Motivation of This Thesis

Several kinds of QoS services are supported in IEEE 802.16e and different kind of QoS has different request for bandwidth. In IEEE 802.16e, there are several methods for SSs to request bandwidth from a BS. In this thesis, bandwidth request mechanisms for OFDMA systems are aimed at. The OFDMA system specified in IEEE 802.16e will be introduced in chapter 2. In chapter 3, some bandwidth request mechanisms in IEEE 802.16 will be introduced first and then the proposed method is illustrated. Simulation results and discussions are shown in chapter 4. Conclusions are made in chapter 5.

Chapter 2

WirelesslessMAN-OFDMA

(Orthogonal Frequency Division Multiple Access) PHY and MAC Overview

In this chapter, the OFDMA PHY and MAC specified in IEEE 802.16e will be introduced.

2.1 OFDMA PHY Overview

In the OFDMA PHY, resources are divided into slots both time and frequency dimension.

Thus, many users can simultaneously transmit data in the same OFDMA symbol and one user can transmit data across several OFDMA symbols.

2.1.1 OFDM Symbol Description

The OFDMA PHY mode based on at least one of the FFT sizes 2048, 1024,512, and 128 shall be supported. This facilitates scalable bandwidth. The MS should scan the DL signal when performing initial network entry to detect the FFT size employed by the BS. After data is modulated onto the subcarriers, Inverse-Fourier-transform is applied to create time domain signal and the time duration is referred to as the useful symbol time Tb. In order to maintain

the orthogonality of the subcarriers, a cyclic prefix is inserted before the actual data samples.

The cyclic prefix is the replication of the last Np samples of the OFDM symbols. The ratio of CP time to useful time shall be 1/32, 1/16, 1/8, or 1/4. As long as the delay spread is shorter than the cyclic prefix duration, ISI is eliminated. Figure 2.1 illustrates this structure.

Figure 2.1 Structure for an OFDM symbol

An OFDMA symbol is made up of subcarriers, and the number of which determines the FFT size. There are three types of subcarriers:

—Data subcarriers for data transmission

—Pilot subcarriers for coherent detection

—Null carriers for guard bands and DC carrier.

In the OFDMA mode, the active subcarriers including data subcarriers and pilot subcarreirs are divided into several subchannels. There are different ways to divide active subcarriers for the downlink and the uplink. The symbol is divided into logical subchannels to support scalability, multiple access, and advanced antenna array processing capabilities. The subcarriers in a subchannel may, but not need to be adjacent.

2.1.2 OFDMA Subcarrier Permutation

Subcarriers of an OFDMA symbol are divided into subchannels. Permutations are applied to do subchannelization. And there are two main categories of subcarrier permutations

in IEEE 802.16e: distributed and adjacent permutation. Distributed subcarrier permutation means that subcarrier indices in a subchannel are spread out across the whole band.

Distributed subcarrier permutation performs well for mobile applications because it makes use of frequency diversity. Distributed subcarrier permutation is used in PUSC (partial usage of subchannels), FUSC (full usage of subchannels), OPUSC (optional PUSC), OFUSC (optional FUSC), TUSC1 (tile usage of subchannels) and TUSC2. For PUSC and OPUSC, some of the subchannels are allocated to the transmitter; for FUSC and OFUSC, all subchannels are allocated to the transmitter. TUSC1 and TUSC2 shall be only used within AAS (Adaptive Antenna System) zone. Adjacent subcarrier permutation means that subcarrier indices in a subchannel are contiguous. Adjacent permutation is used in Band AMC (Adaptive Modulation and Coding) because it is simpler to feedback the channel quality.

2.1.3 OFDMA Zones

A permutation zone is a number of contiguous OFDMA symbols that use the same permutation formula. The OFDMA frame may contain multiple zones. The transition between zones is indicated in the DL-MAP. The PHY parameters may be different from one zone to the next one. The following figure illustrates an OFDMA frame with multiple zones:

Figure 2.2 OFDMA frame with multiple zones

2.1.4 Frame Structure

In licensed bands, the duplex method should be either FDD (Frequency Division Duplex) or TDD (Time Division Duplex). In license-exempt bands, the duplex method shall be TDD.

In TDD, a frame contains DL and UL bursts. The allowed frame durations are 2, 2.5, 4, 5, 8, 10, 12.5, and 20 ms. The frame duration is defined in the DL-MAP. Each frame in the downlink transmission begins with a preamble followed by a DL transmission period and then an UL transmission period. In each frame, the TTG (transmit/receive transition gap) and RTG (receive/transmit transition gap) shall be inserted between the downlink and uplink and at the end of each frame, respectively, to allow the BS to turn around. The following figure is an example of an OFDMA frame in TDD mode:

Figure 2.3 An example of one TDD time frame

DL subframe begins with a preamble and the following 4 subchannels called Frame Control Header (FCH). The FCH carries Downlink Frame Prefix (DLFP) which provides the decoding information for decoding the DL-MAP. The DLFP format is shown in table 2.1. The Preamble

is BPSK modulated by a PN code which is determined by the IDcell which is assigned by the management entity and the sector number. Preamble enables MS to synchronize with the system, maintain the synchronization and do channel estimation. Preamble also enables MS to measure the received power for channel quality reporting and handover. The DL-MAP following the FCH specifies the downlink allocations for MSs；The UL-MAP following the DL-MAP specifies the uplink allocations for MSs. In the DL subframe, Downlink Channel Descriptor (DCD) and Uplink Channel Descriptor are transmitted periodically to broadcast DL and UL system information respectively. The UL subframe contains user-data bursts and ranging channel. The ranging channel is used by the MS to gain access, maintain connection with BS and to request bandwidth.

Syntax Size Notes

Used subchannel bitmap 6 bits Indicate which groups of subchannel are used on the first PUSC zone Reserved 1 bits Shall be set to zero Repetition Coding

Indication

2 bits Indicate 2,4,6 or no repetition coding is used on DL-MAP

Coding Indication 3 bits Indicate which kind of FEC encoding is used on DL-MAP

DL-MAP Length 8 bits Defines the length of the DL-MAP message

Reserved 4 bits Shall be set to zero Table 2.1 DLFP format

2.1.5 Channel Descriptor Message

A Downlink Channel Descriptor (DCD) and an Uplink Channel Descriptor (UCD) shall be broadcast by the BS at a periodic interval to define the characteristics of a downlink and

uplink physical channel respectively. Both the DCD and the UCD message include Configuration Change Count which is incremented by one (modulo 256) by the BS whenever any of the values of this channel descriptor change. If the value of this count in a subsequent DCD remains the same, the MS can quickly decide that the remaining fields have not changed and may be able to disregard the remainder of the message. Downlink and Uplink Burst Profile are also defined in DCD and UCD messages. In UCD, Ranging Backoff Start, Ranging Backoff End, Request Backoff Start and Request Backoff End are also included.

Ranging Backoff Start and End define the initial and final backoff window size for initial ranging contention; Request Backoff Start and End define the initial and final backoff window size for contention bandwidth requests.

2.1.6 Downlink Map (DL-MAP) Message

The DL-MAP is QPSK modulated at 1/2 code rate by FEC specified in DLFP. DL-MAP enables subscriber stations to decode the downlink subframe. The DL-MAP corresponds to the PHY characteristics as defined by the DCD. The DL-MAP provides the Burst Profile of each allocation using a Downlink Interval Usage Code (DIUC) in DL-MAP IEs (Information Elements). Each allocation is assigned by providing the subchannels and the OFDMA-symbol offset from the preamble. The table 2.2 defines the DIUC encoding that should be used in the DL-MAP IEs and table 2.3 shows the DL-MAP format. The content of IEs is different from kind to kind depending on which kind of information the IE refers to (DIUC).

DIUC Usage

0-12 Different burst profiles

13 Gap/PAPR reduction

14 Extended-2 DIUC IE

15 Extended DIUC

Table 2.2 OFDMA DIUC values

Syntax Size Notes Management Message Type=2 8 bits

PHY Synchronization variable for synchronization 、PHY specific

DCD Count 8 bits The configuration change count of the DCD

Base Station ID 48 bits The most significant 24 bits shall be operator ID

DL-MAP IEs variable PHY specific

Padding 0 or 4 bits Padding to reach byte boundary Table 2.3 DL-MAP format

2.1.7 Uplink Map (UL-MAP) message

The UL-MAP defines the uplink bandwidth allocations. The allocations are defined by UL-MAP IEs which shall indicate the allocated number of OFDMA symbols and subchannels, the OFDMA symbol and subchannel offset, etc. The UL-MAP is sent in the first DL burst whose subcarrier permutation, modulation and coding parameters are specified by the DL-MAP. As same as the DL-MAP, Uplink Interval Usage Code (UIUC) is used in the UL-MAP. The table 2.4 defines the OFDMA UIUC encoding and table 2.5 shows the UL-MAP.

UIUC Usage

0 Fast-Feedback Channel

1-10 Different Burst Profile

11 Extended UIUC 2 IE

12 CDMA Bandwidth Request, CDMA Ranging 13 PAPR reduction allocation, Safety Zone

14 CDMA Allocation IE

15 Extended UIUC

Table 2.4 OFDMA UIUC values

Syntax Size Notes

Configuration Change Count of the UCD

Allocation Start Time 32 bits Effective start time of the uplink allocation defined by the UL-MAP UL-MAP IEs variable PHY specific

Padding 4 bits Padding to reach byte boundary Table 2.5 UL-MAP format

UL-MAP IEs with different UIUC values contain different content. For example, UL-MAP IE with UIUC=12 allocates resources for CDMA ranging and bandwidth request. OFDMA symbol offset, subchannel offset, number of OFDMA symbols, number of subchannels and ranging methods are defined in this UL-MAP IE. UL-MAP IE with UIUC=14 allocates bandwidth to a user that requested bandwidth using a CDMA request code. In this IE, duration of the allocation, repetition code used inside the allocated burst, the CDMA code sent by the SS, the ranging code, OFDMA symbol and ranging subchannel used by the SS are indicated.

2.2 MAC Overview

The MAC is connection-oriented and supports various levels of QoS. All services are mapped to a connection based on the associated QoS levels. Each service flow is associated with a single CID (connection identifier). A scheduling service is determined by a set of QoS parameters which are managed using DSA (Dynamic Service Addition) and DSC (Dynamic Service Change). Five services are supported: Unsolicited Grant Service (UGS), Real-time

Polling Service (rtPS), Extended rtPS (ertPS), Non-real-time Polling Service (nrtPS), and Best Effort (BE).

The UGS is designed to support real-time data streams that generate fixed-sized data packets on a periodic basis, such as Voice over IP (VoIP) without silence suppression.

The rtPS is designed to support real-time data streams that generate variable-sized packets at periodic intervals, such as MPEG video.

The Extended rtPS (ertPS) is a scheduling mechanism which builds on the efficiency of both UGS and rtPS. The BS provides unicast grants in an unsolicited manner like in UGS, which saves the latency of a bandwidth request. The allocations for ertPS are dynamic instead of fixed for UGS allocations. ertPS is designed to support real-time service flows that generate variable size data packets on a periodic basis, such as VoIP services with silence suppression.

The nrtPS is designed to support delay-tolerant data streams consisting of variable-sized data packets for which minimum data rate is required, such as FTP.

The BE (Best Effort) service is to provide a scheduling service to support data streams for which no minimum service level is required and may be handled on a space-available basis.

The mandatory QoS service parameters such as Maximum Sustained Traffic Rate, Maximum Latency, Minimum Reserved Traffic Rate, Tolerated Jitter, etc, are defined according to different kind of scheduling service.

Besides, many other techniques are applied in IEEE 802.16 MAC. Mobility is support via hard handover (HO), fast base station switching (FBSS) and macro diversity handover (MDHO), etc. Idle mode and sleep mode are allowed for MSs. The MAC layer also supports ARQ scheme, transport protocols, physical layer HARQ (Hybrid ARQ), adaptive modulation and coding, etc.

Chapter 3

Bandwidth Request Mechanisms in IEEE 802.16e systems

To support different levels of QoS services, changing bandwidth requirements is necessary for all services except incompressible constant bit rate UGS connections. Demand Assigned Multiple Access (DAMA) services are given resources on a demand assignment basis. There are numerous methods by which SSs can request bandwidth in IEEE 802.16e systems. In this chapter, some existed bandwidth request mechanisms are introduced and the proposed bandwidth request method is illustrated.

3.1 Requests

Because the uplink burst profile can change dynamically, all requests for bandwidth shall be made in terms of bytes needed to carry the MAC header and payload, but not the PHY header. The Bandwidth request message may be transmitted during any uplink allocation, except during any initial ranging interval. Bandwidth request may be incremental or aggregate.

A request may come as a stand-alone bandwidth request header or a Piggyback Request (optional) which is carried in the Grant Management subheader. The following figure shows the bandwidth request header.

Figure 3.1 Bandwidth request header format

The length of the bandwidth request header shall always be 48 bytes. The EC field shall be set to 0, which means no encryption. The type field indicates whether the request is incremental or aggregate. The CID (Connection Identifier) indicates the connection which the bandwidth is requested for. The Bandwidth Request (BR) indicates the number of bytes requested.

3.2 Polling

Polling is activated by the BS which allocates bandwidth for SSs to make bandwidth requests. These allocations may be to individual SS or to groups of SSs. The allocations are not in the form of explicit message, but are in the form of IEs within the UL_MAP. Polling may be performed by unicast, multicast, broadcast or PM (Poll Me) bit.

For unicast, each SS is polled individually and allocated with sufficient bandwidth to request bandwidth within the UL-MAP. If the SS does not need bandwidth, the allocation shall be padded.

If no sufficient bandwidth is available to poll each SS individually, some SSs may be polled in multicast groups or a broadcast poll may be issued. As with individual polling, the bandwidth is indicated by IEs within the UL-MAP. For multicast or broadcast polling, the allocation is to a multicast or broadcast CID (connection identifier). To reduce the collision probability, only SSs needing bandwidth reply by Request IEs and contention resolution algorithm is applied. The SS shall assume that the transmission has been unsuccessful if no response has been received before timeout.

SSs with active UGS connections may set the PM bit in the Grant Management subheader in a MAC packet of the UGS connection to tell the BS that they need to be polled.

When the BS detects the polling request, it performs the polling process to satisfy the request.

3.3 Contention-based Bandwidth Requests

IEEE 802.16e also supports contention-based bandwidth requests for OFDM-PHY and OFDMA-PHY. The BS may allocate some resource which is defined in the UL-MAP for SSs to send contention signal. For the UGS and the rtPS, the SS is prohibited from using any contention request opportunities for the connection. The allocation is divided into several transmission opportunities (TOs) and SSs shall send their contention signals in their selected TOs. TO is the basic allocation for a SS to transmit contention signal. These contention signals are allowed to collide. When the BS detects the signals, it will allocate another resource for those SSs to send message to tell the BS the amount of bandwidth they needs.

The detail of contention-based bandwidth requests mechanisms will be illustrated as follows.

3.3.1 Contention-based Focused Bandwidth Requests for WirelessMAN-OFDM

The WirelessMAN-OFDM PHY supports two contention-based Bandwidth Request

mechanisms. The first one allows the SS to send bandwidth request header shown in figure 3.1 during a REQ Region-Full. The other one is that the SS may send a Focused Contention Transmission during a REQ Region-Focused.

In a REQ Region-Full, when subchannelization is not active, the SS shall transmit a short preamble followed by the bandwidth request MAC header shown in figure 3.1.

When subchannelization is active, the allocation is partitioned in to Transmission Opportunities (TOs) both in time and frequency. The width (in subchannels) and length (in OFDM symbols) of each TO is specified in the UCD message. The transmission of an SS shall contain a subchannelization preamble corresponding to the TO chosen, followed by data OFDM symbols using the most robust mandatory coding method (BPSK-1/2).

In a REQ Region-Focused, a SS shall transmit a 4-bit Contention Code over a TO that consists of 4 subcarriers by two OFDM symbols. An OFDM symbol can be divided into 48 contention channels. The selection of the Contention Code and Contention Channel is done with equal probability among the eight possible codes which is shown in table 3.1. Upton detection, the BS shall provide an uplink allocation for the SS to transmit a Bandwidth Request MAC PDU by transmitting an OFDM Focused_Contention_IE, which specifies the Contention Channel, Contention Code, and Transmission Opportunity that were used by the SS. The OFDM Focused_Contention_IE is shown in table 3.2. This allows an SS to determine whether it has been given an allocation by matching these parameters with those it used.

During the first OFDM symbol of the TO, the phase of the four subcarriers is not specified.

During the second OFDM symbol of the TO, the phases shall depend on the corresponding bit in the chosen contention code, and the phase transmitted during the first OFDM symbol on the same subcarrier. If the code bit is +1, the phase shall be the same as that transmitted during the first OFDM symbol. If the code bit is -1, the phase shall be inverted 180 degrees with respect to the phase transmitted during the first OFDM symbol. If b m k0

(

,

)

represents

the kth subcarrier of the mth contention channel in the first OFDM symbol, then the corresponding value during the second OFDM symbol is b m k1

(

,

)

=C k b m k_n( )⋅ 0

(

,

)

where n is the index of the selected contention code.

Code index Bit 0 Bit 1 Bit 2 Bit 3

Table 3.1 OFDM Contention codes

Syntax Size Frame Number Index 4 bits

Transmit Opportunity Index 3 bits Contention Channel Index 6 bits Contention Code Index 3 bits

Table 3.2 OFDM focused contention IE format

3.3.2 Contention-based CDMA Bandwidth Requests for WirelessMAN-OFDMA

3.3.2.1 Transmission Opportunity

For OFDMA systems, the BS will allocate some resources to SSs to perform bandwidth requests. The allocation is called ranging channel. These resources are indicated within the UL-MAP IE with UIUC=12. OFDMA symbol offset, number of OFDMA symbols, subchannel offset, number of subchannels and ranging method are defined in the UL-MAP IE.

Figure 3.2 illustrates the frame structure and the allocation.

Figure 3.2 Frame structure and the allocation of ranging subchannels

The Bandwidth Request allocation is subdivided into transmission opportunities (TOs) of N ₁ OFDMA symbols by N subchannels. The allocation may not be a whole multiple of ₂ N ₁ symbols, so a gap may be formed. This is illustrated in figure 3.3. The SS can send a transmission in two ways. The first one is to modulate one ranging code on the ranging subchannel for a period of one OFDMA symbol. The other one is to modulate three consecutive ranging codes on the ranging subchannel for a period of three OFDMA symbols (one code per symbol). Thus, N is either 1 or 3. Which of these two methods is used is ₁

The Bandwidth Request allocation is subdivided into transmission opportunities (TOs) of N ₁ OFDMA symbols by N subchannels. The allocation may not be a whole multiple of ₂ N ₁ symbols, so a gap may be formed. This is illustrated in figure 3.3. The SS can send a transmission in two ways. The first one is to modulate one ranging code on the ranging subchannel for a period of one OFDMA symbol. The other one is to modulate three consecutive ranging codes on the ranging subchannel for a period of three OFDMA symbols (one code per symbol). Thus, N is either 1 or 3. Which of these two methods is used is ₁

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