Muos Wcdma Air Interface

THE MUOS-WCDMA AIR INTERFACEJohn S. Sadowsky General Dynamics C4 Systems 8201 East McDowell Road, MD H2171 Scottsdale, AZ 85257 480 441 1534 [email protected] ABSTRACT WCDMA is a 3rd generation wireless communication system developed for terrestrial cellular systems. The Mobile User Objective System (MUOS) uses WCDMA as a basis for its waveform definition, thus exploiting the extensive development of this advanced commercial communications technology. MUOS is, however, a geosatellite system which in certain ways is a vastly different communications environment than the terrestrial cellular environment. This paper shall overview key changes to the WCDMA Layer 1 to yield the MUOS waveform definition. INTRODUCTION The Mobile User Objective System (MUOS) is a geosatellite UHF communications system that utilizes the WCDMA air interface developed by the 3rd Generation Partnership Project (3GPP) for terrestrial cellular systems. The geo-satellite link, however, poses a number of different characteristics than terrestrial cellular. These include ~600 ms round trip propagation delay, tight multipath delay spreads (due to satellite elevation), and long coherence time (due to a UHF rather than S-band carrier). The satellite footprint is segmented into 16 beams. Adjacent satellite beams are somewhat analogous to interfering base stations in the terrestrial case; however satellite beams fade in unison as opposed to independent fading seen in terrestrial systems. Thus, MUOS cannot exploit many of the diversity techniques designed into 3GPP WCDMA. This paper is a survey of the various MUOS changes to the 3GPP WCDMA Layer 1 definition. An important topic for the full understanding of the MUOS waveform is power control. This paper shall discussion how the power control feedback is encoded and communicated over physical control channel. We do not, however, discuss any details of the power control algorithm – which is a topic worthy of an entire paper of its own. 3GPP WCDMA was designed to deal with S-band Doppler can result in coherence times less than 1 m sec. This requires rather a rather large dedicated physical control channel (DPCCH) power overhead to support layer 1 signaling and receiver tracking algorithms. The much longer MUOS UHF coherence times makes this costly high DPCCH power overhead unnecessary, and, furthermore, there is a strong desire to minimize control channel overhead in order to maximize data bearing capacity. Thus, the MUOS waveform was designed to operate with significantly less DPCCH and CPICH power than is typical in terrestrial WCDMA. In the user-to-base (U2B) direction, the DPCCH (which included dedicated pilot symbols and physical layer signaling) power is reduced roughly 7 dB. On the B2U direction, the common pilot channel (CPICH) may be reduced as much as 10 dB. The reduction in DPCCH power (in both directions) requires a re-design of physical layer control channel FEC coding. In order to achieve time diversity gains with the very long coherence times, the MUOS coding system shall employ a Dovetail-Interleaving (DTI) scheme. In comparison to the typical 3GPP interleaving span of 40 m sec, MUOS DTI achieves spans of 640 m sec. This is done with a relatively minor augmentation of 3GPP interleaving. THE WCDMA RADIO ACCESS BEARER The Layer 1 and 2 transmit data flow is illustrated in Figure 1 for a WCDMA radio access bearer (RAB). The inter-layer interface consists of logical channels called transport channels (TrCH). Typically there is one TrCH for higher layer control, and there are one or more TrCHs for user dedicated data. TrCH data is presented as transport blocks (ri p “ o s l blocks” A TrCH m y ). transport format is the number of blocks to be transmitted. (In 3GPP WCDMA, block size is also a variable transport format parameter. Each MUOS TrCH has a single fixed block size.) 1 of 6 and a 2nd interleaver mixes data across TrCHs on a per frame basis. which may be 1. and DTX bit insertion in the B2U. in a nutshell. The slot duration is 2/3 m sec. the slot division provides the 1500 bps transmit power control feedback rate. THE USER-TO-BASE WAVEFORM Waveform Construction The MUOS User-to-Base (U2B) waveform is very similar to 3GPP. The MAC communicates its selection to Layer 1 via the TFC indicator (TFCI). The coded and interleaved data from the different TrCHs are then multiplexed into coded composite DPDCH data frames. due to the long round trip delay. Each RAB has a legal list TFCs – all combinations not are allowed. in separate code words. The Layer 1-2 interface operates once per Transmission Time Interval (TTI). in order to exactly match the number of TrCH coded bits to the allocated number of DPDCH bits. MAC TFC selection is constrained by TrCH priority and (in the U2B) available Tx power. It operates on a per TrCH basis. The upper sub-layer is the CCTrCH that performs the following functions separately for each TrCH: CRC attachment (to each transport block). Clearly. WCDMA utilizes Orthogonal Variable Spreading Factor (OVSF) codes for spreading data. Rate matching allocates DPDCH bits between the TrCHs. MUOS cannot support a fast (one bit per slot) feedback rate. spreading and scrambling. which is important for time diversity. and the dedicated physical control channel (DPCCH). The lower sub-layer of Layer 1 is labeled the modulation sub-layer in Figure 1.10) Reed-Muller code. FEC (forward error correction) channel coding (K9 convolutional or rate 1/3 PCCC turbo). is called a transport format combination (TFC). 3GPP allows different TrCHs to have different TTIs within the same CCTrCH. As shown if Figure 1. and a PN code for scrambling. 2. TTI is thus a key parameter for determining the channel coding time span. but is coordinated across TrCHs. Rate matching is an important algorithm. The list of Transport Formats. The main job of the MAC. Layer 1 outputs DPDCH data on a 10 m sec time interval called a frame. There is one instantiation of RLC per TrCH. TFCI Figure 1: The Radio Access Bearer. Both TFCI and PCC are encoded using the 3GPP (32. Layer 1 is partitioned into two sub-layers. 1st interleaving and frame segmentation.Layer 2 has two sub-layers: radio link control (RLC) and media access control (MAC). spread via orthogonal OVSF codes and I-Q multiplexed. Most MUOS RABs have an 80 ms (8 frame) TTI. MUOS TPC feedback is transmitted via a new control channel called the physical control channel (PCC). In 3GPP WCDMA. The TFCI is an important Layer 1 control signal that must be received and correctly decoded at prior to de-interleaving and decoding of dedicated data. which has two parts: the dedicated physical data channel (DPDCH) that consists of coded bits from the Coded Composite Transport Channel (CCTrCH). 2 of 6 .84 Mcps. which consists of the TFCI (from the MAC) and the PCC (power control feedback bits). one from each TrCH. After spreading and scrambling the waveform is constructed using a root raised cosine chip waveform. The DPDCH is on the I-channel and DPCCH is on the Q-channel. It performs packet segmentation and transport block construction. After I-Q multiplexing. MUOS does not implement the 3GPP compressed mode (which is primarily used for inter-frequency and WCDMA  GSM handover measurements). the composite symbols are scrambled using a QPSK Layer 1 constructs the dedicated physical channel (PDCH). The chip rate is 3. DPDCH and the DPCCH are BPSK modulated. 4 or 8 frames. This sub-layer performs modulation. rate matching. Nonetheless. queuing and ARQ. the slot interval is important for MUOS DPCCH construction. but multiple TTIs are not allowed in MUOS. is to examine the blocks ready to transmit in the RLC queues and select the TFC. Frames are divided into 15 slots. and then applies puncturing or repetition (after channel coding). Recall in Figure 1 that the CCTrCH encodes transport blocks once per TTI. see [2]. in powers of 2. strongly stressed channels are purely Rayleigh. For an 8 frame TTI the repetition gain is about 9 dB. which overcomes the 7 dB reduction (as mentioned in the introduction) of DPCCH power level relative to a terrestrial system. the MUOS UHF channel fading characteristics are significantly slower than the terrestrial cellular S-band case. and the base station receiver can perform beam combining – which is similar in implementation to terrestrial cellular antenna diversity (typically referred ta “ot hnoe ) o s sf r advr e ” between sectors. two are coded TFCI and two are coded PCC bits. n t e a w al t s . such as supersonic aircraft. Thus. it is desirable to significantly increase the interleaving time span without breaking the 3GPP CCTrCH flow. The only true diversity that is available to the MUOS U2B link is time diversity. The solution is dovetail interleaving (DTI). The DPDCH SF varies from 256 to 8. not a Doppler spread that would result in a reduced coherence time. This is why the TFCI is encoded and decoded on each frame in 3GPP. unfortunately MUOS does not benefit from the various diversities present in terrestrial cellular. Aircraft velocity results in a large Doppler shift. as discussed below. which differentiates UE signals at the base station receiver. Coherence time. For each TFC. 2 of which are transmitted per slot (as in 3GPP [2]). as explained above. MUOS channels do experience ionospheric scintillation. SF is selected by the CCTrCH for each TFC. MUOS vehicles that are embedded in a terrestrial multipath environment generally do not have the high velocities expected in terrestrial cellular systems. The DPCCH Spreading Factor (SF) is 256. all beams see the same terrestrial/inonspheric fading channel. Likewise. power control controls DPCCH power. the SF is selected so that rate matching is achieved with no more than 100% coded bit repetition.) The MUOS U2B rate matching algorithm is the same as that used in 3GPP. the U2B PCC (power control feedback) is transmitted once every 16 frames. which is inversely related to Doppler sr dr u srmt U ’vl i r av ta es pe . Recall that in 3GPP different TrCHs can have different TTI. This allows the receiver to accumulate TFCI symbols across the TTI prior to TFCI decoding. Each UE is assigned a distinct scrambling code. Satellite elevation results in less terrestrial multipath than that occur from ground propagation in a terrestrial cellular system. While MUOS does support some high velocity vehicles. while DPDCH power is determined by the TFC dependent power ratio. Stressed ionospheric scintillation also has a coherence time. and the resulting number of DPDCH bits is 10x256/SF per slot. In MUOS. TFCI is constant across the (typically 8 frame) TTI. e r e” e s e y r e” Optimal and weakly stressed channels have a strong Ricean line-of-sight component. The MUOS satellite footprint is segmented into 16 beams.scrambling codes as described in [3]. The DPDCH/ DPCCH power ratio is also TFC dependent (in proportion to data rate) as in 3GPP. all is not equal. while the maximum TTI is only 80 m sec. About 85% of the users are expected to experience “p m lionospheric conditions. (The U2B rate matching algorithm strongly discourages puncturing in the U2B direction. All else equal. resulting in ten DPCCH symbols per slot. six are pilots. The 32 bit 3GPP Reed-Muller code is punctured to 30 bits. The MUOS U2B DPCCH has 6 pilot symbols. However. about 1% are ot a i ” “ee lsesd adh r tr “ ek sesd svr y t s . but again. MUOS coherence times can be on the order of 100 m sec or longer. Time Diversity Enhancement via Dovetail Interleaving As noted in the introduction. beam combining is really a beam forming technique that focuses a beam on the UE to reduce multiple access interference (MAI). It is not a true diversity technique. which means that the TFCI can change on frame boundaries. while the modulation sub-layer of Layer 1 operates frame-by-frame. it is typically on the order of 200 m sec or longer. DTI is a frame interleaver that 3 of 6 . these are generally not embedded in a rich multipath environment. Thus. however. Repetition across frames significantly improves TFCI reliability.e l f h Es e cy e t eo dne a st o e o t li multipath environment. Of these ten. As in 3GPP. The 30 bit ReedMuller coded PCC is repeated across these frames and decoded once after accumulating up to 16 frames of repeated PCC. However. While most MUOS channels benefit from a relatively strong line-of-sight Ricean component. However. 2 coded TFCI and 2 coded PCC bits per slot. coherence times are roughly 6 are longer at UHF compared to S-band. The end result is that MUOS coherence times are typically on the order of 100 m sec or longer. or 150x256/SF per frame. which can change on TTI boundaries. depending on data rate. B7 for TTI B. Thus. Optimal Iono. Data from successive TTIs are encoded by the CCTrCH and l e d . DTM = 1 SSN.. and hence. DTM = 1 Urban. resulting in an interleaving time span of 640 m sec. DTI complement power control. The operation of DTI is shown in Figure 3.”The a so total transmission t eo a i n T’data is DTM x i fr g e T I m v s TTI. an important issue in the B2U (as in cellular CDMA and WCDMA) is conservation of the OVSF code space. Most MUOS U2B RABs have TTI = 80 m sec and DTM = 8. where DTM is the dovetail multiplier.. 4 of 6 . and so on. B0. DTM = 8 Maritime. Severe Iono. BLER for 9. B2U MUOS beams are like sectors in a terrestrial cellular system. Severe Iono. Rate matching may employ some puncturing in order to avoid SF reduction.is inserted between the two Layer 1 sub-layers. Rate matching is discussed in more detail below.5 dB improvement on stressed channels.6 kbps RAB 1E+00 Urban. The dovetail interleaver receives all 8 encoded frames of a single TTI at once. DTM = 1 indicates no DTI. Fading that occurs slower than 600 m sec time scale can be mitigated by power control. Optimal Iono. All links transmitted on a common beam are scrambled with the same scrambling code. Unlike the U2B. Recall that the geo-satellite round trip delay is about 600 m sec. . DTM = 8 1E-01 BLER 1E-02 1E-03 0 1 2 3 4 Eb/N0 (dB) 5 6 7 8 Figure 2: DTI enhanced Layer 1. The results show 2-2. and stores most in memory. Thus. It then plays them out over time. . and various links within a beam are distinguished via different orthogonal OVSF codes. DTM = 4. Fading that occurs on a faster than 600 m sec time scale can be mitigated using the time diversity that DTI provides. Severe Iono. As in 3GPP.. . Figure 4: Some DTI performance results. SF is set to support the most heavily loaded TFC. DTM = 1 Maritime.5 dB improvement on one optimal channel. and almost 1. the total number of DPCH bits (which includes both DPDCH and DPCCH bits) per frame is 300x256/SF. Since the SF is fixed. B2U SF is constant (for the RAB) across all TFCs.6 kbps MUOS RAB on three different MUOS channel models. I t example the TTI is 8 frames. as shown in Figure 2. DTM = 8 SSN. conserve OVSF code space. This RAB may be operated with DTM = 8 or with no DTI (DTM = 1). In Figure 3. Figure 3: DTI operation on frames. a l A B C … nh be is The CCTrCH encoded DPDCH frames are labeled A0.. interleaved with DPDCH frames from other TTIs i a dvtlah n n “oe if i . Figure 4 presents some sample DTI performance results for the 9. Severe Iono. the DPCCH and DPDCH are both QPSK modulated and time multiplexed within each slot. THE BASE-TO-USER WAVEFORM The B2U DPCCH The Base-to-User (B2U) DPCCH consists of TFCI and PCC – there are no dedicated pilot symbols (unlike the 3GPP downlink). A7 for TTI A. . each slot has the same number of DPCCH bits. The most heavily loaded TFC will be one of these extended rate TFCs.0} or {3. Using the 3GPP design would result in as much as 40% DPCCH overhead for that RAB. but not for a military system with strict user data rate requirements. P and D. Unlike 3GPP. a P-slot contains PCC bits and DPDCH bits. the entire TFCI is distributed across the frames of the TTI. MUOS supports a 2. Unlike the U2B where DPDCH/DPCCH power ration varies with TFC.10) Reed-Muller code that can be encoded and decoded using the same algorithms. In 3GPP.1} combination may be illegal (per the RAB defined legal TFC list). and probably also the {4. especially beyond TTI TTI Figure 5: DPCCH insertion in the B2U. Figure 5 shows how TFCI and PCC bits are distributed across frames for a hypothetical 4 frame TTI. are required to deliver a guaranteed user data rate. the B2U symbol amplitude is constant across TFCs. with each P-slot bearing 2 PCC bits.1} TFC to prevent control preemption at the specified 32 kbps rate. and decoded once. which sets the puncturing or repetition rate for all TFCs. TFCI and PCC bits are distributed over the TTI (for TFCI) or PCC cycle. consider a RAB with one dedicated data TrCH and one control TrCH. I ae etass m t “de i d dtr es n t r r lyt . a puncturing or repetition rate is set for each TrCH by 5 of 6 .1} TFC. the rate matching rates (puncturing or repetition) will be set by the most heavily loaded TFC. a MUOS RAB must include TFCs that transmit data at a higher rate than the specified rate in order to allow for block errors. the {4. and preemption for control blocks is not accounted frne eta“de i d dtr e o it r r lavrs ” a a . In the 3GPP algorithm. It is usually the case that the control TrCH has a higher MAC priority than does data.4 kbps RAB at SF = 512.1}. The actual data rate that the user sees. r si te a t Military systems. so the four PCC code words shown in Figure 2 are repetitions. For example. N C } where N D is the number of data blocks and N C is the number of control blocks. but not in MUOS. Since there are 32 Reed-Muller coded TFCI bits. a frame is constructed using three slot formats: T. Thus. however. Excessive puncturing has a detrimental impact on performance. Since it is highly desired to constrain SF in order to conserve OVSF codes. Including repetitions. we will need to add {5. and at the receiver decoded once per TTI.0} TFC extended data rate TFC. e avrs ” a a ijust r si e h te a t the maximum rate achieved by the most heavily loaded TFC and does not account for block errors or ARQ retransmissions. the TFCI can be decoded on a frame-by-frame basis. the rate matching algorithm inserts DTX bits in the DPDCH in order to maintain the same TrCH puncturing or repetition rates across all TFCs. to reduce DPCCH overhead. especially for low data rate (hence large SF) RABs. The TFC is indicated as { N D . The first 2 slots of each frame are T-slots. this is an excessive DPCCH overhead. Thus. (Like the rock band Spinal Tap. An “de i d dtr e f 2 bs i tci e b avrs ” a a o 3 kp m g ah vd y te a t h e transmitting N D = 4 blocks of 320 bits in a 40 m sec TTI. In the above example. (Recall that different TrCHs can have different TTIs in 3GPP. The B2U PCC is encoded using a (16. our guitar amps with volume knobs that go up to 11 instead of just 10. This works fine for a terrestrial system. Base-to-User Rate Matching A second significant deviation from the 3GPP is in the B2U rate matching algorithm.) For MUOS. which might be the {4. For under-loaded TFCs (having fewer transport blocks).DPCCH and DPDCH have the same SF. and a D-slot contains only DPDCH bits.) This presents a problem. Alternatively. the PCC codeword is transmitted over 8 P-slots. Thus. Nonetheless. and moreover. the extended rate TFCs are very likely force excessive puncturing.5) bi-orthogonal code. retransmissions and periodic control preemption. which is a sub-code of the (32. The B2U PCC is transmitted every 80 m sec. so the T-slots may have of 4 TFCI bits. There may be integral order repetition. In the example of Figure 5. These are called extended rate TFCs. balancing block error rate (BLER) across TrCHs on the most heavily loaded TFC. and then. So control blocks transmission preempt data. is less due to block errors. such as MUOS. each T-slot must bear at least 4 TFCI bits. A T-slot contains TFCI bits and DPDCH bits. so the most heavily loaded may be either {4. in the B2U power is throttled up or down with data rate via DTX bit insertion. we would have the situation that an infrequently used extended rate TFC constrains the performance across the entire RAB.Mu i ei ad hnecd g 2 tl n n” 3GPP Technical Specification. Tx Amplitude TFCI Figure 3: B2U amplitude profile across one frame. The nominal set contains all other TFCs. and will be transmitted at the nominal set amplitude. 6 of 6 PCC . The TFC set is partitioned into two subsets: the nominal set and the exception set. much like the 3GPP uplink case. 1“hs acanlad ap g f S 5 1. in order to overcome its performance loss. Thus. is applied only to the nominal set. and a modified rate matching algorithm efficiently accommodates extended rate operation.0} TFC achieving 32 kbps in our example) will have no DTX. 2“ lp x g n canloi . again. the exception set TFCs will have a diminished performance relative to the nominal set. This protects the nominal set from excessive extended rate puncturing. F D . puncturing/repletion rates are set using the specified data rate TFC (which would be the {4. Figure 3 illustrates the MUOS solution. P and D slots. SUMMARY AND CONCLUSION This paper has reviewed the MUOS Layer 1 air interface as a modification of the 3GPP WCDMA definition. All nominal set TFC D D Hsm o a t nm td i ai d nm nl P C y bl r r s ie wt f e “o i s ea t h x a set amplitude” n pwer is throttled back for underad o loaded TFCs (in the nominal set) via DTX bit insertion. repetition and puncturing rates are set to completely utilize all available DPDCH bits. The rate matching for each TFC in the exception set is done in a fashion similar to the 3GPP uplink algorithm. A frame is illustrated with T. S 5 1. Once 3GPP sets the puncturing rate for the most heavily loaded TFC. The solution is a MUOS modification to the 3GPP rate matching algorithm.P yi lhne n m pi o 2 c s n transport channels onto physical channels (D ) 3 P T cn aS ecification. the DPCCH fields may be transmitted with inflated amplitude relative to the data.S r i ad dli (D ) 2 an ao ” 3GPP Technical Specification. For each exception set TFC. im i a sh r eo a T C . and within the nominal set the most heavily loaded TFC should be the one that delivers the specified RAB data rate. S 5 1.0} TFC in our example). The main modification for U2B transmission is the insertion of dovetail interleaving that allows MUOS to exploit time diversity in a long coherence time UHF fading environment. G P ehi l p ” c [2] T 2. As in 3GPP. To overcome this performance disadvantage.about 20% puncturing for the 3GPP rate 1/3 turbo code. 3“ped g n Mou t n F D . ht t a tn t sa frl F s T a s ni i t l ’ OK for terrestrial systems because the most heavily l dd F it ‘ ok os’htevrt o e T C sh w r hr t dl e h a e e a i s e advertised data rate. The B2U has been modified in the way DPCCH bits are inserted into the waveform. The exception set contains the extended rate TFCs. with DTX on underloaded TFCs. An extended rate TFC is transmitted with a large amplitude. The nominal set TFC having the specified rate (the {4. which is not acceptable. The 3GPP downlink rate matching algorithm. There is no DTX on exception set. [3] T 2. PCC REFERENCES [1] T 2. For MUOS. these TFC need a power boost. however. Of course. 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