Random access is generally performed when the UE turns on from sleep mode, performs handoff from one cell to another or when it loses uplink timing synchronization. At the time of random access, it is assumed that the UE is time-synchronized with the eNB on the downlink. Therefore, when a UE turns on from sleep mode, it first acquires downlink timing synchronization. The downlink timing synchronization is achieved by receiving primary and secondary synchronization sequences and the broadcast channel as discussed in Chapter 9. After acquiring downlink timing synchronization and receiving system information including information on parameters specific to random access, the UE can perform the random access preamble transmission. Random access allows the eNB to estimate and, if needed, adjust the UE uplink transmission timing to within a fraction of the cyclic prefix. When an eNB successfully receives a random access preamble, it sends a random access response indicating the successfully received preamble(s) along with the timing advance (TA) and uplink resource allocation information to the UE. The UE can then determine if its random access attempt has been successful by matching the preamble number it used for random access with the preamble number information received from the eNB. If the preamble number matches, the UE assumes that its preamble transmission attempt has been successful and it then uses the TA information to adjust its uplink timing. After the UE has acquired uplink timing synchronization, it can send uplink scheduling or a resource request using the resources indicated in the random access response message as depicted in Figure 10.1.

A cellular radio system consists of a collection of fixed eNBs that define the radio coverage areas or cells. Typically, a non-line-of-sight (NLOS) radio propagation path exists between an eNB and a UE due to natural and man-made objects that are situated between the eNB and the UE. As a consequence, the radio waves propagate via reflections, diffractions and scattering. The arriving waves at the UE in the downlink direction (at the eNB in the uplink direction) experience constructive and destructive additions because of different phases of the individual waves. This is due to the fact that, at the high carrier frequencies typically used in the cellular wireless communication, small changes in the differential propagation delays introduce large changes in the phases of the individual waves. If the UE is moving or there are changes in the scattering environment, then the spatial variations in the amplitude and phase of the composite received signal will manifest themselves as the time variations known as Rayleigh fading or fast fading. Traditionally, the time-varying nature of the wireless channel was considered undesirable because it required very high signal-to-noise ratio (SNR) margins for providing the desired bit error or packet error reliability. Therefore, system design efforts focused on averaging out the signal variations due to fast fading by using various forms of diversity schemes such as space, angle, polarization, field, frequency, time or multi-path diversity.

The cellular wireless communications industry witnessed tremendous growth in the past decade with over four billion wireless subscribers worldwide. The first generation (1G) analog cellular systems supported voice communication with limited roaming. The second generation (2G) digital systems promised higher capacity and better voice quality than did their analog counterparts. Moreover, roaming became more prevalent thanks to fewer standards and common spectrum allocations across countries particularly in Europe. The two widely deployed second-generation (2G) cellular systems are GSM (global system for mobile communications) and CDMA (code division multiple access). As for the 1G analog systems, 2G systems were primarily designed to support voice communication. In later releases of these standards, capabilities were introduced to support data transmission. However, the data rates were generally lower than that supported by dial-up connections. The ITU-R initiative on IMT-2000 (international mobile telecommunications 2000) paved the way for evolution to 3G. A set of requirements such as a peak data rate of 2 Mb/s and support for vehicular mobility were published under IMT-2000 initiative. Both the GSM and CDMA camps formed their own separate 3G partnership projects (3GPP and 3GPP2, respectively) to develop IMT-2000 compliant standards based on the CDMA technology. The 3G standard in 3GPPis referred to as wideband CDMA(WCDMA) because it uses a larger 5 MHz bandwidth relative to 1.25 MHz bandwidth used in 3GPP2's cdma2000 system. The 3GPP2 also developed a 5 MHz version supporting three 1.25 MHz subcarriers referred to as cdma2000-3x.

Voice communication and download data services such as web browsing are based on point-to-point (PTP) communication. On the other hand, multicast and broadcast services are based on point-to-multipoint (PTM) communication, where data packets are simultaneously transmitted from a single source to multiple destinations. Examples of broadcast services are radio and television services that are broadcast over the air or over cable networks and the content is available to all the users. Multicast refers to services that are delivered to users who have joined a particular multicast group. The service delivery using point-to-multipoint (PTM) communication is generally more efficient when a large number of users is interested in receiving the same content such as a mobile TV channel. This results in efficient transmission not only over the wireless link but also in the core and access networks. This is because a single multicast broadcast packet travels in the core and access networks and is copied and forwarded to multiple Node-Bs in the multicast broadcast area.

The broadcast services can be delivered to mobile devices either via an independent broadcast network such as DVB-H (digital video broadcast-handheld), DMB (digital multimedia broadcast), MediaFLO or over a service provider's cellular network. The DMB is a South Korean standard derived from the digital audio broadcast (DAB) standard. In the case of an independent broadcast network, dual mode UEs capable of receiving service from both the broadcast network and the cellular network are required.

The LTE network architecture is designed with the goal of supporting packet-switched traffic with seamless mobility, quality of service (QoS) and minimal latency. A packet-switched approach allows for the supporting of all services including voice through packet connections. The result in a highly simplified flatter architecture with only two types of node namely evolved Node-B (eNB) and mobility management entity/gateway (MME/GW). This is in contrast to many more network nodes in the current hierarchical network architecture of the 3G system. One major change is that the radio network controller (RNC) is eliminated from the data path and its functions are now incorporated in eNB. Some of the benefits of a single node in the access network are reduced latency and the distribution of the RNC processing load into multiple eNBs. The elimination of the RNC in the access network was possible partly because the LTE system does not support macro-diversity or soft-handoff.

In this chapter, we discuss network architecture designs for both unicast and broadcast traffic, QoS architecture and mobility management in the access network. We also briefly discuss layer 2 structure and different logical, transport and physical channels along with their mapping.

Network architecture

All the network interfaces are based on IP protocols. The eNBs are interconnected by means of an X2 interface and to the MME/GW entity by means of an S1 interface as shown in Figure 2.1. The S1 interface supports a many-to-many relationship between MME/GW and eNBs.

A major design goal for the LTE system is flexible bandwidth support for deployments in diverse spectrum arrangements. With this objective in mind, the physical layer of LTE is designed to support bandwidths in increments of 180 kHz starting from a minimum bandwidth of 1.08 MHz. In order to support channel sensitive scheduling and to achieve low packet transmission latency, the scheduling and transmission interval is defined as a 1 ms subframe. Two cyclic prefix lengths namely normal cyclic prefix and extended cyclic prefix are defined to support small and large cells deployments respectively. A subcarrier spacing of 15 kHz is chosen to strike a balance between cyclic prefix overhead and robustness to Doppler spread. An additional smaller 7.5 kHz subcarrier spacing is defined for MBSFN to support large delay spreads with reasonable cyclic prefix overhead. The uplink supports localized transmissions with contiguous resource block allocation due to single-carrier FDMA. In order to achieve frequency diversity, inter-subframe and intra-subframe hopping is supported. In the downlink, a distributed transmission allocation structure in addition to localized transmission allocation is defined to achieve frequency diversity with small signaling overhead.

Channel bandwidths

The LTE system supports a set of six channel bandwidths as given in Table 8.1.We note that the transmission bandwidth configuration BWconfig is 90% of the channel bandwidth BWchannel for 3–20 MHz. For 1.4 MHz channel bandwidth, the transmission bandwidth is only 77% of the channel bandwidth. Therefore, LTE deployment in the small 1.4 MHz channel is less spectrally efficient than the 3 MHz and larger channel bandwidths.

Like other 3G systems, the current HSPA system uses turbo coding as the channel-coding scheme. The LTE system supports peak data rates that are an order of magnitude higher than the current 3 G systems. It is therefore fair to ask the question, can the turbo coding scheme scale to data rates in excess of 100 Mb/s supported by LTE, while maintaining reasonable decoding complexity? This question is particularly important as other coding schemes, which offer inherent parallelism and therefore provide very high decoding speeds such as Low Density Parity Check (LDPC) codes, have recently become available. A major argument against turbo coding schemes is that they are not amenable to parallel implementations thus limiting the achievable decoding speeds. The problem, in fact, lies in the turbo code internal interleaver used in the current HSPA system, which creates memory contention among processors in parallel implementation. Therefore, if the turbo code internal interleaver can somehow be made contention free, it becomes possible for turbo code to benefit from parallel processing and hence achieve high decoding speeds.

LDPC codes

Similar to turbo codes, LDPC codes are near-Shannon limit error correcting codes. More recently, LDPC codes have been adopted in standards including IEEE 802.16e wireless MAN, IEEE 802.11n wireless LAN and digital video broadcast DVB-S2. The LDPC codes allow an extremely flexible code design that can be tailored to achieve efficient encoding and decoding. The interest in LDPC codes comes from their potential to achieve very high throughput (due to the inherent parallelism of the decoding algorithm) while maintaining good error-correcting performance and low decoding complexity.

In cellular systems, the wireless communication service in a given geographical area is provided by multiple Node-Bs or base stations. The downlink transmissions in cellular systems are one-to-many, while the uplink transmissions are many-to-one. A one-to-many service means that a Node-B transmits simultaneous signals to multiple UEs in its coverage area. This requires that the Node-B has very high transmission power capability because the transmission power is shared for transmissions to multiple UEs. In contrast, in the uplink a single UE has all its transmission power available for its uplink transmissions to the Node-B. Typically, the maximum allowed downlink transmission power in cellular systems is 43 dBm, while the uplink transmission power is limited to around 24 dBm. This means that the total transmit power available in the downlink is approximately 100 times more than the transmission power from a single UE in the uplink. In order for the total uplink power to be the same as the downlink, approximately 100 UEs should be simultaneously transmitting on the uplink.

Most modern cellular systems also support power control, which allows, for example, allocating more power to the cell-edge users than the cell-center users. This way, the cell range in the downlink can be extended because the Node-B can always allocate more power to the coverage-limited UE. However, in the uplink, the maximum transmission power is constrained by the maximum UE transmission power.

The Global system for mobile communications (GSM) is the dominant wireless cellular standard with over 3.5 billion subscribers worldwide covering more than 85% of the global mobile market. Furthermore, the number of worldwide subscribers using high-speed packet access (HSPA) networks topped 70 million in 2008. HSPA is a 3 Gevolution of GSM supporting high-speed data transmissions using WCDMA technology. Global uptake of HSPA technology among consumers and businesses is accelerating, indicating continued traffic growth for high-speed mobile networks worldwide. In order to meet the continued traffic growth demands, an extensive effort has been underway in the 3G Partnership Project (3GPP) to develop a new standard for the evolution of GSM/HSPAtechnology towards a packet-optimized system referred to as Long-Term Evolution (LTE).

The goal of the LTE standard is to create specifications for a new radio-access technology geared to higher data rates, low latency and greater spectral efficiency. The spectral efficiency target for the LTE system is three to four times higher than the current HSPA system. These aggressive spectral efficiency targets require pushing the technology envelope by employing advanced air-interface techniques such as low-PAPR orthogonal uplink multiple access based on SC-FDMA (single-carrier frequency division multiple access) MIMO multiple-input multiple-output multi-antenna technologies, inter-cell interference mitigation techniques, low-latency channel structure and single-frequency network (SFN) broadcast. The researchers and engineers working on the standard come up with new innovative technology proposals and ideas for system performance improvement.