divided into four mini-slots for frequency hop- Code work order Radio resource mappin ing,and one slot is assigned per frame as a I 104 OFDM blocks in 8 slots I basic radio resource. Therefore the frame struc ture is the same as shown in Fig. 2 except that there are 176( 8x 22)small resources, and each resource bit rate is reduced by additional TDMA overhead needed at the be mini-slot The same control channel can be used to assign both large and small resources using taggered frame dPa § Figure 4 depicts the coding scheme for small esources. Each tone cluster would contain 22 Of the 13 ofdm blocks in each traffic slot. three blocks are used as overhead. This includes a t of two leading blocks(duration of one-half block lord index 13 Time for each of the four mini-slots: this can be real- I Figure 3. Coding of a large radio resource with clustered OFDM and fre ized by using one block in every other tone) for ynchronization and a trailing block (one-fourth quency hopping in a frame; radio resource mapping onto OFDMs time/fre- block for each of the four mini-slots)as guard quency structure provides interleaving, which is required for effective error-correction coding time for separating consecutive mini-slots. A sin- gle radio resource is associated with a f hopping pattern, by which the packets are transmitted using four different tone clusters in For the base station, where uplink transmis- each of the four mini-slots Coding across four ion for all radio resources is asynchronous, a lini-slots for user data, as shown in Fig. 4 receiver may separate 192-kHz clusters with filters exploits frequency diversity. However, it shoul followed by independently synchronized demodu- be noted that when large and small resources are lators. For the mobile terminal, where downlink simultaneously assigned in different clusters of ransmission for base station radio resources is given slot, the frequency range over which small typically synchronous, a receiver may use a single resources can hop to achieve frequency diversity demodulator with receiver windowing to result in ght be limited. Mixed assignment of large and strong attenuation of undesired clusters. Howev- small resources is a topic for further study. Tak er, adjacent clusters may be asynchronous if ing into account overhead for OFDM block guard transmitted by different base stations. The time, synchronization, slot separation, and DPA receiver structure requires further study control, a peak data rate of 1.936(3.3792 x 22/24 x 10/13 x 104/128) Mb/s is available using all 176 LOW-DELAY SERVICES: dio resources, each of 11 kb, SMALL RADIO RESOURCES A FRAME STRUCTURE FOR Similar to the previous section, 528 suchan DYNAMIC PACKET ASSIGNMENT (4. 224 MHz) are organized into 22 clusters of subchannels(192 kHz)each and 8 time slots of The downlink structure is shown in Fig. 5. The 13 blocks each within a 128-block(20 ms)frame. uplink structure is similar, but the control func a difference is that these time slots are further tions are slightly different. At the beginning of each frame. the control channels for both the DPA procedures described earlier sequentially 13 OFDM blocks with a predetermined staggered schedule among adjacent base stations. Some control channe overhead is included to allow three sectors to per form DPA at different time periods, thus obtain BB G ing additional signal-to-interference ratio(SIR) enhancement for the control information for traffic channels, spectrum reuse is achieved by interference avoidance using DPa to avoid slots BBB G hat can cause potential interference; a reuse of 1 旨 is achieved with this intelligent"partial loading. This frame structure permits SiR estimation on all unused traffic slots The desired signal mated by the received signal strength from the two OFDM blocks used for paging, while the 1 mini-slot I interference can be estimated by measuring three 1 slot blocks of received pilot signals. The pilot channels are generated by mapping all the radio resources B: 1 OFDM block currently in use onto corresponding pilot sub- G: Guard equivalent to 0.25 OFDM block in duration channels, thus providing an"interference map I Figure 4. Assignment of a small radio resource with clustered OFDM and [3]. The OFDM the actual traffic subchannels scheme can process many sub- frequency hopping in four mini-slots within a slot. hannels in parallel, which provides a mechanism IEEE Communications Magazine. July 200084 IEEE Communications Magazine • July 2000 For the base station, where uplink transmis￾sion for all radio resources is asynchronous, a receiver may separate 192-kHz clusters with filters followed by independently synchronized demodu￾lators. For the mobile terminal, where downlink transmission for base station radio resources is typically synchronous, a receiver may use a single demodulator with receiver windowing to result in strong attenuation of undesired clusters. Howev￾er, adjacent clusters may be asynchronous if transmitted by different base stations. The receiver structure requires further study. LOW-DELAY SERVICES: SMALL RADIO RESOURCES Similar to the previous section, 528 subchannels (4.224 MHz) are organized into 22 clusters of 24 subchannels (192 kHz) each and 8 time slots of 13 blocks each within a 128-block (20 ms) frame. A difference is that these time slots are further divided into four mini-slots for frequency hop￾ping, and one slot is assigned per frame as a basic radio resource. Therefore, the frame struc￾ture is the same as shown in Fig. 2 except that there are 176 (8 x 22) small resources, and each resource bit rate is reduced by additional TDMA overhead needed at the beginning and end of a mini-slot. The same control channel can be used to assign both large and small resources using staggered frame DPA. Figure 4 depicts the coding scheme for small resources. Each tone cluster would contain 22 individual modulation tones plus 2 guard tones. Of the 13 OFDM blocks in each traffic slot, three blocks are used as overhead. This includes a total of two leading blocks (duration of one-half block for each of the four mini-slots; this can be real￾ized by using one block in every other tone) for synchronization and a trailing block (one-fourth block for each of the four mini-slots) as guard time for separating consecutive mini-slots. A sin￾gle radio resource is associated with a frequency￾hopping pattern, by which the packets are transmitted using four different tone clusters in each of the four mini-slots. Coding across four mini-slots for user data, as shown in Fig. 4, exploits frequency diversity. However, it should be noted that when large and small resources are simultaneously assigned in different clusters of a given slot, the frequency range over which small resources can hop to achieve frequency diversity might be limited. Mixed assignment of large and small resources is a topic for further study. Tak￾ing into account overhead for OFDM block guard time, synchronization, slot separation, and DPA control, a peak data rate of 1.936 (3.3792 x 22/24 x 10/13 x 104/128) Mb/s is available using all 176 radio resources, each of 11 kb/s. A FRAME STRUCTURE FOR DYNAMIC PACKET ASSIGNMENT The downlink structure is shown in Fig. 5. The uplink structure is similar, but the control func￾tions are slightly different. At the beginning of each frame, the control channels for both the uplink and downlink jointly perform the four DPA procedures described earlier sequentially with a predetermined staggered schedule among adjacent base stations. Some control channel overhead is included to allow three sectors to per￾form DPA at different time periods, thus obtain￾ing additional signal-to-interference ratio (SIR) enhancement for the control information. For traffic channels, spectrum reuse is achieved by interference avoidance using DPA to avoid slots that can cause potential interference; a reuse of 1 is achieved with this intelligent “partial loading.” This frame structure permits SIR estimation on all unused traffic slots. The desired signal is esti￾mated by the received signal strength from the two OFDM blocks used for paging, while the interference can be estimated by measuring three blocks of received pilot signals. The pilot channels are generated by mapping all the radio resources currently in use onto corresponding pilot sub￾channels, thus providing an “interference map” without monitoring the actual traffic subchannels [3]. The OFDM scheme can process many sub￾channels in parallel, which provides a mechanism ■ Figure 3. Coding of a large radio resource with clustered OFDM and fre￾quency hopping in a frame; radio resource mapping onto OFDM's time/fre￾quency structure provides interleaving, which is required for effective error-correction coding. χ x x x x x x x x Word index Time 528 1 1 13 Bit index (within a word) 528 tones divided into 22 24-tone clusters Frequency Code work ordering Radio resource mapping 104 OFDM blocks in 8 slots ■ Figure 4. Assignment of a small radio resource with clustered OFDM and frequency hopping in four mini-slots within a slot. B: 1 OFDM block G: Guard equivalent to 0.25 OFDM block in duration 1 slot Time 1 mini-slot B B B BB B G BB B G BB B G Frequency 528 tones divided into 22 24-tone clusters 13 OFDM blocks G