3GPP Picks Femtocell Standards

The 3rd Generation Partnership Project (3GPP) standards body has finally adopted an official architecture and started work on a new standard for home base stations. The specification for the interface between the Home Node B (HNB, the 3GPP term for femtocell) is being decided. The new interface will be called Iu-h and is a blend of existing standards Iu and generic access network (GAN), sometimes referred to as unlicensed mobile access (UMA). The 3GPP chose the solution backed by industry majors Alcatel-Lucent , Kineto Wireless Inc. , and its partners Motorola and NEC Corp. The new standard, which forms part of 3GPP’s Release 8, and interdependent with Broadband Forum extensions to its Technical Report-069 (TR-069), has been completed in just 12 months following close cooperation between 3GPP, the Femto Forum and the Broadband Forum.

Femtocells
The term has already been introduced to in one of the earlier posts. Femtocells are low-power wireless access points that operate in licensed spectrum to connect standard mobile devices to a mobile operator’s network using residential DSL or cable broadband connections.A Home Node B (HNB), is the 3GPP's term for a 3G femtocell. A Node B is an element of a 3G macro Radio Access Network (RAN). A femtocell performs many of the function of a Node B, but is optimized for deployment in the home.

The new standard
The new standard covers the following main areas:

• Network architecture
• Radio & interference aspects
• Femtocell management / provisioning and security

In the proposed network architecture, the interface between femtocells and gateways in the network core re-uses existing 3GPP UMTS protocols and extends them to support the needs of high-volume femtocell deployments. The new standard has adopted the Broadband Forum’s TR-069 management protocol which has been extended to incorporate a new data model for femtocells developed collaboratively by Femto-Forum and Broadband Forum members and published by the Broadband Forum as Technical Report 196 (TR-196).

Single Carrier FDMA - for 4G wireleess

Over the last decade, the bit rates achieved in wireless communications systems have increased steadily.
TDMA and CDMA has been the major technologies in multiple access. The highest bit rates in commercially deployed wireless systems are achieved by means of Orthogonal Frequency Division Multiplexing (OFDM). The next advance in cellular systems, under investigation by the Third Generation Partnership Project (3GPP), also anticipates the adoption of OFDMA to achieve higher bit rates. Single carrier frequency division multiple access (SC-FDMA), a modified form of Orthogonal FDMA (OFDMA), is a promising technique for high data rate up-link communications in future cellular systems.

SC FDMA
An SC system transmits a single carrier, modulated, for example, with QAM, at a high symbol rate. The transmitters use different orthogonal subcarriers to transmit information symbols. The transmission is sequential, which reduces the variations in the transmitted signal envelope. This results in a lower peak-to-average-power ratio. Frequency domain equalization os carried out to counter the severe delay spreads the signal might encounter. The advantages may be listed as:

• Small variations in the instantaneous power of the transmitted signal
• Possibility for low-complexity high-quality equalization in the frequency domain.
• Possibility for FDMA with flexible bandwidth assignment.
• SC-FDMA can be seen as normal OFDM with a DFT-based precoding

SC-FDMA transmitter and receiver
The block diagram of the SC-FDMA receiver and transmitter is given the figure. The figure is self-explanatory. Similar to OFDM modulation, DFTS-OFDM relies on block-based signal generation.
By adjusting the transmitter DFT size and the size of the block of modulation symbols the nominal bandwidth of the DFTS-OFDM signal can be dynamically adjusted.

Throughput
Information throughput is another indication of the system performance. Here the throughput depends on the manner in which information is applied to the subcarriers. The two main methods are localized and distributed. The benefit of distributed system, compared to localized, is the possibility for additional frequency diversity as even a low-rate distributed signal can be spread over a potentially very large overall transmission bandwidth. It has been shown that the SC-FDMA can be tuned to achieve data rates in excess of 40Mbps.

Future
Within a specific SC-FDMA system configuration, there are many design and operational choices that affect performance in a complex manner . The impact of channel estimation error on the throughput performance of SC-FDMA is still not understood clearly. Still, SC-FDMA is a promising technique for high data rate
uplink communication in future cellular systems.

OFDM - Accelerating data rates

Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data. Orthogonal FDM (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own. It is identical to Coded FDM and the Discrete Multitone (DMT) modulation.

Orthogonality
In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. This greatly simplifies the design of both the transmitter and the receiver. A separate requirement for different filters is thus eliminated. This results in high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. But this also means high accuracies in synchronization between transmitter and receiver is required.

OFDM exhibits lower multi-path distortion (delay spread), since the sub-signals are sent at lower data rates. Because of the lower data rate transmissions, multi-path-based delays are not nearly as significant as they would be with a single-channel high-rate system. For example, a narrow band signal sent at a high rate over a single channel will likely experience greater negative effects from delay spread because the transmitted symbols are closer together. In fact, the information content of a narrow band signal can be completely lost at the receiver if the multi path distortion causes the frequency response to have a null at the transmission frequency. The use of the multi-carrier OFDM significantly reduces this problem.

Simple Implementation
The orthogonality allows for efficient modulator and demodulator implementation using the FFT algorithm on the receiver side, and inverse FFT on the sender side. Although the principles and some of the benefits have been known since the 1960s, OFDM is popular for wideband communications today by way of low-cost digital signal processing components that can efficiently calculate the FFT.

Elimination of intersymbol interference
One key principle of OFDM is that since low symbol rate modulation schemes i.e. where the symbols are relatively long compared to the channel time characteristics suffer less from inter symbol interference caused by multi path propagation, it is advantageous to transmit a number of low-rate streams in parallel instead of a single high-rate stream. Since the duration of each symbol is long, it is feasible to insert a guard interval between the OFDM symbols, thus eliminating the inter symbol interference. The cyclic prefix, which is transmitted during the guard interval, consists of the end of the OFDM symbol copied into the guard interval, and the guard interval is transmitted followed by the OFDM symbol. The reason that the guard interval consists of a copy of the end of the OFDM symbol is so that the receiver will integrate over an integer number of sinusoid cycles for each of the multi paths when it performs OFDM demodulation with the FFT.

Simplified equalization
The effects of frequency-selective channel conditions, for example fading caused by multipath propagation, can be considered as constant (flat) over an OFDM sub-channel if the sub-channel is sufficiently narrow-banded, i.e. if the number of sub-channels is sufficiently large. This makes equalization far simpler at the receiver in OFDM in comparison to conventional single-carrier modulation. The equalizer only has to multiply each detected sub-carrier (each Fourier coefficient) by a constant complex number, or a rarely changed value.

Importance of channel coding
Channel coding is used in most cases of digital communication and especially in case of mobile communication. Channel coding implies that each bit of information to be transmitted is spread over several, often very many, code bits. If these coded bits are then, via modulation symbols, mapped to a set of OFDM subcarriers that are well distributed over the overall transmission bandwidth of the OFDM signal, each information bit will experience frequency diversity in case of transmission over a radio channel that is frequency selective over the transmission bandwidth, despite the fact that the subcarriers, and thus also the code bits, will not experience any frequency diversity. Thus, in contrast to the transmission of a single wideband carrier, channel coding (combined with frequency interleaving) is an essential component in order for OFDM transmission to be able to benefit from frequency diversity on a frequency-selective channel.

OFDM for Access control
OFDM can also be used as a user-multiplexing or multiple-access scheme, allowing for simultaneous frequency-separated transmissions to/from multiple mobile terminals. In the downlink direction, OFDM as a user-multiplexing scheme implies that, in each OFDM symbol interval, different subsets of the overall set of available subcarriers are used for transmission to different mobile terminals. Similarly, in the uplink direction, OFDM as a user-multiplexing or multiple access scheme implies that, in each OFDM symbol interval, different subsets of the overall set of subcarriers are used for data transmission from different mobile terminals.

Issues
A drawback of OFDM modulation, as well as any kind of multi-carrier transmission, is the large variations in the instantaneous power of the transmitted signal. Such power variations imply a
reduced power-amplifier efficiency and higher power-amplifier cost. This is especially critical for the uplink, due to the high importance of low mobile-terminal power consumption and cost. Several methods have been proposed on how to reducethe large power variations of an OFDM signal. However, most of these methods have limitations in terms of to what extent the power variations can be reduced. Furthermore, most of the methods also imply a significant computational complexity and/or a reduced link performance.