The communication technology has evolved continually for decades, and mobile video streaming becomes a popular service after the conventional voice connection. As the demand of broadband wireless access (BWA), grows rapidly, people need advanced technologies to provide high speed wireless transmission services under some constraints such as the standby time, product size, and etc. Among all cellular technologies, the WiMax standard (IEEE 802.16 2004/ 2005) is considered as one of important candidates for supporting these new services.
Fig.8a Mobile Wimax
1. Medium access control methods:
The BWA system is a two-way point-to-multipoint wireless network. And it is a kind of shared medium. Therefore, there are several developed MAC technologies below.
(1) Duplex
This technology is developed to decide the relationship between the downlink and uplink. It includes the FDD (frequency division duplex) and TDD (time division duplex). FDD uses a pair of frequency bands for downlinks and uplinks. And TDD uses a frequency slot on a time basis.
(2) Downlink
Downlink is from BS to MSs with a point-to-multipoint operation. The BS broadcasts information to all MSs with a choose multiplexing method. Each MS will receive all information and check out what the signal is belonged to its. FDM (frequency division multiplexing) and TDM (time division multiplexing) are main multiplexing methods.
FDM is for BS to send signals to MS in different frequency slot. And TDM is for BS to allocate several time slots to MS if MS has a requirement.
(3) Uplink
ALL the MS could share some uplink signals to BS who controls the uplink access. There are two kinds of uplink operations, FDMA (frequency division multiple access) and TDMA (time division multiple access). The FDMA is for BS to allocate a frequency slot to MS when MS wants to transmit signals to BS. The TDMA is for MS to transmit its
signals to BS in an allocated time slot.
(4) Combination
(i)FDMA/FDM/FDD is for fixed rate access service, but not suitable for large number of MSs because of the limit of bandwidth utilization;
(ii)FDMA/TDM/FDD is for the high rate in the downlink and fixed rate in the uplink, but not suitable for the flexible rate in the uplink;
(iii)TDMA/TDM/FDD (iv)TDMA/TDM/TDD
They are appropriate for high speed, flexible rate and large number of MSs.
(5) Request/grant mechanism
The bandwidth request/grant is a key method in BWA system. there are three man methods in this operation.
(i) Polling: The BS broadcasts an ID for the MS in each downlink frame, and then the MS is the only one that can use the coming
slot in next frame to transmit its bandwidth request to the BS.
(ii) ALOHA: Each MS send its request randomly in a fixed channel.
(iii) Piggybacking: on the current transmission, the MS send the piggybacking to reduce overhead bandwidth allocation. But when the MS has no current transmission, the system must provide an additional operation to establish the first transmission by using polling or ALOHA.
2. Processing Units at MAC and PHY
We introduce all connection communicating with the BS using time-division multiplexing/time-division multiple access (TDM/TDMA). The wireless link of each connection from BS to MS is depicted in Fig.8b. At PHY, multiple transmission modes are available for each user. Those represent a pair of a specific modulation format based on channel estimation obtained form receiver, as shown in Fig.8c. The AMC selector determines the modulation-coding scheme whose index is sent back to the transmitter through a feedback channel for the AMC controller to update the transmission mode.
Fig.8b Wireless link with adaptive modulation and coding scheme
Fig.9c Transmission modes in the IEEE 802.16 standard
Fig.8d Processing units at MAC and PHY
At the MAC, the processing unit is a packet consisting of multiple information bits which include packet header, payload, and cyclic redundancy check (CRC) bits. At the PHY, the processing unit is a frame consisting of multiple transmission symbols.
And data are transmitted frame by frame through wireless channel.
Based on channel estimation at the receiver, the adaptive modulation and coding (AMC) selector determines a suitable scheme.
After modulation and coding with appropriate transmission mode and transmission rate, each packet is mapped to a symbol block consisting of multiple symbols, as shown in Fig.8d.
3. Time Plane of 802.16e OFDMA Systems
Fig.8e Time plane of IEEE 802.16 frame structure The frame structure consists of the following:
A preamble using the first symbol, FCH with fixed number of subchannel, DL_MAP and UP_MAP message for resource allocation of downlink and uplink data bursts, and some uplink control channel for ranging, uplink ACK and etc.
DL_MAP can present resource allocation information for burst or each MS. It will cause processing overhead when MS must search its own packet among many packet combined in a burst. And however, each MS can be effectively allocated resource by using DL_MAP, it causes another overhead due to transmission of many DL_MAP IE message. Therefore, the 802.16 system defines several kinds of MAP message to reduce the size of MAP messages.
Additionally, the 802.16 system defines two kinds of subchannel establishing methods to support various types of channel condition.
4. Diversity Subchannels:
It is expected to achieve the MSs with high velocity or low signal-to-interference ratio (SINR) for avoiding deep fading and averaging intercell interference by selecting subcarriers pseudo
randomly.
5. Band AMC Subchannels:
In this channel structure, the channel response can be seen as a flat fading channel. Due to the flat fading nature of this subchannel, the system can good at the multi-user diversity as the channel situation does not significantly change during the scheduling procedure.
Therefore, it is expected to appropriate for MSs with low velocity or high SINR.
6. Uplink Control Channel For a Cross-Layer Protocol
The Mobile Wimax standard provides several uplink control channels for fast exchange of information, such as physical channel information and ACK/negative ACK (NACK), for cross-layer operation. And the system performance can be improved by carefully utilizing those uplink control channels to exchange cross-layer information.
1) CQICH—The channel quality information channel (CQICH) is allocated to an MS using a CQICH control IE management message, and is used to report the downlink carrier-to-interference-plus-noise ratio (CINR) for either diversity subchannels or band AMC subchannels. This channel occupies one uplink slot in the FAST-FEEDBACK region allocated through UL_MAP message. For diversity subchannels, the MS reports the average CINR of the BS preamble from which the BS is able to determine the DL modulation and coding scheme (MCS) level.
Here, a CINR measurement is quantized into 32 levels and encoded into five information bits. On the other hand, for band AMC subchannels, a mobile station (MS) can report the differential of CINR values of five selected frequency bands (increment: 1 and decrement: 0 with a step of 1 dB) on this CQICH after reporting the CINR measurements of the five best bands using a MAC management message such as REP-RSP.
2) Fast Feedback Channel—Fast feedback channels may be allocated individually to MSs for the transmission of PHY-related information that requires a fast response from the MS. One fast feedback channel occupies one UL slot in the FAST-FEEDBACK region allocated through a UL_MAP message. Using these fast feedback channels, the MS can report the followings:
‧ Variable information for MAC operation, such as the anchor BS selection information for macro diversity handover and the request for UL rate adaptation
of VoIP service
‧ PHY-related information, such as DL channel measurement information for multiple-input multiple-output (MIMO )operation, the MIMO coefficient for the best DL reception (e.g., antenna weight), and MIMO mode selection (e.g., space-time transmit diversity [STTD], spatial multiplexing [SM], and beamforming).
3) UL ACK Channels—A HARQ ACK channel region for the inclusion of one or more ACK channel(s) for HARQ support of MSs is allocated using a HARQ ACK region allocation IE. The UL ACK channel occupies one half-slot in this HARQ ACK channel region, which may override the fast feedback region. This UL ACK channel is implicitly assigned to each HARQ enabled burst according to the order of the HARQ-enabled DL bursts in the DL MAP. Thus, the MS can quickly transmit ACK or NACK feedback for DL HARQ-enabled packet data using this UL ACK channel.
4) UL sounding—The 802.16e OFDMA system defines UL sounding to support smart antenna or MIMO, and this UL sounding is a kind of UL pilot signal. The BS measures the UL channel response from UL sounding waveforms transmitted by each MS, and translates the measured UL channel response to an estimated DL channel response under the assumption of TDD reciprocity. In order to allocate resources for the transmission of UL channel sounding, the BS allocates a sounding zone through a UL_MAP message. In this sounding zone each MS can transmit its UL sounding signal, maintaining signal orthogonality among multiple multiplexed MS sounding transmissions.