The spectrum

Regarding the spectrum, LTE becomes quite flexible, allowing bandwidths of 1.25MHz, 1.6MHz, 2.5MHz, 5MHz, 10MHz, 15MHz and 20MHz in the downlink and uplink [4]. Furthermore, it supports broadcast transmission in downlink and uplink-downlink modes, on the other hand, radio resources for broadcast transmissions can be modified according to the operator needs.

The various scenarios which may present between the interaction of different service providers and other networks that they have, are not largely affected because manufacturers have planned coexistence, within the same geographic area, of the EUTRAN with other networks such as 3G and coexistence between adjacent operators, so too, is the case of the overlapping in countries boundaries [4].

The LTE spectral efficiency exceeds largely to HSPA +, these are the results of Telefónica [9], the study provides a scenario of urban centers with high density of buildings, 2x2 MIMO antenna configuration for both cases and using 64QAM as modulation scheme, with this, the study says that LTE spectral efficiency exceeds to HSPA + by 20% at full load. It is apparent from Figure # 1 than for rural or suburban centers LTE benefits will outweigh, furthermore along the chart, the LTE superiority is denoted.

Figure # 1: Spectral efficiency in terms of resource use [9].

Macro-cells are not very efficient [1] in the residential area due to penetration problems that have been mentioned in a previous article, also in the macro-cells there are many users and is more difficult to provide QoS to all of them. For these reasons Internet Service Providers (ISP) have opted for femto-cell implementation at residential level, in order to increase the quality of services provided.

Femto-cells are low-power base stations that provide low cost and high quality wireless services to residential, in this area offer a coverage of approximately 10 m [2], which are designed to integrate automatically with macro-cell networks. Femto-cells integrate with mobile operator through a broadband connection, typically DSL [2] and [3], as can be seen in Figure # 1. However it can also be performed by a wireless link as mentioned in [4]. In General, the femto-cell redirects the traffic from the home cellular system, through the broadband connection, releasing the resource consumption of the macro-cell.

Figure # 1. Typical Femto-cell [3].

Duplex Systems transmission: LTE-FDD and LTE-TDD

FDD (Frequency Division Duplex) is a scheme of transmitting and receiving signals, this system allows full duplex communication using two different frequencies, one for the downlink and one for uplink, maintaining a separation band between said frequencies in order no overlap of channels, it makes that the FDD spectral efficiency is not very good. However, one advantage of this multiplexing scheme is that it does not introduce additional delays or latency.

On the other hand, TDD (Time Division Duplex) is another multiplexing technique for communication that uses a single channel or frequency, for information transmission. In this case, the transmission and reception is performed by the same frequency but with differences in time and a temporal separation between the two directions of communication, making more efficient use of spectrum. The TDD multiplexing takes a temporary assignment to the communication directions, and the guard time, which makes it more sensitive to delays and latency.

Some comparisons:

• Regarding FDD distance has better characteristics at longer distances than TDD, so TDD has more acceptation in scenarios where distances are shorter.

• The signal propagation also has different characteristics, since TDD uses a single frequency, this makes the channel as such, presents the same characteristics in transmission and reception.

• In FDD each channel has different propagation characteristics depending on the frequency used.

• The frequency difference of the FDD channels, causes that the capacity of each channel depends on the frequency allocated by regulatory authorities.

• In TDD is easier to make a dynamic distribution of the uplink and downlink capacity in order to meet the demand characteristics of resources.

Figure # 1 shows the frequency assignments set by 3GPP for the use of TDD and FDD:

Figure # 1: Frequency bands for LTE FDD and TDD [1].

With so much information that comes every day around the fourth generation mobile (4G), it is very common that we like to be updated, we care about general aspects of the architecture of this new technology. Well, "System Architecture Evolution" (SAE) or also known as EPC ("Evolved Packed Core") has its development from the year 2004 until 2009, during that time many studies have been deployed which allowed to define the standards that describes the architecture of the core network. The specifications are in chronological order in the following figure:

Figura # 1: SAE evolution [1].