can receive and transmit data, but not at the same time. The communication is two-way and only one frequency (channel) is used. Examples of half duplex communication are walkie-talkies or other two-way radio systems.

3. Full Duplex – Each party can transmit and receive data simultaneously. The communication is two-way and two frequencies (channels) are used – one for transmitting and one for receiving. In the case of cellular networks, a limited shared resource (spectrum) needs to be shared with all users so full duplex communication is possible (Note that full duplex service, like regular phone conversation, can be carried over a half duplex channel). The two main methods used are:

1. Time Division Duplexing (TDD) – The communication is done using one frequency, but the time for transmitting and receiving is different. This method emulates full duplex communication using a half duplex link.

The key advantages of TDD (known also as TD-LTE) are usually seen in conditions where the uplink and downlink data transmissions are not symmetrical. Moreover, since the transmitting and receiving is done using one frequency, the channel estimations for beamforming (and other smart antenna techniques) apply for both the uplink and the downlink. A typical disadvantage of TDD is the need to use guard periods between the downlink and uplink transmissions.

2. Frequency Division Duplexing (FDD) – The communication is done using two frequencies and the transmitting and receiving of data is simultaneous.

The advantages of FDD are typically observed in situations where the uplink and downlink data transmissions are symmetrical (which is not usually the case when using wireless phones). On top of that, when using FDD, the interference between neighboring Radio Base Stations (RBSs) is lower than when using TDD. Aside from that, the spectral efficiency (which is a function of how well a given spectrum is used by certain access technology) of FDD is greater than TDD.

Differences between FDD-LTE and TDD-LTE

The two versions of LTE are very similar. In fact, they differ only in the physical layer and, as a result, the version implemented is transparent to the higher layers. This means that UEs will be able to support both TDD-LTE and FDD-LTE with one chipset with only minor modifications required. All major chipset vendors including ST-Ericsson (M700/M710 chipsets), Altair Semiconductor (FourGee-6150 chipset), and Qualcomm (MDM9200/MDM9600 chipsets) have already released chipsets that support both LTE flavors. UEs based on those chipsets are (or will soon be) available from Sony Ericsson, Huawei, Samsung, Nokia, and others.

The following features are unique to TDD-LTE:

1. Frame structure – 3GPP has specified a special subframe that allows switching between downlink and uplink transmission.

2. Random access – Several additional random access formats exist in certain subframes. Also, several random access channels exist in every subframe.

3. Scheduling – The scheduling for the uplink is multi-frame.

4. HARQ – The number of HARQ processes depends on the uplink/downlink resource allocation.

5. ACK/NACK – Multiple acknowledgements and negative acknowledgements are combined on the uplink control channels. This ultimately leads to increased control signaling and lower spectrum/resource utilization.

6. Guard periods – These are used in the center of special subframes. They allow for the advance of the uplink transmission timing.

Another difference between FDD-LTE and TDD-LTE is that in FDD-LTE every downlink subframe can be associated with an uplink subframe. In TDD-LTE the number of downlink and uplink subframes is different and such association is not possible. Additionally, the uplink coverage with respect to a specific data rate in TDD-LTE is generally worse than FDD-LTE due to the fact that the uplink transmission is not continuous. The percentage of coverage for control and data channels is, however, very similar to that of FDD-LTE.

In terms of spectrum efficiency, the performances of TDD-LTE and FDD-LTE are similar for non-delay sensitive traffic. The lower performance of TDD-LTE is due to the guard periods mentioned above.

Finally, TDD-LTE and TD-SCDMA work together with minimum interference issues, even if both technologies are deployed in the same frequency band (assuming that the TD-LTE UL:DL configurations are chosen correctly and both systems are synchronized to the same time source).

LTE TDD and FDD modes have been greatly harmonized in the sense that both modes share the same underlying framework, including radio access schemes OFDMA in downlink and SC-FDMA in uplink, basic subframe formats, configuration protocols, etc.. As clear indication of the harmonization, the TDD mode is included together with the FDD mode in the same set of specifcations, including the physical layer where there are just a few differences due to the uplink/downlink switching operation. In terms of architecture there are no differences between FDD and TDD and the very few differences in the MAC and higher layer protocols relate to TDD specifc physical layer parameters. Procedures are kept the same. Thus there will be high implementation synergies between the two modes allowing for effcient support of both TDD and FDD in the same network or user device. Coexistence would of course still require careful analysis.

The uplink coverage with respect to a specific data rate in TDD-LTE is generally worse than FDD-LTE due to the fact that the uplink transmission is not continuous. The percentage of coverage for control and data channels is, however, very similar to that of FDD-LTE. In terms of spectrum effciency, the performances of TDD-LTE and FDD-LTE are similar for non-delay sensitive traffic. The lower performance of TDD-LTE is due to the guard periods mentioned above. Overall, TDD-LTE offers operators a great alternative to FDD. Its natural suitability for asymmetric applications, low latency, high throughput, and security make it a flexible and cost-effective solution for the next generation wireless networks. TDD is more flexible than FDD in meeting the need to dynamically reconfigure the allocated upstream and downstream bandwidth in response to customer needs. In summary, TDD is a more desirable duplexing technology that allows system operators to receive the most from their investment in spectrum and telecom equipment, while meeting the needs of each individual customer.

REFERENCES
1. Holma, H. and A. Toskala, LTE for UMTS: OFDMA and SC-FDMA Based Radio Access, 267, John Wiley & Sons Ltd., United Kingdom, 2009.
2. Dahlman, E., S. Parkvall, and J. SkÄold, 4G LTE/LTE-Advanced for Mobile Broadband, 100-137, Elsevier Ltd., UK, 2011.
3. Dahlman, E., S. Parkvall, J. SkÄold, and P. Beming, 3G Evolution: HSPA and LTE for Mobile Broadband, 2nd Edition, 318, Elsevier, Department in Oxford, UK, 2008.
4. Parkvall, S. and D. Astely, “The evolution of LTE towards IMT-advanced,” Journal Of Communications,
Vol. 4, No. 3, 146-153, Apr. 2009.
5. Progri, I., Geolocation of RF Signals: Principles and Simulations, 115, Springer, USA, 2011.
6. ASCOM.

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